Technical Report

RITA Bureau of Transportation Statistics

TR-008

September 2010

Factors Involved in Fatal Vehicle Crashes

by Jenny Guarino and Amrut Champaneri

For every vehicle crash, there are numerous factors contributing to its severity. In 2008, 37,261 people were killed 

in an estimated 5.81 million police-reported motor vehicle 

crashes1

. While fatal crashes are not the most prevalent, 

they are the most consequential from a safety standpoint, 

making it vital to understand factors common to fatal incidents. 

This report examines factors that contribute to fatal crashes 

involving a motor vehicle (e.g., car, truck, or bus)2. Accident 

level data was obtained from the National Highway Traffic 

Safety Administration’s (NHTSA’s) Fatality Analysis Reporting System (FARS) database for 2004–20083

. Analysis of 

the FARS database shows:

• Single-vehicle crash fatalities are the most prevalent.

• The main factors in single-vehicle fatal crashes involve 

the functionality of traffic controls and the first harmful 

event in a crash (e.g., rollover, traffic barrier, or ditch).

• With regard to two-vehicle fatal crashes, main factors 

are traffic controls and the characteristics of the roadway (e.g., roadway alignment or surface type).

• In multi-vehicle (three or more) fatal crashes, road 

characteristics, vehicle speed, and road type (e.g., interstate or country road) are the main factors involved.

The FARS Database

FARS has been operational since 1975 and is a yearly 

census of fatal traffic crashes within the 50 States, the 

District of Columbia, and Puerto Rico4. Every vehicle crash 

on a public roadway that results in at least one fatality is 

recorded in the FARS database with information retrieved 

from Police Accident Reports (PARs). FARS analysts utilize 

the PARs to determine which variables to record for a 

1

 NHTSA 2008 Traffic Safety Facts Annual Report - http://www-nrd.nhtsa.dot.gov/

pubs/811172.pdf

2

 For this report, a motor vehicle is defined as a vehicle in transport (i.e., not towed, 

pushed, or parked).

3

 Refer to ftp://ftp.nhtsa.dot.gov/FARS for FARS database.

4

 Further analysis excludes data from Puerto Rico

given crash. NHTSA has a cooperative agreement with an 

agency in each state government to provide information 

on fatal crashes in a standard format based on data from 

PARs and accident investigations.

For this analysis, data were broken into three groups, 

dependent on the number of vehicles involved in the crash5

(see figure 1). Single-vehicle crashes account for more 

than half (58.5 percent) of all fatal crashes in the time 

period studied. In contrast, two-vehicle crashes account 

for 34.9 percent and multi-vehicle crashes account for 6.5 

percent during the same time span.

Recently, many researchers have focused on single-vehicle 

crashes to define factors related to fatal crashes. However, analyzing factors by the number of vehicles involved 

reveals overall similarities and differences among the crash 

groups; allowing the relative importance of each factor to 

be explored further.

5

 All following mentions of “crash” are defined as crashes where at least one fatality 

occurred.

Figure 1: Fatal Crashes by Crash Size, 2004–2008

Single vehicle Two vehicle Multi-vehicle

0

20,000

40,000

60,000

80,000

100,000

120,000

109,936

65,604

12,256

Fatal crashes

Crash size

SOURCE: U.S. Department of Transportation, National Highway Traffic 

Safety Administration, FARS Database as of December 2009.
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FARS Analysis

Principal component analysis, a statistical analysis technique, was used to define factors that influence fatal 

crashes. Table 1 depicts the top eight factors for each of 

the three groups and for all crashes combined. Here, factor order corresponds to the weight the factor has on fatal 

crashes. For example, when examining all fatal crashes, 

the functionality of traffic controls carries the most weight, 

and potential, in understanding fatal crashes.

As seen in table 1, several factors appear frequently across 

the board, regardless of the number of vehicles involved. 

This report will focus on the factors that load highly across 

all fatal crash sizes as they have the most potential influence in understanding the precursors of fatal crashes. 

These common factors will be defined as either transportation-related or human-related (e.g., due to human error 

or behavior) and their relationship with fatal crashes will be 

examined further. 

Transportation-Related Factors

Transportation-related factors generally refer to either the 

elements of the roadway infrastructure or external factors such as weather conditions. Rather than examine all 

transportation-related factors listed in table 1, the five leading factors (based on weight) will be evaluated further. 

Table 1: Top Eight Factors Involved in Fatal Crashes by Crash Size, 2004–2008

Factor All fatal crashes Single vehicle Two vehicle

Multi-vehicle

(three or more)

1 Traffic controls Crash characteristics Traffic controls Road characteristics

2 Speed and route type Traffic controls Road characteristics Speed and route type

3 Road characteristics Speed and route type Speed and route type Road classification

4 Weather impacts Weather impacts Road classification Traffic controls

5 Traffic flow Road characteristics Location of crash Weather Impacts

6 Crash characteristics Traffic flow Weather impacts Drunk driving/light condition

7 Road classification Road classification Drunk driving/light condition Persons

8 Persons Persons Persons Location of crash

NOTES: “Crash characteristic” relates to the crash’s relation to the roadway and the first harmful event which occurred; “Persons” relates to 

the relationship between the number of persons involved in a fatal crash and fatality counts; “Drunk driving/light condition” is driven by the 

relationship between the FARS variables associated with drunk driving and the time of day.

SOURCE: U.S. Department of Transportation, National Highway Traffic Safety Administration, FARS Database as of December 2009

Figure 2: Traffic Controls Present at Crash Location by Crash Size, 2004–2008 

(as a percent of fatal crashes by crash size)

Traffic 

signal

no pedestrian 

signal

Traffic signal

unknown 

pedestrian 

signal

Traffic 

signal with 

pedestrian 

signal

Other Other sign Stop sign Unknown Warning 

sign

0

10

20

30

40

50

60

Single vehicle Two vehicle Multi-vehicle

Percent

Traffic controls

NOTES: “Other” includes all traffic controls accounting for less than 1 percent of total. Figure 2 omits data where no traffic controls were present.

SOURCE: U.S. Department of Transportation, National Highway Traffic Safety Administration, FARS Database as of December 2009.
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Technical Report

These factors are defined as being the most influential 

across all groups (see table 1): 

• traffic controls,

• speed and route type,

• road characteristics,

• weather impacts, and 

• road classification. 

Traffic Controls

In the FARS database, a traffic control device is defined as 

any type of signage or signal with the goal of influencing 

traffic flow (e.g., stop, yield, or warning signs). In singlevehicle crashes, 86.3 percent occured in areas where 

there are no traffic controls; in contrast to 63.9 percent in 

two- and 70.9 percent in multi-vehicle crashes. Where present, at least 95 percent of traffic controls were functioning 

properly (95.9 percent single, 98.2 percent two, and 98.2 

percent multi-vehicle). Figure 2 depicts the type of traffic control present at the time of a crash. Where a traffic 

control existed, a high proportion of crashes occurred in 

areas where a stop sign or traffic signal (e.g., traffic light) 

was present.

Speed and Route Type

As seen in figure 3, a large proportion of fatal crashes, regardless of size, occurred on a state highway. With regard 

to speed limits, the greatest percentage of fatal crashes 

occurred in a 55 mile-per-hour (mph) zone: 27.7, 33.1, and 

29.9 percent of single-, two- and multi-vehicle fatal crashes, 

respectively. Following in frequency, a high proportion of 

fatal crashes took place in a 45 mph zone (13.5, 15.7, and 

14.9 percent, respectively).

While the speed and route type factor is transportation 

related, it is also heavily influenced by human behavior. 

Transportation officials can set appropriate speed limits 

for routes or roadways, however it is ultimately the driver 

who controls the vehicle and the speed. Further analysis 

is needed to understand the role human behavior plays in 

driving this factor.

Figure 3: Route Type by Crash Size, 2004–2008

(as a percent of fatal crashes by crash size)

County 

road

Interstate Municipality Other State 

highway

Township U.S. 

highway

0

5

10

15

20

25

30

35

Single vehicle Two vehicle

Multi-vehicle

Percent

SOURCE: U.S. Department of Transportation, National Highway Traffic Safety Administration, FARS Database as of December 2009. 

Table 2: Alignment of Roadway, 2004–2008

Roadway alignment Frequency Percent

Single vehicle

Straight 72,109 65.7

Curved 36,984 33.6

Unknown 843 0.8

Two vehicle

Straight 54,006 82.3

Curved 11,345 17.3

Unknown 253 0.4

Multi-vehicle

Straight 10,457 85.3

Curved 1,738 14.2

Unknown 61 0.5

NOTE: Percents may not add to 100 due to rounding.

SOURCE: U.S. Department of Transportation, National Highway Traffic 

Safety Administration, FARS Database as of December 2009.
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Road Characteristics

The road characteristics factor could potentially be influenced by federal or state highway policies. The main variables driving the road characteristics factor are:

• roadway alignment,

• roadway profile, and 

• pavement type.

As seen in table 2, most fatal crashes occurred on straight 

roadways. While fewer than 20 percent of fatal crashes 

involving two or more vehicles occurred on a curved 

roadway, one-third of single-vehicle crashes took place 

on a curve. A large proportion of crashes—86.9 percent of 

single-, 88.7 percent of two- and 85.8 percent of multi-vehicle crashes—-occured on roadways paved with bituminous 

(coded in FARS as “blacktop”)6.

Figure 4 looks at the distribution of roadway profiles among 

the three crash sizes (see box A). A large percentage of 

fatal accidents occurred on level or grade roadways. With 

regard to single-vehicle crashes, 44.5 percent occurred on 

roadways that were straight, level, and paved with bituminous (compared to 57.4 percent of two- and 57.3 percent of 

multi-vehicle crashes). An additional 16.5 percent of singlevehicle crashes occurred on curved, level, bituminous 

roadways; in contrast to 8.2 percent of two- and 6.6 percent 

of multi-vehicle crashes. 

6

 A large percent of roadways are defined simply as ‘paved’ or ‘unpaved.’ Based on 

2008 roadway miles: 32.6% of roadways were unpaved and 67.4% were paved. Of 

those paved, 32% were bituminous (blacktop), 2% concrete and the final 65% were 

paved but unknown if with bituminous or concrete. (See: http://www.fhwa.dot.gov/

policyinformation/statistics/2008/hm12.cfm)

Weather Impacts

The weather impacts factor depicts the relationship between weather and roadway surface conditions. While the 

majority of fatal crashes (82.7, 82.6, and 81.6 percent for 

the three fatal crash sizes, respectively) occurred on dry 

roads under normal conditions, further analysis by region 

may provide a deeper understanding of how this factor 

relates to fatal accidents.

Road Classification

Figure 5 depicts the distribution of fatal crashes by roadway 

function and crash size. In both single- and two-vehicle 

crashes, a greater proportion of crashes occurred in rural 

than in urban settings. With multi-vehicle crashes, a greater 

percentage occurred in urban settings.

Figure 4: Roadway Profiles at Fatal Crash Site by 

Crash Size, 2004–2008

(as a percent of fatal crashes by crash size)

Grade Hillcrest Level Unknown

0

10

20

30

40

50

60

70

80

Single vehicle Two vehicle

Multi-vehicle

Percent

Roadway profile

NOTE: “Sag” removed as it accounts for less than 1% of total crashes

SOURCE: U.S. Department of Transportation, National Highway Traffic 

Safety Administration, FARS Database as of December 2009. 

In FARS, roadways that fatal crashes occur on are assigned one of five profiles:

1. Grade – a sloped roadway

2. Level – a flat roadway

3. Hill crest – the top of a hill

4. Sag – a drop in the roadway

5. Unknown

SOURCE: U.S. Department of Transportation, National Highway Traffic Safety Administration, FARS Analytic Reference Guide 1975-2008 

(http://www-nrd.nhtsa.dot.gov/Pubs/811137.PDF)

Box A: Roadway Profiles

Figure 5: Roadway Function by Crash Size, 

2004–2008

(as a percent of fatal crashes by crash size)

Rural Urban

0

10

20

30

40

50

60 Single vehicle Two vehicle

Multi-vehicle

Percent

SOURCE: U.S. Department of Transportation, National Highway Traffic 

Safety Administration, FARS Database as of December 2009. 
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The road classification factor is also influenced by whether 

or not the road is part of the National Highway System 

(NHS). The NHS includes the Interstate Highway System 

as well as other roads important to the nation’s economy, 

defense, and mobility7. The majority of fatal crashes occurred on roads not classified as NHS – 72.6, 65.96, and 

50.4 percent for single-, two-, and multi-vehicle crashes 

respectively. 

Human-Related Factors

This analysis defines human-related factors as events 

contributing to a crash due to human influence, behavior, 

or error. As stated above, factors such as speed and route 

type are heavily influenced by the actions of individual 

drivers. The relative impact of human behavior should be 

considered when exploring possible solutions for similar 

transportation-related factors. Only two of the factors listed 

in table 1 can be defined as human-related: 

• persons, and 

• drunk driving/light condition. 

Persons

The persons factor depicts the relationship between the 

number of persons in a crash and the number of fatalities. Figure 6 examines the number of persons involved 

(includes all fatalities, injured parties, or otherwise involved 

persons such as uninjured occupants) by fatal crash size. 

In both single and two vehicle fatal crashes, the majority 

7 For more information on NHS, refer to http://www.fhwa.dot.gov/planning/nhs// 

of vehicles involved were single occupancy (47.8 and 42.7 

percent, respectively). With regards to fatalities, the majority of fatal crashes resulted in only one fatality (94.5 percent 

in single-vehicle, 88.2 percent in two-vehicle and 83.95 

percent in three-vehicle fatal crashes).

Drunk Driving/Light Condition

The drunk driving/light condition factor involves the relationship between driving under the influences of alcohol 

and current light conditions. In the FARS database, “drunk 

Figure 7: Light Conditions at Time of Crash by 

Crash Size, 2004–2008

(as a percent of fatal crashes by crash size)

Daylight Dark Dark but 

lighted

Dawn Dusk

0

10

20

30

40

50

60

70

Single vehicle Two vehicle

Multi-vehicle

Percent

Light conditions

SOURCE: U.S. Department of Transportation, National Highway Traffic 

Safety Administration, FARS Database as of December 2009. 

Figure 8: Percent of Drunk Drivers by Crash 

Size, 2004–2008 

(as a percent of fatal crashes by crash size)

0

10

20

30

40

50

60

70

80

Single vehicle Two vehicle

Multi-vehicle

0 1 2

Percent

Number of drunk drivers

NOTES: Categories greater than 2 were omitted as they accounted for 

less than 1 percent. Alcohol data is underreported in FARS, true distribution may vary.

SOURCE: U.S. Department of Transportation, National Highway Traffic 

Safety Administration, FARS Database as of December 2009.

Figure 6: Number of Persons Involved in Crash 

by Crash Size, 2004–2008

(as a percent of fatal crashes by crash size)

0

10

20

30

40

50

60

Single vehicle Two vehicle Multi-vehicle

1 234 5 6+

Percent

Number of persons

NOTE: Involved Persons includes all persons involved in a crash; fatalities, injured parties, and otherwise involved persons such as uninjured 

occupants.

SOURCE: U.S. Department of Transportation, National Highway Traffic 

Safety Administration, FARS Database as of December 2009. 
Technical Report

6

driver” is a derived variable and is pulled from police reports; for this reason such alcohol data may be underrepresented in the database. Figure 7 examines the occurrence 

of fatal crashes by light conditions. As seen, regardless of 

crash size, the greatest proportion of fatal crashes occurred 

during the daylight hours (39.9, 61.2, and 64.7 percent for 

the three crash groups, respectively). 

Figure 8 examines the number of reported drunk drivers 

involved in a crash by group. As seen, a large proportion of 

fatal crashes did not involve a drunk driver. Single-vehicle 

crashes were more likely to involve a drunk driver than 

those involving two or more vehicles. 

Table 3 portrays, for single-, two-, and multi-vehicle fatal 

crashes, the percent that occur in each lighting condition 

with no or one drunk driver. For each group, the majority of 

fatal crashes occur during the daylight and do not involve 

drunk drivers (31.4, 53.23, and 55.9 percent, respectively, 

for the three crash size groups). Fatal crashes involving 

drunk drivers occurred most frequently in the dark (28.3, 

13.2, and 11.6 percent, respectively, for the three crash size 

groups)8

. Single-vehicle crashes after dark were more likely 

than not to involve a drunk driver (28.3 v. 23.0 percent of all 

single-vehicle crashes). In contrast, both two- and multi-vehicle crashes were more likely to not involve a drunk driver.

Data Issues

Both human-related factors - persons and drunk driving/

light condition - can greatly influence fatal crashes. However, they do not carry as much weight as transportation-related factors in table 1. This, in part, reflects that variables 

which influence human-related factors are often under-reported in the FARS database due to the way crash data are 

obtained—a key limitation of the database. NHTSA9 notes 

that while many variables on the police reports are concrete 

8

 Includes both the Dark and Dark but Lighted categories. Dark but Lighted refers to 

crashes that occur during hours of darkness, on a stretch of roadway that is artificially 

lighted (e.g.-street lights.)

9

 NHTSA Research Note: An Examination of Driver Distraction as Recorded in 

NHTSA Databases- http://www-nrd.nhtsa.dot.gov/Pubs/811216.pdf

across the jurisdictions, it is possible to have inconstancies. 

Often these inconsistencies are found in variables regarding human behavior, such as distraction, which are hard to 

quantify. 

There is still much to learn to get a complete picture of the 

factors that influence fatal crashes. Unlike transportationrelated factors, such as traffic controls, human-related 

factors cannot be categorized and are difficult to measure. 

Further examination of variables that address human 

behavior can provide more information on the role humanrelated factors play in fatal crashes. Additionally, further 

analysis of higher ranked factors, such as roadway characteristics, may prove useful in developing strategies to 

address roadway fatalities.

For related BTS data and publications: www.bts.gov

About this report

This report was prepared by Jenny Guarino, Mathematical Statistician at the Bureau of Transportation Statistics 

(BTS) and Amrut Champaneri, Director of the Office of 

Statistical Quality at BTS. BTS is a component of the U.S. 

Department of Transportation’s Research and Innovative 

Technology Administration (RITA).

For questions about this, or other BTS special reports, call 

1-800-835-1351, email answers@bts.gov, or visit www.bts.gov. 

Data –

This report is based on data from NHTSA’s Fatality Analysis 

Reporting System (FARS) Database. 

• ftp://ftp.nhtsa.dot.gov/FARS

Related RITA Reports –

•	 Improving Highway Safety by Identifying High Risk

Rural Road Segments and Safety Countermeasures, 

April 2010

•	 Urban Sprawl and Delayed Emergency Services Arrival, June 2010

Table 3: Percentage of Drunk Drivers by Light 

Condition

Single 

vehicle

Two 

vehicle

Multivehicle

Daylight

No drunk drivers 31.39 53.29 55.93

1 drunk driver 7.79 7.22 7.81

Dark

No drunk drivers 15.05 11.39 9.89

1 drunk driver 19.43 7.56 6.11

Dark but Lighted

No drunk drivers 7.92 6.9 6.71

1 drunk driver 8.89 5.58 5.49

Dawn

No drunk drivers 1.35 1.57 1.72

1 drunk driver 0.48 0.26 0.32

Dusk

No drunk drivers 1.24 1.68 1.48

1 drunk driver 0.8 0.56 0.57

NOTE: Percents may not add to 100 as fatal crashes can occur under 

any light conditions with more than one drunk driver.

SOURCE: U.S. Department of Transportation, National Highway Traffic 

Safety Administration, FARS Database as of December 2009