Highway Safety
Research Continues on a Variety of Factors That Contribute to Motor Vehicle Crashes Gao ID: GAO-03-436 March 31, 2003Nearly 6.3 million motor vehicle crashes occurred in the United States in 2001, or one crash every 5 seconds. On average, a person was injured in these crashes every 10 seconds, and someone was killed every 12 minutes. Since the 1970s, progress has been made in reducing the number of fatalities and injuries on our nation's roads. From 1975 through 2001, fatalities decreased from 44,525 to 42,116, while the rate of fatalities per 100 million vehicle miles traveled decreased from 3.35 to 1.51. However, the decline in fatalities has leveled off in recent years. In the 1970s, Indiana University conducted one of the most significant studies to date on the factors that contribute to motor vehicle crashes. This study examined human, environmental, and vehicle factors that contribute to crashes. As requested, this report provides more recent information from data, experts, and studies about the factors that contribute to motor vehicle crashes and information about major ongoing and planned Department of Transportation research into factors that contribute to crashes.
Many factors combine to produce circumstances that may lead to a motor vehicle crash--there is rarely a single cause of such an event. Three categories of factors contribute to crashes: human factors, roadway environment factors, and vehicle factors. Human factors involve the actions taken by or the condition of the driver of the automobile, including speeding and violating traffic laws, as well as being affected by alcohol or drugs, inattention, decision errors, and age. Roadway environment factors include the design of the roadway, roadside hazards, and roadway conditions. Vehicle factors include any failures that may exist in the automobile or design of the vehicle. Human factors are seen as the most prevalent, according to data, experts, and studies, in contributing to crashes, followed by roadway environment and vehicle factors. Agencies within the Department of Transportation have research projects underway or planned that address the factors that contribute to crashes. For example, the Federal Motor Carrier Safety Administration and the National Highway Traffic Safety Administration are conducting a study on the causes and contributing factors to large truck crashes. In addition, the National Highway Traffic Safety Administration is conducting a 100-Car Naturalistic Driving Study and the Drive Atlanta Study. The 100-Car Naturalistic Driving Study involves collecting data from vehicles equipped with sensors and cameras to obtain better information on crashes and near misses. The Drive Atlanta Study involves collecting data from 1,100 vehicles equipped with data recorders to develop information about how excessive speed contributes to crashes. In addition, the Transportation Research Board has proposed a broad, 6-year, $180 million research program focused on making significant improvements in highway safety. This study, among other things, would involve installing sensors and other data collection devices on over 5,000 vehicles.
GAO-03-436, Highway Safety: Research Continues on a Variety of Factors That Contribute to Motor Vehicle Crashes
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Report to Congressional Requesters:
United States General Accounting Office:
GAO:
March 2003:
Highway Safety:
Research Continues on a Variety of Factors That Contribute to Motor
Vehicle Crashes:
GAO-03-436:
GAO Highlights:
Highlights of GAO-03-436, a report to congressional requesters
Why GAO Did This Study:
Nearly 6.3 million motor vehicle crashes occurred in the United States
in 2001, or one crash every 5 seconds. On average, a person was
injured in these crashes every 10 seconds, and someone was killed every
12 minutes. Since the 1970s, progress has been made in reducing the
number of fatalities and injuries on our nation‘s roads. From 1975
through 2001, fatalities decreased from 44,525 to 42,116, while the
rate of fatalities per 100 million vehicle miles traveled decreased
from 3.35 to 1.51. However, the decline in fatalities has leveled off
in recent years. In the 1970s, Indiana University conducted one of the
most significant studies to date on the factors that contribute to
motor vehicle crashes. This study examined human, environmental, and
vehicle factors that contribute to crashes. As requested, this report
provides more recent information from data, experts, and studies about
the factors that contribute to motor vehicle crashes and information
about major ongoing and planned Department of Transportation research
into factors that contribute to crashes.
What GAO Found:
Many factors combine to produce circumstances that may lead to a motor
vehicle crash”there is rarely a single cause of such an event. Three
categories of factors contribute to crashes: human factors, roadway
environment factors, and vehicle factors. Human factors involve the
actions taken by or the condition of the driver of the automobile,
including speeding and violating traffic laws, as well as being
affected by alcohol or drugs, inattention, decision errors, and age.
Roadway environment factors include the design of the roadway, roadside
hazards, and roadway conditions. Vehicle factors include any failures
that may exist in the automobile or design of the vehicle. Human
factors are seen as the most prevalent, according to data, experts, and
studies, in contributing to crashes, followed by roadway environment
and vehicle factors.
Agencies within the Department of Transportation have research projects
underway or planned that address the factors that contribute to
crashes. For example, the Federal Motor Carrier Safety Administration
and the National Highway Traffic Safety Administration are conducting a
study on the causes and contributing factors to large truck crashes.
In addition, the National Highway Traffic Safety Administration is
conducting a 100-Car Naturalistic Driving Study and the Drive Atlanta
Study. The 100-Car Naturalistic Driving Study involves collecting data
from vehicles equipped with sensors and cameras to obtain better
information on crashes and near misses. The Drive Atlanta Study
involves collecting data from 1,100 vehicles equipped with data
recorders to develop information about how excessive speed contributes
to crashes. In addition, the Transportation Research Board has
proposed a broad, 6-year, $180 million research program focused on
making significant improvements in highway safety. This study, among
other things, would involve installing sensors and other data
collection devices on over 5,000 vehicles.
www.gao.gov/cgi-bin/getrpt?GAO-03-436
To view the full report, including the scope
and methodology, click on the link above.
For more information, contact Peter Guerrero, (202) 512-2834,
guerrerog@gao.gov.
[End of section]
Contents:
Letter:
Results in Brief:
Background:
Human, Roadway Environment, and Vehicle Factors Contribute to Motor
Vehicle Crashes:
Federal Research Directed at Better Understanding of Factors That
Contribute to Crashes:
Agency Comments and Our Evaluation:
Appendix I: Objectives, Scope, and Methodology:
Analyzing NHTSA Data:
Identifying Studies:
Interviewing Federal Officials and Experts:
Ongoing and Planned Transportation Research:
Appendix II: Tri-Level Study of the Causes of Traffic Accidents:
Objectives, Scope, and Methodology for the Tri-Level Study:
Results of the Tri-Level Study:
Appendix III: Roadway Design Features:
Figures:
Figure 1: Fatality Statistics, 1975-2001:
Figure 2: Crash Causes Found by the Tri-Level Study:
Figure 3: Speeding Drivers in Fatal Crashes, by Age and Gender, 1997-
2001:
Figure 4: Drivers in Alcohol-Related Fatal Crashes, by Age and Gender,
1997-2001:
Figure 5: Inattentive Drivers Involved in Crashes by Age, 1997-2001:
Figure 6: Number and Rate of Driver Involvement in Fatal Crashes by
Age, 1997-2001:
Figure 7: Fatality Rates by Type of Road System, 2001:
Figure 8: Vehicle Crash Rates, 2001:
Figure 9: Passenger Vehicle Rollovers, 2001:
Figure 10: Factors Contributing to Crashes Identified by the Tri-Level
Study:
Figure 11: Impact of Access Points on Traffic Crashes:
Abbreviations:
AAA: former American Automobile Association
AASHTO: American Association of State Highway and Transportation
Officials
BAC: blood alcohol content
CDS: Crashworthiness Data System
DOT: Department of Transportation
FARS: Fatality Analysis Reporting System
FHWA: Federal Highway Administration
FMCSA: Federal Motor Carrier Safety Administration
F-SHRP: Future Strategic Highway Research Program
GES: General Estimates System
NHTSA: National Highway Traffic Safety Administration
SUV: sport utility vehicle
VMT: vehicle miles traveled:
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United States General Accounting Office:
Washington, DC 20548:
March 31, 2003
The Honorable Carl Levin
United States Senate:
The Honorable George V. Voinovich
United States Senate:
Nearly 6.3 million motor vehicle crashes occurred in the United States
in 2001, or one crash every 5 seconds. On average, a person was injured
in these crashes every 10 seconds, and someone was killed every 12
minutes. While there have been significant improvements in motor
vehicle safety over the past several decades, decreases in injuries and
fatalities have leveled off since the early 1990s.
In the 1970s, Indiana University conducted a major study that examined
the human, environmental, and vehicle factors that contribute to
traffic crashes.[Footnote 1] You asked us to (1) provide more recent
information on the factors that contribute to motor vehicle crashes,
and (2) identify major ongoing and planned Department of Transportation
research into factors that contribute to crashes.
To provide information on factors that contribute to motor vehicle
crashes, we obtained and analyzed crash data from three Department of
Transportation databases. In addition, we interviewed experts from
academia, insurance organizations, and advocacy groups. To identify
recent studies on factors that contribute to motor vehicle crashes, we
conducted a literature search, explored the Transportation Research
Information System, and reviewed periodicals. This effort resulted in
numerous studies being identified on various aspects of motor vehicle
crashes. We then, with input from a number of experts and agency
officials, judgmentally selected studies that would provide additional
information on the particular factors being discussed. For each of the
selected studies that are used in this report, we determined whether
the studies' findings were generally reliable. We evaluated the
methodological soundness of the studies using common social science and
statistical practices. To identify the major ongoing and planned
research into factors that contribute to crashes, we interviewed
officials from the National Highway Traffic Safety Administration, the
Federal Highway Administration, and the Transportation Research Board.
Appendix I provides more details on our scope and methodology.
Results in Brief:
Many factors can combine to produce circumstances that lead to a motor
vehicle crash--there is rarely a single cause of such an event. Three
categories of factors contribute to crashes: human factors, roadway
environment factors, and vehicle factors. Human factors involve the
actions taken by or the condition of the driver of the motor vehicle,
including speeding and violating traffic laws, as well as being
affected by alcohol or drugs, inattention, decision errors, and age.
Roadway environment factors that contribute to, or are associated with,
crashes include the roadway design (for example, medians, narrow lanes,
the lack of shoulders, curves, access points, or intersections);
roadside hazards (for example, poles, trees, or embankments adjacent to
the road); and roadway conditions (for example, rain, ice, snow, or
fog). Vehicle factors include any vehicle-related failures that may
exist in the automobile or design of the vehicle. In general, human
factors are considered to be the most prevalent factors contributing to
crashes, followed by roadway environment and vehicle factors.
Various agencies within the Department of Transportation have research
projects underway or planned that address the factors that contribute
to crashes. For example, the Federal Motor Carrier Safety
Administration and the National Highway Traffic Safety Administration
are studying the causes of, and factors contributing to, large truck
crashes. In addition, the National Highway Traffic Safety
Administration's 100-Car Naturalistic Driving Study involves
collecting data from vehicles equipped with sensors and cameras to
obtain better information on crashes and near misses. Another project,
the Drive Atlanta Study, involves collecting data from 1,100 vehicles
equipped with data recorders to develop information about how speeding
contributes to crashes. A number of follow-on studies to these efforts
are also being considered. In addition, the Transportation Research
Board has proposed a broad, 6-year, $180 million research program
focused on making significant improvements in highway safety. This
program, among other things, could involve installing sensors and other
data collection devices on over 5,000 vehicles. The final phase of the
research program would use the results of the instrumented vehicle
study to identify countermeasure improvements.
We provided copies of a draft of this report to the Department of
Transportation for its review and comment. In discussing this report,
agency officials provided technical clarification and information,
which we incorporated in the report as appropriate. In addition,
National Highway Traffic Safety Administration officials provided
information comparing light truck and passenger car crash rates, which
we also incorporated in the report.
Background:
Since the 1970s, progress has been made in reducing the number of
fatalities and injuries on our nation's roads, but the numbers are
still significant. From 1975 through 2001, annual fatalities decreased
from 44,525 to 42,116, or about 5 percent. During the same period, the
fatality rate per 100 million vehicle miles traveled, a common method
of measurement, dropped from 3.35 to 1.51, or about 55 percent. This
reduction in fatalities was considerable, given the growth in the
number of drivers and vehicles on the road. For example, from 1975
through 2001, licensed drivers increased from about 130 million to
about 191 million, and the number of registered vehicles increased from
about 126 million to about 221 million. Figure 1 shows the yearly
number of fatalities and the rate of fatalities per 100 million vehicle
miles traveled. Injury and property-damage-only crashes also fell,
going from about 6.8 million in 1988, the earliest year of available
data, to about 6.3 million in 2001.
Figure 1: Fatality Statistics, 1975-2001:
[See PDF for image]
[End of figure]
The fatal, injury, and property-damage-only crashes have significant
economic cost. The National Highway Traffic Safety Administration
(NHTSA) recently calculated the economic costs for motor vehicle
crashes in 2000 at more than $230 billion, or the equivalent of over
$800 for every person living in the United States. NHTSA's estimate of
economic costs includes productivity losses, property damage, medical
costs, rehabilitation costs, travel delay, legal and court costs,
emergency services, insurance administration costs, and costs to
employers.
One of the most significant studies to date on the factors that
contribute to motor vehicle crashes was the Tri-Level Study of the
Causes of Traffic Accidents, conducted in the 1970s by the Indiana
University at Bloomington Institute for Research in Public Safety.
Referred to as the Tri-Level study, it investigated how frequently
various factors contributed to traffic crashes. According to NHTSA
officials, the Tri-Level study has been the only study in the past 30
years to collect large amounts of on-scene crash causation data. To
provide researchers with insight into the factors that contribute to
traffic crashes, collision data were collected on three levels, each
providing an increasing level of detail, including 13,568 police-
reported crashes; 2,258 crashes investigated by on-scene technicians;
and 420 crashes investigated in depth by a multidisciplinary team. The
study assessed causal factors as either definite, probable, or
possible. The study found that crashes were caused by human (or driver-
based) factors, environmental (roadway or weather-related) factors, or
vehicle-related factors. As shown in figure 2, the study concluded that
the human factors were definite or probable causes in about 93 percent
of crashes, while environmental and vehicle factors contributed to
about 33 and 13 percent, respectively. See appendix II for a more
detailed discussion of the Tri-Level study.
Figure 2: Crash Causes Found by the Tri-Level Study:
[See PDF for image]
[End of figure]
NHTSA's mission is to reduce deaths, injuries, and economic losses
resulting from motor vehicle crashes. As part of this responsibility,
NHTSA conducts or sponsors research into the causes of motor vehicle
crashes. NHTSA also conducts research on driver behavior and traffic
safety to develop more efficient and effective means to improve safety.
Three principal databases provide information about traffic crashes:
the Fatality Analysis Reporting System (FARS), the Crashworthiness Data
System (CDS), and the General Estimates System (GES). The FARS database
contains information provided by the states on all vehicle crashes that
result in the death of an occupant or nonmotorist within 30 days of
the
incident. The CDS database contains information from a detailed sample
of about 4,000 minor, serious, or fatal tow-away crashes, annually. To
obtain this information, teams of trained crash investigators visit the
crash site and collect data on such elements as the damage to the
vehicle and interior locations struck by the occupants. The GES
database contains information from a nationally representative sample
of police accident reports. This is NHTSA's largest crash database,
with information collected on over 50,000 crashes each year.
The Federal Highway Administration's (FHWA) safety mission is to reduce
highway fatalities and injuries through development and implementation
of a program of nationally coordinated research and technology
innovations. Research is conducted in areas that address FHWA's highway
safety goals related to roadway departure, intersections, and
pedestrians. FHWA is also conducting research in a number of areas that
will partially focus on crash causation, including rollovers, speed
management, intersection safety, and pedestrian and bicyclist safety.
FHWA annually produces a highway statistics report, which consists of
data on motor fuel, motor vehicles, driver licensing, highway-user
taxing, state and local government highway finance, highway mileage,
and federal aid for highways. FHWA also maintains a database, called
the Highway Safety Information System. The system uses data on crash,
roadway, and traffic variables collected by eight states to analyze a
number of highway safety problems. These analyses range from
identifying basic problems, to identifying the size and extent of a
safety issue, to modeling efforts that attempt to predict future
crashes from roadway characteristics and traffic factors.
Human, Roadway Environment, and Vehicle Factors Contribute to Motor
Vehicle Crashes:
Motor vehicle crashes are complex events that rarely have a single
cause. For example, it would be challenging to identify a single cause
of a crash that occurred on a narrow, curvy, icy road when an
inexperienced driver, who had been drinking, adjusted the radio or
talked on a cell phone. It would likely be the combined effect of a
number of these factors that contributed to the crash.
In examining the causes of motor vehicle crashes, a number of experts
and studies identified three categories of factors that contribute to
crashes--human, roadway environment, and vehicle factors. Human factors
involve the actions taken by or the condition of the driver of the
automobile, including speeding and other traffic violations, as well as
the effects of alcohol or drugs, inattention, decision errors, and age.
Roadway environment factors that contribute to or are associated with
crashes include the design of the roadway (for example, medians, lane
width, shoulders, curves, access points, or intersections); roadside
hazards (for example, poles, trees, or embankments adjacent to the
road); and the roadway conditions (for example, rain, ice, snow, or
fog). Vehicle factors include vehicle-related failures and vehicle
design issues that contribute to a crash. In general, human factors are
considered to be the most prevalent factor contributing to crashes,
followed by roadway environment and vehicle factors. Although this
report discusses these categories separately, they should be viewed in
terms of how they can concurrently contribute to an unstable situation
that results in a crash.
Human Factors Contribute to Motor Vehicle Crashes:
Human factors involve actions taken by or the condition of the driver
of the vehicle. They are considered the most prevalent factors by data,
experts, and studies in traffic crashes. Human factors that can
contribute to crashes include speeding and other traffic violations, as
well as the effects of alcohol or other drugs, inattention, driver
decision errors, and age.
Speeding:
Driving either faster than the posted speed limit or faster than
conditions would safely dictate can contribute to traffic crashes.
Speeding reduces a driver's ability to steer safely around curves or
objects in the roadway, extends the distance necessary to stop a
vehicle, and increases the distance a vehicle travels while the driver
reacts to a dangerous situation.
According to our analysis of NHTSA's databases, from 1997 through 2001,
speeding was identified as a contributing factor in about 15 percent of
all crashes and about 30 percent of all fatal crashes. In addition,
almost 64,000 lives were lost in speeding-related crashes.[Footnote 2]
As shown in figure 3, we found that for every age category of drivers
involved in fatal crashes, males were more likely than females to be
involved in a fatal speed-related crash. In addition, younger drivers,
regardless of sex, are the most likely to be involved in a speed-
related fatality. From 1997 through 2001, 36 percent of male drivers
and 24 percent of female drivers 16 to 20 years old who were involved
in fatal crashes were speeding at the time of the crash. The percentage
of speeding-related fatal crashes decreases with increasing driver
age.[Footnote 3]
Figure 3: Speeding Drivers in Fatal Crashes, by Age and Gender, 1997-
2001:
[See PDF for image]
[End of figure]
A 1998 study by NHTSA and FHWA indicates that fatal crashes increased
in states that raised speed limits.[Footnote 4] When Congress enacted
the National Highway System Designation Act of 1995 (P.L. 104-59),
which repealed the national maximum speed limit, the Secretary of
Transportation was required to study the impact of states' actions to
raise speed limits above
55 and 65 miles per hour. The study found that states with increased
speed limits in 1996 experienced approximately 350 more Interstate
fatalities than would have been expected based on historical trends--
about 9 percent above expectations. Concurrently, the Interstate
fatalities experienced in states that did not increase speed limits in
1996 were consistent with pre-1996 trends. The Insurance Institute for
Highway Safety also assessed the effects of speed limit
increases.[Footnote 5] Its researchers found an increase in fatalities
for a 9-month period in 1996 on Interstate highways and freeways, as
compared with the previous 6 years--about 16 percent in 12 of the
states that had raised maximum speed limits to at least 70 miles per
hour by March 1996. In contrast, occupant fatalities increased only 4
percent on Interstate highways and freeways in the comparison group of
states that did not raise speed limits. However, both of these studies
are limited because they cover short time periods.
According to a Transportation Research Board official, studies have
confirmed a direct relationship between speed and crash
severity.[Footnote 6] Once a crash has occurred--that is, a vehicle has
hit another vehicle or a stationary object--the vehicle undergoes a
rapid change in speed. While the vehicle decelerates rapidly, its
occupants continue to move at the vehicle's speed prior to impact until
they are stopped by striking the interior of the vehicle, by impact
with objects external to the vehicle if ejected, or by being restrained
by a safety belt or an airbag that deploys.
According to the FHWA Director of the Office of Safety Programs, while
absolute speed clearly relates to injury and fatality outcomes,
speeding is the real issue. The Director pointed out that despite their
lower volumes, almost half of all speeding-related fatalities occur on
local or collector roads--low-speed roads found in residential and
business areas. In addition, the Director said that speed variance is
also a factor. When vehicles driving down a particular roadway are
traveling at very different speeds, the probability of a crash
increases. The relative crash-involvement rate increases for vehicles
that are traveling above or below the average speed of traffic.
Traffic Control Violations:
Drivers who fail to follow prescribed traffic control laws also
contribute to crashes. This includes running red lights or failing to
stop at stop signs. Our analysis of NHTSA's data found that from 1997
through 2001, about 36 percent of motor vehicle crashes occurred at
traffic control devices. Of those crashes, 59 percent occurred at
traffic lights while an additional 28 percent occurred at stop signs.
A study performed by the Insurance Institute for Highway Safety and the
Preusser Research Group identified characteristics of red light-running
crashes and the drivers involved.[Footnote 7] It found that drivers'
noncompliance with traffic control devices, such as traffic signals and
stop signs, is a major cause of motor vehicle crashes. The study
examined the prevalence of red light-running crashes on a national
basis to identify the characteristics of such crashes and the drivers
involved.[Footnote 8] The study estimated that almost 260,000 red
light-running crashes occurred in 1996, of which 809 resulted in
fatalities. It also found that, as a group, red light runners involved
in crashes were more likely than other drivers to be younger than age
30, to be male, to have prior moving violations and convictions for
driving while intoxicated, to have invalid driver's licenses, and to be
reported by police as having consumed alcohol prior to the crash.
According to an official from Northwestern University, red light-
running might also partly reflect driver frustration with poor traffic
operations. For example, a driver might feel the need to speed through
a red light because of previous experience of being held at that light
too long, or of being subjected to a series of unsynchronized stop
lights. A 1999 study funded by DaimlerChrysler Corporation surveyed
over 5,000 people regarding their behavior at red lights.[Footnote 9]
The study found that those respondents who reported speeding up to beat
a red light would most often do so because they were in a rush and
wanted to save time.
Alcohol and Other Drugs:
Alcohol and other drugs are contributing factors in many motor vehicle
crashes. It is illegal in every state and the District of Columbia to
drive a motor vehicle while under the influence of, impaired by, or
with a specific blood content of alcohol or drugs. In addition, all
states but Massachusetts have blood alcohol laws that make it illegal
to drive with a specified level of alcohol in their blood.[Footnote 10]
As of January 2003, 17 states had set the standard at .10 percent blood
alcohol content (BAC)--the level at which a person's blood contains 1/
10 of 1 percent alcohol.[Footnote 11] The remaining states have more
stringent laws, setting the limit at .08 percent BAC. According to
NHTSA, on average, a 170-pound man reaches .08 percent BAC after
consuming five 12-ounce beers (4.5 percent alcohol by volume) over a 2-
hour period. A 120-pound woman reaches the same level after consuming
three beers over the same period.
In analyzing NHTSA's databases, we found that from 1997 through 2001,
there were about 76,000 alcohol-related fatal crashes (41 percent of
all fatal crashes), 980,000 alcohol-related injury crashes (10 percent
of all injury crashes), and 2.3 million alcohol-related crashes (7
percent of all crashes).[Footnote 12] During this 5-year period, nearly
85,000 people died in alcohol-related crashes. Eighty-six percent of
these fatalities occurred in crashes where the highest recorded BAC was
.08 percent or above, while 14 percent occurred in crashes where the
highest recorded BAC was between .01 percent and .07 percent. In
addition, we found that male drivers were more likely to be involved in
alcohol-related fatal crashes than female drivers. Figure 4 shows that,
for each age category, there were a greater number of male than female
drivers in fatal crashes that involved alcohol.
Figure 4: Drivers in Alcohol-Related Fatal Crashes, by Age and Gender,
1997-2001:
[See PDF for image]
[End of figure]
While research has shown that everyone's driving is impaired at blood
alcohol levels of .10 percent and higher, recent research has shown
that lower levels of alcohol also affect performance. In a study by the
Southern California Research Institute, 168 test subjects were tested
at zero BAC; then at the highest BAC for their drinking classification
of either light, moderate, or heavy drinker; and then at .02 percent
BAC intervals, as their alcohol levels decreased.[Footnote 13] For this
study, the researchers defined impairment by comparing the subjects'
performance on a given test while under the influence of alcohol versus
their performance on the same test after being given a placebo.
According to the resulting report, alcohol impaired the driving-related
skills for these volunteers at .02 percent BAC,
the lowest tested alcohol level. The magnitude of impairment increased
consistently at BACs through .10 percent, the highest level
tested.[Footnote 14] According to a Southern California Research
Institute official, this study is significant because it provided
important, previously unknown findings that certain driving-related
skills are impaired at any departure from zero BAC.
A recent study by Westat examined the relative risk of fatal crash
involvement as a function of the BAC of fatally injured or surviving
drivers.[Footnote 15] By combining crash data from FARS with exposure
data from the 1996 National Roadside Survey, the researchers determined
that, in general, the relative risk of involvement in a fatal passenger
vehicle crash increased steadily with increased driver's BAC.[Footnote
16] For example, the study found that a .02 percent BAC increase among
16-through-20-year-old male drivers was estimated to more than double
the relative risk of a fatal single-vehicle crash injury. The study
also found that among drivers aged 21 through 34, those with a BAC of
.03 percent have twice the risk of fatalities as compared with drivers
with zero BAC. Furthermore, among drivers aged 21 through 34, those
with a BAC of .10 percent have over 10 times the risk of a fatality
compared with drivers with zero BAC.
All states restrict driving while under the influence of, being
impaired by, or being incapable of safely driving because of illegal
drugs or prohibited substances in the driver's body. As of January
2003, eight states have statutes that make it unlawful for a driver to
have any amount of an illegal drug or prohibited substance in his or
her body while operating a motor vehicle, regardless of how the drug
affects the driver's driving ability.[Footnote 17] Additional states
have varying legislation that also allows zero tolerance to driving
under the influence of drugs.[Footnote 18]
Studies have shown that drugs can affect driving-related skills. For
example, a study by Maastricht University, the Netherlands, indicated
that the combined use of marijuana and alcohol impairs driving
performance.[Footnote 19] For a small number of subjects who were
somewhat frequent users of marijuana, the study found that either
marijuana doses alone or alcohol alone impaired the subjects' test-
driving performances. However, subjects who used marijuana in
combination with alcohol demonstrated impairment in several aspects of
driving performance. Another study by Maastricht University also found
the combined use of marijuana and alcohol to produce similar effects on
a small, limited group of subjects. The study showed that under the
influence of low doses of either marijuana or alcohol, the drivers were
less able to detect peripheral traffic and instead focused on the
central driving task.[Footnote 20]
Driver Inattention:
Driver inattention occurs when there is a delay in recognition of
information needed to safely accomplish the driving task. Two
categories of driver inattention are distraction and drowsiness.
Drivers may become distracted when they direct their attention
elsewhere because of some occurrence inside or outside of the vehicle.
NHTSA defines four categories of distraction: visual distraction (for
example, looking away from the roadway), auditory distraction (for
example, responding to a noise, such as a ringing cell phone),
biomechanical distraction (for example, manually adjusting the radio
volume), and cognitive distraction (for example, being lost in
thought). Many distracting activities that drivers engage in can
involve more than one of these components. Driver drowsiness is also a
type of driver inattention, in that a tired or fatigued driver may
exhibit behaviors typically associated with inattentive drivers.
Our analysis of 1997 through 2001 data from NHTSA found that, overall,
about 2.5 million drivers of passenger vehicles that were towed away
from crashes were identified as inattentive. Of these, about 1.3
million were distracted, about 871,000 "looked but did not see" (an
aspect of being inattentive), and about 348,000 were sleepy or asleep.
In addition, about 7.6 million drivers were identified as "attentive"
at the time of the crash.[Footnote 21] We also conducted a more
detailed analysis of inattentive drivers. As figure 5 shows, overall,
more drivers between ages 16 and 44 were involved in inattentive-type
crashes than drivers aged 45 and above. More drivers aged 16 to 20 were
inattentive than any other age group.
Figure 5: Inattentive Drivers Involved in Crashes by Age, 1997-2001:
[See PDF for image]
Note: This includes only those drivers involved in crashes where at
least one passenger vehicle had to be towed away.
[End of figure]
We also analyzed NHTSA's databases to determine specific sources of
distraction. We found that some outside person, object, or event was
identified as contributing to 27 percent of the distractions. Other
common sources of distractions included another occupant in the
vehicle, followed by adjusting a radio, cassette, or CD.
A recent study by the AAA Foundation for Traffic Safety analyzed 1995
through 1999 NHTSA crash data on driver attention status and sources of
distraction and found that 8 percent of drivers were identified as
distracted, 5 percent as "looked but did not see," and 2 percent as
sleepy or asleep, while 49 percent of the drivers were identified as
attentive at the time of the crash.[Footnote 22] The remaining 36
percent were either unknown or had no driver present. Without the
unknowns, the percentage of drivers identified as distracted increases
to 13 percent. The study also identified specific sources of
distraction. Some external person, object, or event caused almost 30
percent of such distractions.
Drowsiness and fatigue are also aspects of inattention and can
contribute to crashes. Drowsiness is a basic physiological state,
brought about by the restriction or interruption of sleep. It also
results from natural changes in the body's level of alertness during
each 24-hour sleep-wake cycle. According to the National Sleep
Foundation, our internal body clocks program us to be sleepy twice a
day: first during the early morning hours between midnight and dawn,
and again between 1:00 p.m. and 4:00 p.m. For the driver, the main
effect of drowsiness or fatigue is a progressive withdrawal of
attention from the road and traffic demands, leading to impaired
performance behind the wheel. Drivers can become so fatigued that they
are slow to perceive risky situations and are unable to respond quickly
enough to avoid a crash. Fatigue can also arise because of medication
or illness.
According to an official from the National Sleep Foundation, studies
have shown that sleep-deprived individuals are less likely to be able
to concentrate on the task at hand. In addition, as people get tired
they engage in behaviors that lead to other distractions, such as
smoking, drinking or eating, turning up the radio, or employing other
"tricks" to try to stay awake. The official also told us that the
foundation's national polls and international studies support the
perspective that driver fatigue is a much larger problem than what the
federal statistics show. Recently, the National Sleep Foundation
conducted a telephone survey and found that about 51 percent of the
respondents reported that they had driven a car or another vehicle
while feeling drowsy, and about 17 percent had dozed off while driving
within the past year.[Footnote 23] The study found that male
respondents were more likely than female respondents to say they had
driven while feeling drowsy. In addition, respondents with children and
respondents aged 18 to 29 were at the highest risk for driving while
feeling drowsy. The study also found that older respondents, 65 and
over, are less likely to drive drowsy or to fall asleep at the wheel.
Driver Decision Errors:
Driver decision errors involve misjudgments made while driving. These
include improperly judging stopping distances, improperly judging
distances of cars traveling behind the vehicle, and other misjudgments
of distance between cars that result in a crash. Decision errors also
include crashes that result from traveling the wrong way on a one-way
street. A driver decision error differs from an error that may have
resulted from inattention or distraction because the driver sees a
hazard, such as an oncoming car making a left-hand turn, but makes the
wrong decision concerning the proper action to take.
A recent study by Veridian Engineering examined unsafe driving acts in
severe crashes in four sites across the United States to determine the
specific driver behaviors and unsafe driving acts that lead to crashes,
along with the situational, driver, and vehicle characteristics
associated with these behaviors.[Footnote 24] According to the study,
in 717 of the 723 crashes analyzed (99 percent), a driver behavioral
error caused or contributed to the crash. Of the 1,284 drivers involved
in these crashes, 732 drivers
(57 percent) contributed in some way to the cause of their crashes.
Age:
There is a strong relationship between a driver's age and the
likelihood of being involved in a crash. While age, in itself, would
not be the cause of the crash, some of the characteristics displayed at
various ages can lead to a higher probability of being involved in
traffic crashes.
Our analysis of NHTSA's databases found that younger and older drivers
become involved in a greater number of crashes, especially fatal
crashes, than do other age groups. Figure 6 shows that drivers aged 16
through 20 and those aged 75 or more have a greater chance of being
involved in fatal crashes per vehicle mile traveled (VMT) than do other
age groups.[Footnote 25]
Figure 6: Number and Rate of Driver Involvement in Fatal Crashes by
Age, 1997-2001:
[See PDF for image]
[End of figure]
According to the Insurance Institute for Highway Safety, teenagers'
crash rates are disproportionately high mainly because of the drivers'
youth combined with driving inexperience. A recent study by the
Insurance Institute for Highway Safety showed that the age factor plays
out in a more risky driving style among adolescents.[Footnote 26] The
study, which reviewed and summarized other research on the risks
associated with younger drivers, found that increased crash risk comes
immediately on licensure and drops very rapidly in the first few
months. Compared with older drivers, this study concluded that young
people are more likely to drive at excessive speeds, follow too
closely, violate traffic signs and signals, overtake other vehicles in
a risky manner, allow too little time to merge, and fail to yield to
pedestrians. Risky driving leads young people into hazardous
situations, and inexperience makes it more difficult to cope with such
situations. The researchers also found that driving at night is
associated with an increased risk of serious crashes for young drivers.
The driving task is more difficult for young drivers when it's dark,
and the risky driving that involves younger drivers, generally
associated with recreational activities, is more likely to occur at
night. Fatigue and alcohol are also more likely to contribute to
younger drivers' crashes during nighttime hours. The study also found
that there is a heightened crash risk when teenage drivers have
passengers in their vehicles. The study found that this increased risk
is present only for teenage drivers, and it increases incrementally
with each additional passenger.
Older drivers are also at increased risk, because the elderly have
higher rates of fatal crashes per vehicle mile traveled than all but
the youngest drivers. According to a recent study by the Insurance
Institute for Highway Safety, this is largely attributable to their
increased fragility.[Footnote 27] In the study, fragility started at
age 60 to 64 and continued to rise with advancing age. In addition, a
recent study by Dr. Leonard Evans found that given similar crash
severity, older drivers are more likely to sustain fatal injuries than
younger drivers.[Footnote 28] The author suggested that if populations
of 70-year-old males and 20-year-old males were subjected to the
identical mixes of blunt trauma, the population of older males would
sustain over two times more fatalities. A similar comparison of female
populations would yield almost two times more fatalities for older
females. In addition, a literature review conducted by the University
of Michigan Transportation Research Institute found that older drivers
are more likely to suffer from medical disabilities that could impair
their driving, and they may use medications that could affect their
driving performance.[Footnote 29] The study also found that with
increasing age, most drivers experience some loss of visual perception
and decreased cognitive and psychomotor functions. For example, a 1988
AAA Traffic Safety Foundation study tested a small group of volunteers
and found that older adults with less joint flexibility exhibited
poorer driving ability than those with wider ranges of motion.[Footnote
30] In addition, according to the FHWA Director of the Office of Safety
Programs, frailty is not the sole factor in older driver fatality
rates, noting that drivers 85 and older have more than twice the
overall crash rate of middle-aged drivers aged 40 through 44.
Roadway Environment Contributes to Motor Vehicle Crashes:
The roadway environment is generally cited as the second most prevalent
factor contributing to crashes by data, experts, and studies. It can be
defined as those factors external to the driver and the vehicle that
increase the risk of a crash. Roadway environment factors that
contribute to, or are associated with, crashes include the design of
the roadway (for example, medians, narrow lanes, the lack of shoulders,
curves, access points, or intersections); roadside hazards (for
example, poles, trees, or embankments adjacent to the road); and the
roadway conditions (for example, rain, ice, snow, or fog).
Roadway Design:
The principal guidance on roadway design is the American Association of
State Highway and Transportation Officials' (AASHTO) Policy on
Geometric Design of Highways and Streets. This guidance provides
recommendations on constructing the nation's roadways, including such
features as the sharpness of curves, the slope of roadways, the width
of lanes, and the design of medians and barriers. In general, different
functional road systems are constructed for specific purposes. For
example, Interstate highways are intended for high mobility and
therefore have limited access points, while local roads are designed
for increased access, which can limit mobility. Design principles
generally suggest that as average daily traffic increases, additional
design elements should be adopted that increase safety, including wider
lanes, paved shoulders, and clear zones (areas free of roadside hazards
next to the roadway).
Based on FHWA's data, we found that fatal crashes were more frequent on
rural roads than on urban roads.[Footnote 31] In 2001, rural roads
handled only about 40 percent of all vehicle miles traveled, yet more
than 60 percent of all fatalities occurred on these roads. Figure 7
shows that fatality rates are higher on rural roads in comparison with
urban roads, regardless of the road type.
Figure 7: Fatality Rates by Type of Road System, 2001:
[See PDF for image]
[End of figure]
Figure 8: Note: The urban Interstate fatality rate also includes
fatalities from other urban freeways and expressways.
[See PDF for image]
[End of figure]
A recent FHWA study developed relationships between roadway features
and crash rates on two-lane rural highways.[Footnote 32] For this
study, FHWA developed predictive models to estimate the safety impacts
of roadway design features. Studies have found that the following
roadway design features can affect crash rates. Appendix III contains
additional information on roadway design features.
* Medians --Medians (that is, the physical separations between opposite
lanes of traffic) provide a recovery area for out-of-control vehicles
and reduce head-on crashes by separating traffic driving in opposite
directions.
* Lane width --Wider lanes may reduce crashes by allowing for greater
separation between vehicles traveling in adjacent lanes as well as
providing additional space to recover from near-crash situations.
* Shoulders --Wide roadway shoulders that are paved provide an
opportunity for drivers to recover from errors that cause a vehicle to
stray out of a lane.
* Curves --Curves have been shown to contribute to crashes, whether
horizontal curves (left or right) or vertical curves (up and down).
Crash rates on curves are associated with their design features
(including degree, length, and angle) and cross-sectional curve
elements (lane width, shoulder size and type, and median
characteristics).
* Access points --As the number of access points, or locations where
vehicles can gain entry to the roadway, increases, the more likely it
is that a traffic crash will occur.
* Intersections --Intersections, or at-grade locations where vehicles
may transfer between roads, are among the most complex roadway designs
a driver encounters. This is the result of increased points of conflict
between vehicles, and between vehicles and pedestrians.
FHWA's Chief Highway Safety Engineer told us that it is important that
a roadway be designed to allow a driver the time and space to make and
recover from various errors without crashing. For example, two-lane
rural roads are often characterized by sharp horizontal and vertical
curves, narrow lanes, no shoulders or narrow ones, and roadside hazards
such as utility poles or trees adjacent to the road. These design
elements can be associated with higher fatality rates. This contrasts
with the multilane highways, which generally have gradual horizontal
and vertical curves, wider lanes and shoulders, and wide, clear zones
adjacent to the road.
FHWA's Director of Office of Safety Research and Development pointed
out that there are some data limitations associated with crashes and
roadway design. For example, the Director noted that NHTSA's crash
databases contain very limited data on roadway design features at the
crash location or immediately preceding the crash location.
Accordingly, detailed analysis comparable to what is possible for the
driver is not possible for the roadway. The Director also stated that
efforts are underway to provide the means to more precisely locate the
point of a crash and to relate that location to detailed roadway and
roadside information databases.
Roadside Hazards:
Roadside hazards are physical features that a vehicle can crash into if
it leaves the roadway. Each year, about 14,000 persons are killed and
almost 1 million persons are injured when vehicles run off the road and
crash. Many of these deaths and injuries result from crashes into poles
and trees, which are often located close to the edge of the roadway.
Our analysis of NHTSA's data found that 16 percent of all crashes from
1997 through 2001 involved striking a roadway object as the first
property-damaging or injury-producing event in the crash. In addition,
we found that that in these crashes, posts or poles were the most
common fixed objects for a vehicle to hit after leaving the roadway
(about 20 percent), followed by ditches (14 percent), trees (14
percent), and guardrails (11 percent).
The Washington State Transportation Center conducted a study examining
roadside crashes on a single section of roadway.[Footnote 33] Models
were created to predict the frequency and severity of run-off-the-road
crashes related to a variety of roadway environmental factors. The
study found, for example, that both a decreased distance from the
outside shoulder edge to roadside objects and an increased number of
trees near the roadway increased the likelihood of a crash. Overall,
the study supported the enlargement of roadside recovery space to
decrease the occurrence and severity of run-off-the-road crashes.
The chairperson of AASHTO's Task Force on Roadside Safety addressed the
importance of roadside hazards. The AASHTO official said that, in order
of preference, the four methods for addressing roadside hazards are to
(1) remove it, (2) relocate it, (3) redesign it, and (4) shield the
roadside hazards (for example, a guardrail or impact barrier).
Roadway Conditions:
Roadway conditions can contribute to crashes through both road surface
conditions and reduced visibility. Surface conditions that can impair a
driver's ability to control the vehicle include standing water, snow,
ice, and oil, in addition to such road surface features as holes, ruts,
paved edge drop-offs, and worn surfaces. Crashes can also result when
visibility is somehow reduced, preventing a driver from receiving the
proper visual driving cues. Reduced visibility can occur because of
weather-related events or the presence or absence of natural or
artificial lighting.
Surface conditions. Common road surface conditions that can create
slippery roads are rain, snow, and ice. Slippery road conditions lead
to a loss of friction between a vehicle's tires and the roadway. This
loss of friction may lead to the reduced controllability of the
vehicle, ultimately resulting in a crash.
Our analysis of NHTSA's data from 1997 through 2001 found that about 23
percent of crashes occurred when road surface conditions were either
wet, snowy, slushy, or icy. In addition, a recent study by Iowa State's
Center for Transportation Research and Education examined the weather's
impacts on safety.[Footnote 34] The researchers examined the impact of
more severe winter storms on volume, safety, and speed characteristics
on seven segments of Interstate highways in Iowa. Their analysis of 54
storm events concluded that crash rates increased by over 1,000 percent
during winter storm events with high snowfall rates.
Reduced visibility. Reduced visibility can occur during nighttime hours
(including dawn and dusk) and during weather-related events, such as
fog, rain, or snow. Reduced visibility can decrease a driver's ability
to receive the proper visual cues to successfully navigate the road.
Our analysis of NHTSA's data found that overall, while 15 percent of
all crashes took place under limited light conditions, about 34 percent
of all traffic fatalities occurred at that time. Although other factors
are involved during nighttime crashes, such as alcohol or fatigue, the
reduction of visual cues for the driver also appears to play a role.
The primary purpose of roadway lighting is to provide increased
visibility of the roadway and its immediate environment, to allow a
person to drive more efficiently and safely. An FHWA study examined the
impact of lighting options on urban freeways in Minnesota.[Footnote 35]
The study used data on crashes, roadways, and traffic volume to compare
the safety of continuously lighted urban freeways with that of urban
freeways having interchange lighting only. Using data from between 1985
and 1990, the study determined that 12 percent more crashes occur on
sections with interchange-only lighting than on road sections with
continuous lighting, assuming all other factors remain the same. The
study concluded that there was a positive relationship between urban
freeway lighting and highway safety.
Weather-related phenomena can also inhibit driver visibility. While fog
crashes are proportionally small compared with all other crashes, they
can involve numerous vehicles in a chain-reaction pileup. A recent
example of this occurred in Wisconsin in October 2002, where a
fog-related crash involving 51 vehicles resulted in 10 deaths. The
National Transportation Safety Board has concluded that major fog-
related incidents generally occur because drivers have not maintained
uniform reduced speeds during times of limited visibility.
According to NHTSA's Director of the Office of Human-Centered Research,
the significance of adverse weather, including both slippery roads and
reductions in driver visibility, is not fully understood because there
are no measurements (for example, vehicle miles traveled under adverse
weather conditions) available to make comparisons between crash rates
under various conditions. A researcher at the University of Michigan's
Transportation Research Institute said that pedestrian-related crashes
are particularly sensitive to light conditions. The researcher pointed
out that, unlike vehicles and roads that may have lighting or
reflective markings, pedestrians are generally not highly visible and
are more likely to be involved in crashes during nighttime hours.
Vehicle Factors Contribute to Motor Vehicle Crashes:
Vehicle factors can contribute to crashes through vehicle-related
failures and vehicle design characteristics (attributes that may
increase the likelihood of being involved in certain types of crashes).
While such recent events as the number of crashes involving tire
separations have highlighted the importance of vehicle factors, it is
generally shown by data and studies and believed by experts that
vehicle factors contribute less often to crashes than do human or
roadway environment factors.
Vehicle-Related Failures:
Two types of vehicle-related failures can contribute to traffic
crashes: equipment-related and maintenance-related. Equipment-related
failures include both original manufacturer and aftermarket-installed
vehicle equipment that function improperly. If not corrected, some
equipment-related failures might lead to the loss of a vehicle's
handling capabilities, resulting in traffic crashes. The widely
publicized tire separations are a recent example of an equipment-
related failure. Equipment-related failures can be identified by the
manufacturer or by NHTSA, and may result in a recall. In 2002, NHTSA
reported 413 recalls involving over 18 million vehicles, over 1 million
pieces of equipment, about 675,000 tires, and over 1 million child
safety seats. NHTSA's Director of the Office of Defects Investigations
told us that its investigations have identified 143 fatalities
associated with recalls from 1990 through 2000. Maintenance-related
failures result from an operator's improper maintenance of vehicle
components, which may impair the function of the vehicle's equipment.
Examples of maintenance-related failures include inadequate tire tread
depth, worn brakes, unchecked or unchanged vehicle fluids, and
underinflated tires.
Our analysis of NHTSA's data found that from 1997 through 2001, there
were about 778,000 crashes in which police identified that a specific
vehicle-related failure might have contributed to the crash. Where
these failures were identified, brake systems and tires were identified
most frequently, at 29 percent and 27 percent, respectively. Data is
not collected by NHTSA in a manner that provides information on whether
these crashes were caused by equipment or maintenance-related failures.
One vehicle factor that NHTSA believes may contribute to crashes is
underinflated tires. In 2001, NHTSA conducted a study that found that
27 percent of passenger cars and 33 percent of light trucks were being
driven with one or more underinflated tires. To reduce this problem,
Congress passed the Transportation Recall Enhancement, Accountability,
and Documentation Act of 2000 (P.L. 106-414), which will require motor
vehicles to be equipped with a tire-pressure monitoring system to warn
the driver if a tire is significantly underinflated. In May 2002, NHTSA
issued part one of a two-part final rule requiring this system. It
requires that between November 1, 2003, and October 31, 2006, auto
manufacturers phase in one of two different tire-monitoring systems.
The second rule, which has yet to be finalized, is scheduled to be
issued March 1, 2005.
Several officials told us that vehicle-related failures and their
effect on crashes are difficult to quantify. For example, NHTSA's Chief
of Information Services stated that the central problem with
identifying vehicle factors is that police officers are not necessarily
qualified to identify vehicle defects.
Vehicle Design:
The design of a vehicle has been shown to affect handling in particular
types of maneuvers. For example, high-performance sports cars have very
different handling characteristics from those of sport utility vehicles
(SUV). Recent changes in the composition of the nation's vehicle fleet,
in part attributable to the purchase of many SUVs, have resulted in an
overall shift toward vehicles with a higher center of gravity (more
top-heavy), which can roll over more easily than some other vehicles.
Rollover crashes are particularly serious because they are more likely
to result in fatalities. NHTSA has developed rollover ratings for
vehicles by calculating their static stability. This factor is a static
metric that is determined by dividing a vehicle's track width, or
distance between wheels from side to side, by twice the height of its
center of gravity.
As shown in figure 8, our analysis of NHTSA's 2001 data showed that
vans were the least likely to be involved in a crash, with about 432
crashes per 100 million vehicle miles traveled (VMT).[Footnote 36]
Passenger cars were the most likely to be involved in a crash, with a
rate of 655 crashes per 100 million VMT. The figure also shows that
both vans and SUVs had the lowest fatal crash rate, at 1.9 and 2.3
fatal crashes per 100 million VMT, respectively.
Figure 9: Vehicle Crash Rates, 2001:
[See PDF for image]
[End of figure]
In 2001, rollover crashes killed 10,118 occupants in passenger cars,
pickup trucks, SUVs, and vans. This represents almost one-third of the
year's 31,875 occupant deaths in these types of vehicles. Figure 9
shows the percentage of rollover occurrence by vehicle type in 2001.
Passenger cars were the vehicle type least likely to roll over in a
crash; passenger cars rolled over in about 2 percent of all crashes,
and rolled over nearly 16 percent of the time in fatal crashes. In
comparison, our analysis shows that SUVs were over three times more
likely to roll over in a crash than were passenger cars; that is,
occurring in almost 6 percent of all crashes. The proportion of SUVs
that rolled over in fatal crashes was over twice as high when compared
with passenger cars. In 2001, SUVs rolled over in fatal crashes over 35
percent of the time.
Figure 10: Passenger Vehicle Rollovers, 2001:
[See PDF for image]
[End of figure]
The National Transportation Safety Board (NTSB) recently examined
rollovers in 15-passenger vans from 1991 through 2000.[Footnote 37] The
NTSB found that 15-passenger vans with 10 to 15 passengers had a
rollover rate about three times greater than that of vans seating 5 or
fewer passengers. In addition, NTSB found that the 15-passenger vans
carrying 10 to 15 passengers rolled over in 96 of the 113
single-vehicle crashes (85 percent). However, they also found that the
vans rolled over only 28 percent of the time, or 69 times out of the
244 single-vehicle crashes, when there were fewer than 5 occupants in
the van. Additional analysis showed that higher speeds were also
strongly correlated with a greater chance of rollovers. NTSB
recommended that 15-passenger vans be rated by NHTSA for rollover
propensity. Although NHTSA has established a rollover resistance
rating system and is currently developing dynamic rollover tests, 15-
passenger vans will not be evaluated for rollover propensity because
they exceed the weight criteria for the testing program.
A study by the Insurance Institute for Highway Safety examined single-
vehicle rollover crashes.[Footnote 38] The study concluded that the
combined rollover crash rate for pickup trucks and SUVs was more than
twice the rate for passenger cars. The higher rollover rate for pickup
trucks and SUVs was present even when taking into consideration a
variety of crash circumstances, including location, roadway alignment,
and the driver's age. The study concluded that both pickup trucks and
SUVs are more prone to rollover crashes than are passenger cars.
A recent NHTSA study addressed rollovers from 1991 through
2000.[Footnote 39] One of its findings was that of all vehicle types
considered in the study, SUVs are the only type in which the number of
occupant fatalities in rollover crashes exceeds the number of occupant
fatalities in nonrollover crashes; in 2000, nearly two-thirds of SUVs'
occupant fatalities occurred in rollover crashes. One of the report's
conclusions was that, despite declines in passenger car occupant
fatalities, the increasing influence of light truck fatal crashes in
general, and rollover crashes in particular, is instrumental in
maintaining the level of traffic crash fatalities. NHTSA's Division
Chief of Math Analysis stated that reducing rollovers is one of the
NHTSA Administrator's top five priorities.
In commenting on a draft of this report, NHTSA provided an analysis
comparing the crash rates for both passenger cars and light trucks
using VMT based on FHWA's Highway Statistics Series.[Footnote 40] The
analysis indicates that passenger cars had a lower fatal crash rate at
1.73 per 100 million VMT, as compared with a rate of 2.13 for light
trucks. With regard to the vehicle involvement rate in all crashes,
NHTSA's analysis shows that passenger cars had a rate of 423 crashes
per 100 million VMT, which is slightly higher than that for the light
trucks (401 crashes per 100 million VMT).
In February 2003, the Alliance of Automobile Manufacturers, a trade
group that represents the three major U.S. automobile manufacturers and
a number of foreign manufacturers, published analyses examining
occupant fatality rates by vehicle type. One analysis used registered
vehicles as a method to compare fatality rates between vehicle types.
Its results indicate that in 2001, SUVs had a slightly higher occupant
fatality rate than had passenger cars--16.25 and 15.70 per 100,000
registered vehicles, respectively. The alliance points out, however,
that 72 percent of people killed in SUV rollover crashes were not
wearing safety belts, which can reduce a driver's risk of fatal injury
in a rollover by 80 percent. They further stated that in 2000, 35
percent of SUV single-vehicle rollover fatalities were alcohol-related.
Federal Research Directed at Better Understanding of Factors That
Contribute to Crashes:
Various modal agencies within the Department of Transportation have
research projects underway and planned that address aspects of crash
causes. For example, the Federal Motor Carrier Safety Administration
and NHTSA are conducting a study on the causes and contributing factors
to large truck crashes. NHTSA is also funding the 100-Car Naturalistic
Driving Study, which involves collecting data about crashes and near
misses from 100 vehicles equipped with sensors. Further, NHTSA is
funding a project called the Drive Atlanta Study that involves
collecting data from 1,100 vehicles equipped with data recorders. In
addition to possible follow-on research on the above projects, planned
research includes a Transportation Research Board proposal for a 6-year
program that would, among other things, involve installing sensors and
other data collection devices on over 5,000 vehicles.
Large Truck Crash Causation Study:
In 1999, Congress established the Federal Motor Carrier Safety
Administration (FMCSA) within DOT and mandated that it study the causes
of and contributing factors in large truck crashes. In 2001, large
truck crashes resulted in about 5,000 fatalities and 131,000 injuries.
FMCSA partnered with NHTSA to implement the 4-year, $18 million Large
Truck Crash Causation Study. The study's goal is to develop a greater
understanding of the factors leading to large truck crashes, so that
cost-effective countermeasures can be developed to decrease the number
and severity of these crashes.
To conduct this study, FMCSA and NHTSA built on the existing crash
investigation system that NHTSA had established to collect data for the
Crashworthiness Data System database. For this effort, researchers at
24 locations collect information on a sample of large truck crashes by
visiting the crash sites shortly after they occur and completing a
response protocol that was developed for this project. NHTSA's Director
of the National Center on Statistics and Analysis told us that the most
informative crash causation data is often collected at the site of the
crash while the vehicles and participants are still present.
Cooperative agreements were established between the police, FMCSA, and
NHTSA to use an established, on-scene investigative approach. These
cooperative agreements were based on previous agreements set up between
NHTSA and police for data collection for the CDS database, but they
were modified to accommodate the other parties involved and a faster
time frame for the crash investigations. A NHTSA official stated that
this multiagency partnering is important for the success of the study,
and that establishing rapid notification procedures requires the
cooperation of state and local police along with their police dispatch
personnel.
The researchers expect to investigate at least 1,000 crashes by the end
of 2003. FMCSA and NHTSA officials said that the results will yield
findings about critical pre-crash events, the reasons for these events,
and relative risks in truck crashes. They also said that this
information should significantly help to create proven countermeasures
to decrease the number and limit the severity of truck crashes.
As a follow-on to this study, NHTSA requested $10 million in its fiscal
year 2004 budget to begin a National Motor Vehicle Crash Causation
Survey. This study would develop and conduct a nationally
representative effort to collect on-scene crash causation data. The
Large Truck Crash Causation Study would be used as the model for the
proposed study. The on-scene methodologies and procedures developed for
the Large Truck Crash Causation Study would also be applicable to this
proposed effort. NHTSA officials said that start-up costs and
implementation timing would be reduced by making use of the
infrastructure in place for the truck study, which is scheduled to
complete data collection by the end of 2003.
One Hundred-Car Naturalistic Driving Study:
NHTSA is currently conducting the 100-Car Naturalistic Driving Study,
whose purpose is to help develop better crash-avoidance warning
systems. This 1-year, $3 million driving research study involves
collecting data from 100 vehicles equipped with various sensors and
cameras. NHTSA has partnered with FHWA, Virginia, and the Virginia
Polytechnic Institute and State University (Virginia Tech) to fund the
study. Virginia Tech is responsible for conducting the study.
NHTSA has equipped 100 cars (80 individually owned and 20 leased) with
five video cameras and a variety of sensors to track proximity and
relationships to other vehicles and objects. In addition, the vehicles
have sensors that detect glare and whether the driver is using a cell
phone in the car. Volunteers will use the vehicles for their everyday
driving in the metropolitan Washington, D.C., area for the duration of
the study, which began in early 2003. The cameras and sensors are to
provide data for studying crashes as well as near misses. In the event
of a crash, NHTSA will send a team of researchers to the site to
investigate.
NHTSA officials told us that they are considering a follow-on to this
study, if it is successful. An expanded version of the study could
include a representative sample of up to 10,000 equipped cars around
the country. The official said that after completion of the initial
study, researchers should have greater knowledge about which sensors
and equipment provided the most relevant information on contributing
factors to motor vehicle crashes, and would install only that equipment
in the larger fleet of vehicles. NHTSA told us that they might seek
funding from auto manufacturers and other entities to supplement their
funding.
Drive Atlanta Study:
Later this year, NHTSA will begin a 2-year, $3.1 million Drive Atlanta
Study, which involves installing data recorders in 1,100 vehicles to
develop information on situations and circumstances where excessive
speed contributes to crashes. Drive Atlanta is primarily funded by a
$1.9 million contract with NHTSA and $1.2 million from Safety
Intelligence Systems, Inc. FHWA is also contributing money to the
study. The private company is providing the development costs and is
prototyping and testing the MACBOX, the data recorder that will be used
by the Georgia Institute of Technology to conduct the study.
In this study, the data recorder information will be combined with
three other types of data. Data will be contributed by the Atlanta
Traffic Management Center on prevailing traffic conditions, the
National Oceanic and Atmospheric Administration on weather, and the
Georgia Department of Transportation on roadway characteristics.
According to a program official, this combination of data will enable
the researchers to know when and where the driving occurred, what were
the posted speed limits along the drivers' routes, what were the roads'
characteristics, and numerous other data. The researchers plan to
create speed profiles for all of the study's participants at the
conclusion of the study, to examine exactly how speed is involved in
crashes. NHTSA estimates that at least 100 crashes will occur over the
next 2 years involving these vehicles.
Future Strategic Highway Research Program:
In the Transportation Equity Act for the 21st Century (P.L. 105-178),
Congress requested that the Transportation Research Board conduct a
study to determine the goals, purposes, research agenda and projects,
administrative structure, and fiscal needs for a new strategic highway
research program. In response to this request, a committee of highway
industry leaders was formed to develop recommendations. The committee
engaged in an outreach process to gather input from the highway
community regarding strategic priorities and promising research
approaches. The committee's report was published in October 2001 and
recommended a 6-year, $450 million to $500 million Future Strategic
Highway Research Program (F-SHRP) focused on the following areas: (1)
accelerating the renewal of America's highways; (2) making a
significant improvement in highway safety; (3) providing a highway
system with reliable travel times; and (4) providing highway capacity
in support of the nation's economic, environmental, and social goals.
FHWA contributed $1.5 million over fiscal years 2002 and 2003 toward
the F-SHRP planning activities.
The F-SHRP objective of "making a significant improvement in highway
safety" includes three major areas: (1) methodology development using
existing data, (2) large-scale research studies of multiple factors
related to the risk of collisions and casualties for high priority
roadway safety issues, and (3) analysis of the field data for
countermeasure implications. A key aspect of this project is the use of
in-vehicle and roadside technologies to gather data to examine crash
rates and pre-crash conditions on a large scale to perform risk
analyses. The study recommended that $180 million to $200 million be
committed to this safety objective.
The F-SHRP safety plan includes using the data from NHTSA's national
crash databases as well as other studies that have used instrumented
vehicles and roadside technologies. The F-SHRP study would involve
collecting data from 5,000 to 6,000 instrumented vehicles and roadside
technologies for over 2 to 3 years. According to the contractor who
developed the implementing plan for the F-SHRP safety goal, analysis of
the previous study data will enable the F-SHRP researchers to first
test risk measures and analysis methods before implementation of the F-
SHRP field study. The contractor said that NHTSA's 100-Car Naturalistic
Driving Study and Drive Atlanta Study and FHWA's Road Departure Study
would be good sources of the type of instrumented vehicle data the
early methodology projects need.[Footnote 41] The proposed F-SHRP
instrumented vehicle study would involve data collection for two high-
priority highway safety problems: run-off-the-road and intersection
crashes. The fleet of instrumented vehicles would be split between at
least two geographic areas, with volunteer drivers using the vehicles
for their everyday driving. The final phase of the research would be to
use the results of the large-scale instrumented vehicle study to
identify appropriate countermeasure improvements.
Agency Comments and Our Evaluation:
We provided copies of a draft of this report to the Department of
Transportation for its review and comment. In discussing this report,
NHTSA and FHWA officials provided technical clarification and
information, which we incorporated in the report as appropriate. In
addition, NHTSA provided information comparing light truck and
passenger car crash rates, which we also incorporated in the report.
As arranged with your offices, unless you publicly announce its
contents earlier, we plan no further distribution of this report until
30 days after the date of this letter. At that time, we will send
copies of this report to cognizant congressional committees and to the
Honorable Norman Y. Mineta, Secretary of Transportation; the Honorable
Dr. Jeffrey W. Runge, Administrator of the National Highway Traffic
Safety Administration; and the Honorable Mary E. Peters, Administrator
of the Federal Highway Administration. We will also make copies
available to others upon request. In addition, the report will be
available at no charge on the GAO Web site at http://www.gao.gov.
If you have questions about the report, please contact me at (202)
512-2834. Key contributors to this report were Michele Fejfar, Glenn C.
Fischer, Bonnie Pignatiello Leer, Sara Ann Moessbauer, Elsie Picyk,
Beverly Ross, and Glen Trochelman.
Peter Guerrero
Director, Physical Infrastructure Issues:
Signed by Peter Guerrero
[End of section]
Appendix I: Objectives, Scope, and Methodology:
To provide information on the factors that contribute to motor vehicle
crashes, we obtained and analyzed crash data from National Highway
Traffic Safety Administration (NHTSA) databases, obtained and reviewed
research studies on the topic, and interviewed a variety of experts and
federal officials. To identify major ongoing and planned Department of
Transportation (DOT) research into factors that cause crashes, we
obtained documents from and interviewed officials of NHTSA and the
Federal Highway Administration (FHWA).
Analyzing NHTSA Data:
For each factor contributing to traffic crashes, we obtained and
analyzed data for calendar years 1997 through 2001 from NHTSA's crash
reporting systems--the most recent 5-year period for which these data
are available. Our analysis involved the use of three of NHTSA's
databases: the Fatality Analysis Reporting System, the General
Estimates System, and the Crashworthiness Data System. Each database
contained different levels of crash data.
* Fatality Analysis Reporting System (FARS) - This database provides
information on all traffic-related fatalities. A crash must result in
the death of an occupant or nonmotorist within 30 days of the incident
to be included in this database. Each of the states provides the data
to NHTSA in a standardized format. The states generally obtain this
information through data from reports that police officials prepare at
the scene of the crash as well as state vehicle registration files,
state driver licensing files, state highway department data, death
certificates, coroner or medical examiner reports, hospital medical
records, and emergency medical service reports. NHTSA created the
database to identify traffic safety problems, develop and implement
countermeasures, and evaluate vehicle safety standards and highway
safety programs. We used this database to present overall information
on traffic deaths as well as to provide an understanding of crashes
involving speed, alcohol, age, and vehicle design. It should be noted
that while fatality data is useful in understanding crashes, other
factors, in addition to those involved in causing the crash, might have
contributed to the fatality. This would include such factors as whether
safety belts or other occupant protection measures were used and
operated properly.
* General Estimates System (GES) - This database is created from a
nationally representative sample of police accident reports completed
for crashes. Other criteria necessary for inclusion in the database are
that the crash must involve at least one motor vehicle traveling on a
traffic way, and that the crash must result in property damage, injury,
or death. This database was created to identify traffic safety problem
areas, provide a basis for regulatory and consumer initiatives, and
form the basis for cost and benefit analyses of traffic safety
initiatives. This is NHTSA's largest crash database, with information
collected on over 50,000 randomly sampled police accident reports each
year. We analyzed the GES data to provide information on speed, traffic
control violations, roadside hazards, roadway conditions, and vehicle
defects. In addition, for the analysis in which we compared fatalities
with other types of crashes (reduced visibility, vehicle design, age),
we combined FARS and GES data.
* Crashworthiness Data System (CDS) - This database contains
information from a detailed sample of about 4,000 minor, serious, and
fatal crashes annually. The criterion necessary for inclusion in the
database is that at least one passenger vehicle must be damaged
severely enough to require towing from the crash site. Teams of trained
crash investigators visit the crash site and collect data elements such
as vehicle crash damage and interior vehicle locations that the
occupants struck. The investigators also generally locate and interview
crash victims and review medical records to determine the types of
crash-related injuries. A goal of this database includes having the
ability to examine the crashworthiness of vehicles; that is, how
vehicles perform in crashes with respect to protecting their occupants.
We used the CDS database to provide information on crashes involving
driver inattention.
In commenting on a draft of our report, NHTSA officials said that the
FARS, GES, and CDS databases, although providing useful information,
rely on data from police accident reports or on data collected days or
weeks after the crash, making it difficult to obtain causation data.
Therefore, NHTSA relies on the Indiana Tri-Level study data, which is
almost a quarter of a century old. They noted that since the Tri-Level
study was completed, cars, drivers, highways, technology, and
lifestyles in the United States have changed dramatically. As
previously discussed, NHTSA has proposed to develop and conduct a
nationally representative survey to collect on-scene crash causation
data--the National Motor Vehicle Crash Causation Survey. NHTSA
officials indicated that these on-scene, real-time data are needed to
best understand crash causation.
In addition to using these three databases, for some analyses we also
calculated frequency rates using vehicle miles traveled. We used
vehicle miles traveled data from two different sources--the 2001 FHWA
Highway Statistics Series data and the 2001 National Household Travel
Survey. For example, we used FHWA's vehicle miles traveled data in
examining crash rates by road type. FHWA obtains its data on vehicle
miles traveled by counting the number and types of vehicles passing
particular points around the country. Because FHWA's statistics do not
include data on age or gender, we used vehicle miles traveled from the
2001 National Household Travel Survey for some analyses.[Footnote 42]
This survey of about 26,000 households in the United States was
conducted from March 2001 through May 2002. It provides data on
personal travel behavior at the national level to use as a benchmark
for a variety of applications. Although the overall response rate for
sampled households was low (41 percent), there are few other sources
for information on U.S. travel patterns. We used the most recent
available data, preliminary release (version 1), to get estimates of
annual vehicle miles traveled for our tables on rates of driver
involvement and types of vehicles involved in crashes.
We assessed the reliability of FARS, CDS, and GES by reviewing existing
information about the data and performing electronic tests of the data.
There are certain limitations associated with using these databases for
our analysis. For example, the source of the GES data is police
accident reports that are prepared at the scene of the crash. Although
the GES has procedures to ensure that data reflect information in the
accident reports, we did not verify the accuracy of the accident
reports themselves. In addition, since GES, CDS, and the National
Household Travel Survey are based on samples, any estimates derived
from these databases are subject to sampling errors. A sampling error
indicates how closely the results of a particular sample would be
reproduced if a complete count of the population were taken with the
same measurement methods. The estimated sampling errors (at the 95
percent confidence level) do not exceed plus or minus 11 percentage
points.
Identifying Studies:
To identify recent studies on factors that contribute to motor vehicle
crashes, we conducted a literature search, explored the Transportation
Research Information System, and reviewed periodicals. This effort
resulted in numerous studies being identified on various aspects of the
motor vehicle crashes. We then, with input from a number of experts and
officials from NHTSA and FHWA, judgmentally selected studies that would
provide additional information on the particular factors being
discussed. For each of the selected studies that are used in this
report, we determined whether the study's findings were generally
reliable. To do so, we evaluated the methodological soundness of the
studies using common social science and statistical practices. For
example, we examined each study's methodology, including its
limitations, data sources, analyses, and conclusions.
Interviewing Federal Officials and Experts:
In conducting this review we interviewed a wide variety of federal
officials and other experts. Within DOT, we interviewed officials from
the Volpe Center, the National Highway Traffic Safety Administration,
and the Federal Highway Administration. We also spoke with individuals
affiliated with academic institutions, including the University of
North Carolina's Highway Safety Research Center, the University of
Michigan's Transportation Research Institute, Northwestern
University's Center for Public Safety, Texas A&M University's Texas
Transportation Institute, and the Johns Hopkins School of Public
Health. In addition, we interviewed officials from the Insurance
Institute for Highway Safety. We also spoke with automobile industry
representatives at the Alliance of Automobile Manufacturers, a trade
group that represents the three major U.S. automobile manufacturers and
a number of foreign manufacturers. We spoke with officials from the
Transportation Research Board, Advocates for Highway and Auto Safety,
AAA (formerly the American Automobile Association), American
Association of State Highway and Transportation Officials (AASHTO), the
National Sleep Foundation, the Midwest Research Institute, and the
Southern California Research Institute. In general, the officials and
experts provided information about major factors that contribute to
motor vehicle crashes and research on these factors.
Ongoing and Planned Transportation Research:
To identify major ongoing and planned DOT research into factors that
contribute to motor vehicle crashes, we interviewed officials from
NHTSA, FHWA, and the Transportation Research Board. These agencies have
a great deal of ongoing and planned research on a wide variety of motor
vehicle safety issues, such as research to mitigate accident severity
and safety system issues. However, to respond to this objective, we
selected ongoing and planned studies that (1) represented major
research studies, (2) examined multiple factors contributing to
crashes, (3) examined causal factors rather than countermeasures, and
(4) collected original data, rather than analyzed existing data. We
also obtained documents describing the research projects and reviewed
federal budgetary documents on the projects.
We performed our review from July 2002 through February 2003 in
accordance with generally accepted government auditing standards.
[End of section]
Appendix II: Tri-Level Study of the Causes of Traffic Accidents:
Researchers at the Indiana University Bloomington's Institute for
Research in Public Safety conducted the Tri-Level Study of the Causes
of Traffic Accidents from 1972 through 1977. The study investigated how
frequently various human, environmental, and vehicle factors were
involved in traffic crashes. According to NHTSA officials, the Tri-
Level study has been the only study in the past 30 years to collect on-
scene crash causation data. The study, conducted for NHTSA,
incorporated 13,568 police-reported crashes, including on-scene
investigation of 2,258 crashes, and an in-depth investigation of 420
crashes. The investigation teams assessed causal factors as definite,
probable, or possible. The in-depth team identified human errors as
definite or probable causes in 93 percent of the crashes, environmental
factors in 34 percent, and vehicle factors in 13 percent. In 20 percent
of the crashes studied in depth, no definite cause could be identified.
Objectives, Scope, and Methodology for the Tri-Level Study:
Indiana University conducted the study to satisfy a broad range of
NHTSA's needs for data on traffic crash causation. Two of the main
objectives for the research were to:
* Identify those factors that are present and serve to initiate or
influence the sequence of events resulting in a motor vehicle crash.
* Determine the relative frequency of these factors and their causal
contribution within a defined crash and within the driving population.
Researchers collected collision data on three levels (A, B, and C),
each providing an increasing amount of detail. Data collection for
level A involved examining police reports for 13,568 crashes and
collecting other baseline data, such as vehicle registration files,
driver license files, roadway inventories, and local surveys. For level
B, teams of technicians conducted on-site investigations of 2,258
crashes immediately following their occurrence. For level C, a
multidisciplinary team conducted independent, in-depth investigations
of 420 of the crashes. The crashes investigated on-scene and in-depth
were generally representative of all police-reported crashes occurring
in Monroe County, Indiana, during the study period.
In the clinical assessments of crash causation in Monroe County, a
traffic crash was viewed as the last event in a chain of events and
conditions that preceded it. A crash cause was defined as an event or
condition but for which the crash would not have occurred. Emphasis was
placed on events and conditions that immediately preceded the crash
because they may be viewed as the final links of a casual chain that
culminates in the crash.
According to the study, during in-depth investigations, the researchers
attempted to acquire as much relevant information as possible, and then
made clinical case-by-case determinations of the causal factors
involved, based on all of the information obtained. An assessment
system permitted each identified factor to be evaluated as definitely,
probably, or possibly involved as either a causal or severity-
increasing factor. A causal factor was defined as a factor necessary or
sufficient for the occurrence of the crash; had the factor not been
present in the crash sequence, the crash would not have occurred. A
severity-increasing factor was defined as a factor that was neither
necessary nor sufficient for the occurrence of the crash, but its
removal from the crash sequence would have lessened the speed of the
initial impact. The causal assessment process for each crash involved
two major steps: first, identifying relevant deficiencies of drivers,
vehicles, and the driving environment that were present in the crash
sequence; and second, assessing the investigation team's certainty that
the crash would not have occurred had each deficiency been corrected to
its minimally acceptable state.
In addition, data on Monroe County drivers, vehicles, roads, and
crashes were compared with available national data. It was found that
for Monroe County the severity distribution of reported crashes; the
proportion of crashes occurring on dry, wet, or snow-or ice-covered
roads; the proportion occurring in urban or rural areas; and the age
distribution of the vehicles were nearly the same as for the United
States as a whole. The most notable difference was that young drivers
were overrepresented. However, the effects of this overrepresentation
on the overall causal results were found to be minimal. Thus, it was
found that while the results from Monroe County, Indiana, do not
represent the United States as a whole in a statistical sense, they
indicate factors that are likely to be important on a national level
and their relative involvement.
Results of the Tri-Level Study:
Human factors were the most frequently implicated of the three
categories, and vehicle factors the least frequently implicated. As
figure 10 shows, the in-depth team concluded that human factors were
definite causal factors in 71 percent of the crashes; environmental
factors in 13 percent; and vehicle factors in 4 percent. Similarly, the
in-depth team concluded that these same three categories were definite
or probable causal or severity-increasing factors in 93 percent, 34
percent, and 13 percent, respectively.
Figure 11: Factors Contributing to Crashes Identified by the Tri-Level
Study:
[See PDF for image]
[End of figure]
The on-site team concluded that human factors were definite causal
factors in 64 percent of the crashes; environmental factors in 19
percent; and vehicle factors in 4 percent. The on-site team concluded
that these same three categories were definite or probable causal or
severity-increasing factors in 90 percent, 35 percent, and 9 percent,
respectively. The in-depth team could not establish a definite cause
for 20 percent of the crashes they investigated, while the on-site
technicians could not establish a cause for 26 percent of the crashes.
However, the on-site team identified one or more probable causes in
nearly all the crashes. Also, more than one factor was implicated as a
cause in many of the crashes.
Human Factors:
The study categorizes human direct causes based on an information-
processing model of the driver as vehicle controller. This model
assumes that drivers are continuously engaged in perceiving and
comprehending information, making decisions, and taking actions to
achieve necessary control responses. The "perception" and
"comprehension" categories were combined as "recognition errors"
because of the difficulty in distinguishing errors in these functions
through crash investigation. A "critical nonperformance" category was
added to reflect instances where a driver ceases to perform as an
information processor. A "noncrash" category was included to
accommodate any intentional crash involvements. Recognition errors were
cited as the most prevalent human causal factor, followed by decision
errors, performance errors, and critical nonperformance errors.
More specific human-direct-cause categories were grouped in the causal
hierarchy, under these major headings. With regard to specific human
errors, improper lookout was cited as the most prevalent error. Other
specific human errors cited included excessive speed, inattention,
improper evasive action, and internal distraction.
The researchers separately recorded human conditions and states that
impeded the ability of the driver to function as an information
processor. These factors, which included fatigue, driver experience,
and alcohol impairment, were viewed as potential "reasons behind the
reasons." Alcohol impairment was cited as the most prevalent human
condition, followed by other drug impairment and fatigue.
Environmental Factors:
The study categorized environmental factors as involving highway-
related factors, slick roads, or other ambience-related factors. Among
these, highway-related factors predominated; the in-depth team
identified them as definite causes in 7 percent of crashes. Slick roads
were definite causes in 4 percent of crashes, and other
ambience-related factors in 2 percent. More specific environmental
causes were defined under these three broad headings. The most commonly
cited specific environmental factors were view obstructions and slick
roads.
Vehicle Factors:
The study categorized vehicle factors according to major vehicle
systems, and then according to more specific categories. The most
commonly cited deficiency in these systems was with the brake system,
followed by the tires and wheels. The most commonly cited vehicle
deficiency causal factor was gross brake failure, followed by
inadequate tread depth.
[End of section]
Appendix III: Roadway Design Features:
A number of roadway design studies and experts we spoke with addressed
how various aspects of roadway design might contribute to traffic
crashes. These included medians, lane widths, shoulders, curves, access
points, and intersections.
Medians:
Medians are physical separations between opposite lanes of traffic that
provide a recovery area for out-of-control vehicles. They also serve to
separate traffic driving in opposite directions, thereby minimizing
their interactions and likelihood of being involved in catastrophic
head-on crashes. Some considerations regarding medians include their
presence or absence (that is, divided vs. undivided roads), the width
of the median, and whether a barrier is placed in the median.
Presence or Absence of Medians:
An analysis of NHTSA's databases showed that from 1997 through 2001, 44
percent of all traffic fatalities occurred on undivided, rural, two-
lane roads. This represents 73 percent of all traffic fatalities in
rural areas. In urban areas, 35 percent of traffic fatalities occurred
on two-lane undivided roadways. In addition, a study conducted by the
Kentucky Transportation Center examined the impact of converting two-
lane undivided rural roads to four-lane divided roads at 25
locations.[Footnote 43] They found that, on average, there was a
reduction in crash rate after the road's conversion from a two-lane
undivided rural road to a four-lane divided road.
Width of Medians:
According to experts with whom we spoke, medians provide safety
benefits by allowing vehicles enough room to recover from various
vehicle or human factors that could contribute to a crash. A study
published in the Transportation Research Record used Highway Safety
Information System data from Illinois and Utah to assess the
relationship between median width and crash rates.[Footnote 44] The
study was based on a total of 3,055.1 miles of roadway, with speed
limits of at least 35 miles per hour. The study attempted to isolate
only the median's width as the predictive factor for crash rates, but
it acknowledged that there could be other elements influencing the
crash rates as well. Overall, the study concluded that crash rates
decrease with increasing median widths greater than 25 to 30 feet, and
increasing widths continue to provide additional benefits up to widths
of approximately 65 to 80 feet.
Existence of Barriers:
Some experts told us that although the installation of median barriers
can reduce head-on crashes, their presence may increase the number of
total crashes. This might occur because the median barrier reduces the
amount of space a vehicle has to recover within the median. The
Washington State Department of Transportation recently conducted a
study of cross-median crashes on multilane and divided state highways
with full-access control.[Footnote 45] One goal of the study was to
revise the guidelines for the installation of median barriers. The
study examined cross-median crashes from 1996 through 2000 from a
sample of 677 miles of road. Using a benefit-cost analysis, the study
recommended installing median barriers on all full-access control,
multilane highways with posted speed limits of 45 miles per hour or
greater where the median width was 50 feet or less.
Lane Widths:
Wider lanes increase the separation between vehicles traveling in
adjacent lanes as well as provide additional space to recover from
near-crash situations. In a recent study, FHWA addressed the
relationship between lane width and crashes on two-lane rural highways
based on expert assessments and previous studies.[Footnote 46] The
study included an analysis of the combined effects of lane width and
average daily traffic on crash rates, and it predicted that lane width
has only a slight impact on crash rates at low volumes of
traffic.[Footnote 47] However, the study also predicted that at
high-average daily traffic volumes, the two-lane rural roads with 9-
foot lanes have a 50 percent greater chance of having crashes than have
similar roads with 12-foot lanes. In discussing lane width with
experts, we were told by one academic researcher that while wider lanes
provide additional space between vehicles, wider lanes may give drivers
an increased perception of safety resulting in higher rates of speed,
possibly leading to other safety problems.
Roadway Shoulders:
Roadway shoulders provide a clear space for drivers to recover from
errors. A recent FHWA study examined the relationship between shoulder
width, average daily traffic, and crash rates for two-lane rural
highways and predicted that, in general, at low-average daily traffic
rates, shoulder width only slightly affects the crash rate but as the
average daily traffic rate increases, so does the influence of shoulder
width on crash rates.[Footnote 48] For example, at high-average daily
traffic volumes, the study predicted that a 50 percent greater number
of crashes occur on two-lane rural highways with no shoulders than on
similar roads with 6-foot shoulders. Experts told us that while wider
shoulders are generally better than narrow ones, the benefits that
shoulders provide are also influenced by the material from which they
are constructed. A researcher at the University of North Carolina's
Highway Research Safety Center told us that paved shoulders are
associated with fewer crashes at lower rates than those with gravel or
grass shoulders. Another expert pointed out that soft shoulders can
lead to a loss of vehicle control both through the uneven edges between
the driving lane and the shoulder or through a differential of friction
between the driving lane and the shoulder.
Curves:
Curves have been shown to contribute to crashes, whether horizontal
curves (left or right) or vertical curves (up and down). Various
elements of curves may affect the likelihood of a crash, including
features of the curve (for example, degree, length, and angle of the
curve) and cross-sectional curve elements (for example, lane width,
shoulder size and type.) A 1991 FHWA study identified factors more
strongly associated with curves than adjacent straightaways in
Washington State.[Footnote 49] These factors included a higher
percentage of fatal crashes, head-on and opposite sideswipe crashes,
fixed-object and rollover crashes, crashes at night, and crashes
involving drinking drivers. Vertical curves have also been associated
with higher crash rates, though, according to an American Association
of State Highway and Transportation Officials chairperson, not as much
as compared with horizontal curves. An important design element
regarding vertical curve safety is the need to provide drivers with
adequate stopping sight distance.
Access Points:
Access points are locations where vehicles enter a roadway, such as
residential and business driveways and exit and entrance ramps on
highways. A 1998 study completed for the Minnesota Department of
Transportation found that as access points to roads increase, so do the
number of crashes.[Footnote 50] For example, on four-lane urban
conventional roadways, with no left turns, the researchers found that
in Minnesota there were an average of 2.22 crashes per million vehicle
miles traveled when there were from zero to ten access points per mile.
(See fig. 11.) However, the rate of crashes increased to 7.38 when the
number of access points was greater than 50 per mile. Additionally,
traffic safety experts supported the conclusion that more access points
generally lead to higher crash rates.
Figure 12: Impact of Access Points on Traffic Crashes:
[See PDF for image]
[End of figure]
Intersections:
According to FHWA, intersections are among the most complex roadway
designs a driver encounters. A recent report for NHTSA found that in
2001, intersection and intersection-related crashes represented 22.5
percent of total fatal crashes and 43 percent of overall
crashes.[Footnote 51] There are four major crash types at
intersections: crossing, rear-end, improper lane changing, and
pedestrian and bike. Multiple factors contribute to intersection
crashes, including: poor physical design, inadequate traffic
engineering, failure of driver licensing and education to train drivers
in negotiating intersections, and driver disregard for traffic control
devices. For example, a poorly designed intersection might provide
inadequate sight distance, which could limit a driver's response time
to react to vehicles or pedestrians at that intersection. Additionally,
incorrectly timed or inconspicuous traffic control devices can also
contribute to a crash.
FOOTNOTES
[1] J.R. Treat et al., Tri-Level Study of the Causes of Traffic
Accidents (Washington, D.C.: Institute for Research in Public Safety,
May 30, 1979), for the U.S. Department of Transportation.
[2] NHTSA defines a crash as speed-related if the driver was charged
with a speed-related offense or if an officer indicated that the driver
was racing, driving too fast for conditions, or exceeding the posted
speed limit.
[3] Some analyses in this report discuss fatality data associated with
specific factors. It should be noted that other elements, in addition
to the factor discussed, might have also contributed to the fatalities.
These would include circumstances such as the use of safety belts or
other occupant protection measures.
[4] NHTSA/FHWA, Report to Congress: The Effect of Increased Speed
Limits in the Post-NMSL Era (Washington, D.C.: February 1998).
[5] Charles M. Farmer, Richard A. Retting, and Adrian K. Lund, Effect
of 1996 Speed Limit Changes on Motor Vehicle Occupant Fatalities
(Washington, D.C.: Insurance Institute for Highway Safety, October
1997). This study focused on 12 states that raised maximum speed limits
to at least 70 miles per hour between December 8, 1995, and April 1,
1996.
[6] The Transportation Research Board is a unit of the National
Research Council, a private, nonprofit institution that is the
principal operating agency of the National Academy of Sciences and the
National Academy of Engineering.
[7] Richard A. Retting and Allan F. Williams for the Insurance Institute
for Highway Safety, and Robert G. Ulmer for the Preusser Research
Group, "Prevalence and Characteristics of Red Light Running Crashes in
the United States," Accident Analysis and Prevention, vol. 31 (1999),
pp. 687-94.
[8] The study reviewed intersection crashes in both the Fatality
Analysis Reporting System and the General Estimates System during the
5-year period from 1992 through 1996.
[9] Bryan E. Porter, Thomas D. Berry, Jeff Harlow, and Tancy Vandecar,
A Nationwide Survey of Red Light Running: Measuring Driver Behaviors
for the "Stop Red Light Running" Program, June-August 1999.
[10] Blood alcohol content of .08 percent in Massachusetts is evidence
of alcohol impairment, but it is not illegal per se.
[11] Of these states, Louisiana, New York, and Tennessee have .08
percent BAC laws that will be effective during the latter half of 2003.
[12] NHTSA indicates that a fatality is alcohol-related if it occurred
in a crash where any one of the actively involved persons in the crash
had a BAC of .01 percent or greater.
[13] H. Moskowitz, M. Burns, D. Fiorentino, A. Smiley, and P. Zador,
Driver Characteristics and Impairment at Various BACs (Los Angeles, CA:
Southern California Research Institute, August 2000).
[14] The study subjects were examined only as their BAC was declining
and, according to the study, the results would underestimate the
magnitude of impairment expected during alcohol consumption when BAC
was rising.
[15] P.L. Zador, S.A. Krawchuk, and R.B. Voas, Relative Risk of Fatal
Crash Involvement by BAC, Age and Gender (Rockville, MD: Westat, April
2000).
[16] The 1996 National Roadside Survey was a national survey of
weekend, nighttime drivers in the 48 contiguous states. The survey
consisted of interviewing and breath-testing over 6,000 noncommercial
four-wheel vehicle operators between September and November 1996.
[17] These eight states include Arizona, Georgia, Indiana, Illinois,
Iowa, Minnesota, Rhode Island, and Utah. The Georgia Supreme Court has
determined that the Georgia statute, Ga. Code Ann. 40-6-391(a)(6)
(2002), is an unconstitutional denial of equal protection. See Love v.
State, 271 Ga. 398 (1999). Accordingly, the enforceability of the
Georgia statute is questionable.
[18] Examples of zero tolerance to driving under the influence of drugs
include laws that prohibit drug addicts or habitual users of drugs from
driving vehicles (found in California, Colorado, Idaho, Kansas, and
West Virginia) or statutes that make it illegal for minors to drive
with any amount of a prohibited drug in their bodies (found in North
Carolina and South Dakota).
[19] Hindrik W. J. Robbe and James F. O'Hanlon, Marijuana, Alcohol, and
Driving Performance (The Netherlands: Institute for Human
Psychopharmacology, July 1999).
[20] C.T.J. Lamers and J.G. Ramaekers, Visual Search and Urban City
Driving under the Influence of Marijuana and Alcohol (The Netherlands:
Maastricht University, June 2001). Specifically, both studies examined
the effects of delta-9-tetrahydrocannabinol (THC), the primary active
ingredient of cannabis (marijuana).
[21] About 6 million were identified as "unknown" or "no driver
present."
[22] Jane C. Stutts, Donald W. Reinfurt, Loren Staplin, and Eric A.
Rodgman, The Role of Driver Distraction in Traffic Crashes (Washington,
D.C.: University of North Carolina for the AAA Foundation for Traffic
Safety, May 2001). This study examined drivers involved in crashes
where at least one vehicle had to be towed away.
[23] The National Sleep Foundation commissioned WB&A Market Research to
conduct the 2002 "Sleep in America" telephone poll of 1,010 adults at
least 18 years old between October 1 and December 10, 2001. The margin
of error is plus or minus 3.1 percent.
[24] D. L. Hendricks, M. Freedman, P.L. Zador, and J.C. Fell, The
Relative Frequency of Unsafe Driving Acts in Serious Traffic Crashes
(Washington, D.C.: Veridian Engineering, Westat, Inc., and Star
Mountain, Inc., January 2001). A sample of 723 crashes involving 1,284
drivers was investigated at four different sites in the country between
April 1, 1996, and April 30, 1997.
[25] For this VMT analysis, we used data from 2001 National Household
Travel Survey. The National Household Travel Survey consists of
household-based travel surveys conducted every 5 years by DOT. Survey
data are collected from a sample of U.S. households and expanded to
provide national estimates of trips and miles by travel mode, purpose,
and a host of other characteristics. The survey collects information on
daily, local trips and on long-distance travel in the United States.
[26] A.F. Williams and S.A. Ferguson for the Insurance Institute for
Highway Safety, "Rationale for Graduated Licensing and the Risks It
Should Address," Injury Prevention, vol. 8 (2002), pp. 9-16.
[27] Guohua Li, Elisa R. Braver, and Li-Hui Chen, Exploring the High
Driver Death Rates per Vehicle-Mile of Travel in Older Drivers:
Fragility versus Excessive Crash Involvement (Arlington, VA: Insurance
Institute for Highway Safety, August 2001).
[28] Leonard Evans, "Age and Fatality Risk from Similar Severity
Impacts," Journal of Traffic Medicine, vol. 29, 2001, pp. 10-19.
[29] David W. Eby, Deborah A. Trombley, Lisa J. Molnar, and Jean T.
Shope, The Assessment of Older Drivers' Capabilities: A Review of the
Literature (Ann Arbor, MI: University of Michigan Transportation
Research Institute, August 1998).
[30] Kenard McPherson, Jeffrey Michael, Andrew Ostrow, and Peter
Shaffron, Physical Fitness and the Aging Driver, Phase I (Washington,
D.C.: AAA Foundation for Traffic Safety, 1988).
[31] The term "urban" is used to denote the federal-aid legislation
definition of an area. Such areas include, at a minimum, a census place
with an urban population of 5,000 to 49,999, or a designated urbanized
area with a population of 50,000 or more. Rural areas are those areas
outside urban areas.
[32] D.W. Harwood, F.M. Council, E. Hauer, W.E. Hughes, and A. Vogt,
Prediction of the Expected Safety Performance of Rural Two-Lane
Highways, FHWA-RD-99-207 (December 2000).
[33] Jinsun Lee and Fred Mannering, Analysis of Roadside Crash
Frequency and Severity and Roadside Safety Management, Washington State
Transportation Center (December 1999).
[34] Keith K. Knapp, Dennis Kroeger, and Karen Giese, The Mobility and
Safety Impacts of Winter Storm Events in a Freeway Environment Final
Report, Iowa State Center for Transportation Research and Education
(February 2000).
[35] Federal Highway Administration, Comparison of the Safety of
Lighting Options on Urban Freeways, FHWA Public Roads On-Line (Autumn
1994).
[36] For this VMT analysis we used data from the 2001 National
Household Travel Survey.
[37] National Transportation Safety Board, Evaluation of the Rollover
Propensity of 15-Passenger Vans, NTSB SR-02/03 (October 2002).
[38] Charles Farmer and Adrian Lund, "Rollover Risk of Cars and Light
Trucks after Accounting for Driver and Environmental Factors," Accident
Analysis and Prevention, vol. 34 (2002), pp. 163-73. The study examined
all single-vehicle fatal crashes for 4 years, along with single-vehicle
injury crashes involving rollovers from three states by vehicle type
for 4 years. The study used vehicle registration as a means to make
comparisons among vehicle types.
[39] William Deutermann, Characteristics of Fatal Rollover Crashes, DOT
HS 809 438 (April 2002).
[40] This VMT analysis used the FHWA's Highway Statistics Series. FHWA
obtains its data on vehicle miles traveled by counting the number and
types of vehicles passing particular points around the country.
[41] In the Road Departure Study, the University of Michigan
Transportation Research Institute will develop and test a new crash
avoidance warning system in 11 passenger cars. The system, designed to
prevent road departure and run-off-the-road crashes, will alert the
driver when the vehicle begins to wander off the road or when the
vehicle is traveling too fast for an upcoming curve.
[42] 2001 National Household Travel Survey User's Guide, Version
1(Preliminary Release) January 2003.
[43] Kenneth Agent and Jerry Pigman, Safety Impacts of Rural Road
Construction, Kentucky Transportation Center KTC-01-01(February 2001).
[44] M.W. Knuiman, F.M. Council, and D.W. Reinfurt, The Effect of
Median Width on Highway Accident Rates, Transportation Research Record
1401, 1993.
[45] Washington State Department of Transportation, Median Treatment
Study on Washington State Highways (March 2002).
[46] D.W. Harwood, F.M. Council, E. Hauer, W.E. Hughes, and A. Vogt,
Prediction of the Expected Safety Performance of Rural Two-Lane
Highways, FHWA-RD-99-207 (December 2000).
[47] This factor applies to single-vehicle run-off-the-road, multiple-
vehicle same-direction sideswipe crashes, and multiple-vehicle
opposite-direction crashes.
[48] D.W. Harwood, F.M. Council, E. Hauer, W.E. Hughes, and A. Vogt,
Prediction of the Expected Safety Performance of Rural Two-Lane
Highways, FHWA-RD-99-207 (December 2000).
[49] C. Zegeer, J. Stewart, F. Council, and D. Reinfurt, Cost Effective
Geometric Improvements for Safety Upgrading of Horizontal Curves,
Federal Highway Administration Report FHWA-RD-90-021 (October 1991).
[50] Minnesota Department of Transportation, Statistical Relationship
Between Vehicle Crashes and Highway Access (August 1998).
[51] National Highway Traffic Safety Administration, Traffic Safety
Facts, 2001.
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