DOT HS 811 182 August 2009

The Long-Term Effect of 

ABS in Passenger Cars 

and LTVs

This document is available to the public from the National Technical Information Service, Springfield, Virginia 22161
This publication is distributed by the U.S. Department of Transportation, National Highway Traffic 

Safety Administration, in the interest of information exchange. The opinions, findings, and 

conclusions expressed in this publication are those of the authors and not necessarily those of the 

Department of Transportation or the National Highway Traffic Safety Administration. The United 

States Government assumes no liability for its content or use thereof. If trade or manufacturers’ 

names or products are mentioned, it is because they are considered essential to the object of the 

publication and should not be construed as an endorsement. The United States Government does not 

endorse products or manufacturers. 
                                                

i 

 

 

Technical Report Documentation Page 

1. Report No. 

DOT HS 811 182 

2. Government Accession No. 3. Recipient’s Catalog No. 

4. Title and Subtitle 

The Long-Term Effect of ABS in Passenger Cars and LTVs 

5. Report Date 

August 2009 

6. Performing Organization Code 

7. Author(s) 

Charles J. Kahane and Jennifer N. Dang 

8. Performing Organization Report No. 

9. Performing Organization Name and Address 

Evaluation Division; National Center for Statistics and Analysis 

National Highway Traffic Safety Administration 

Washington, DC 20590 

10. Work Unit No. (TRAIS) 

11. Contract or Grant No. 

12. Sponsoring Agency Name and Address 

National Highway Traffic Safety Administration 

1200 New Jersey Avenue SE. 

Washington, DC 20590 

13. Type of Report and Period Covered 

NHTSA Technical Report 

14. Sponsoring Agency Code 

15. Supplementary Notes 

16. Abstract 

Statistical analyses based on data for calendar years 1995 to 2007 from the Fatality Analysis Reporting 

System (FARS) and the General Estimates System (GES) of the National Automotive Sampling System 

(NASS) estimate the long-term effectiveness of antilock brake systems (ABS) for passenger cars and 

LTVs (light trucks and vans) subsequent to the 1995 launch of public information programs on how to 

use ABS correctly. ABS has close to a zero net effect on fatal crash involvements. Fatal run-off-road 

crashes of passenger cars increased by a statistically significant 9 percent (90% confidence bounds: 3% to 

15% increase), offset by a significant 13-percent reduction in fatal collisions with pedestrians (confidence 

bounds: 5% to 20%) and a significant 12-percent reduction in collisions with other vehicles on wet roads 

(confidence bounds: 3% to 20%). ABS is quite effective in nonfatal crashes, reducing the overall crashinvolvement rate by 6 percent in passenger cars (confidence bounds: 4% to 8%) and by 8 percent in LTVs 

(confidence bounds: 3% to 11%). The combination of electronic stability control (ESC) and ABS will 

prevent a large proportion of fatal and nonfatal crashes. 

17. Key Words 

NHTSA; FARS; NASS; GES; FMVSS; crashavoidance; effectiveness; fatality reduction; lives 

saved; brakes; statistical analysis; benefits 

18. Distribution Statement 

This report is free of charge from the NHTSA Web 

site at www.nhtsa.dot.gov 

19. Security Classif. (Of this report) 

Unclassified 

20. Security Classif. (Of this page) 

Unclassified 

21. No. of Pages 

89 

22. Price 

Form DOT F 1700.7 (8-72) 
TABLE OF CONTENTS 

List of abbreviations ....................................................................................................................iv 

Executive summary......................................................................................................................vi 

1. History of ABS and previous effectiveness findings........................................................... 1 

1.1 Rationale for ABS....................................................................................................... 1 

1.2 Stopping test findings ................................................................................................. 2 

1.3 Market and regulatory history..................................................................................... 5 

1.4 Initial effectiveness analyses....................................................................................... 7 

1.5 The safety community’s response............................................................................. 10 

1.6 The second generation of effectiveness analyses...................................................... 12 

1.7 Developments since 2002 ......................................................................................... 14 

1.8 Goals of the evaluation ............................................................................................. 15 

2. Effect of ABS in fatal crashes: analyses of 1995-2007 FARS data................................... 17 

2.0 Summary................................................................................................................... 17 

2.1 Analysis for passenger cars....................................................................................... 17 

 2.1.1 FARS calendar year range .............................................................................. 17 

 2.1.2 Control group involvements versus response group....................................... 17 

 2.1.3 Cars without ABS versus cars with ABS........................................................ 19 

2.1.4 Adjusting the case counts for air bag effectiveness........................................ 22 

2.1.5 Results............................................................................................................. 24 

2.1.6 Comparative results for four cohorts of cars................................................... 28 

 2.1.7 Drinking versus non-drinking drivers............................................................. 30 

 2.1.8 Net effect by specific road-surface condition ................................................. 33 

2.2 Analysis of rear-wheel ABS for LTVs ..................................................................... 33 

 2.2.1 LTVs without ABS versus LTVs with rear-wheel ABS................................. 33 

2.2.2 Results............................................................................................................. 36 

2.3 Analysis of four-wheel ABS for LTVs..................................................................... 39 

 2.3.1 LTVs with rear-wheel ABS versus LTVs with four-wheel ABS ................... 39 

2.3.2 Results............................................................................................................. 41 

 2.3.3 Drinking versus non-drinking drivers............................................................. 45 

ii 
3. Effect of ABS in all crashes: analyses of 1995-2007 GES data ........................................ 47 

3.0 Summary................................................................................................................... 47 

3.1 Analysis for passenger cars....................................................................................... 47 

 3.1.1 GES calendar year range................................................................................. 47 

 3.1.2 Control group involvements versus response group....................................... 48 

 3.1.3 Cars without ABS versus cars with ABS........................................................ 49 

3.1.4 Results............................................................................................................. 50 

3.1.5 Comparative results for four cohorts of cars................................................... 55 

 3.1.6 Net effect by specific road-surface condition ................................................. 56 

 3.1.7 Net effect on injury crashes ............................................................................ 56 

3.2 Analysis of rear-wheel ABS for LTVs ..................................................................... 57 

3.2.1 Results............................................................................................................. 57 

3.3 Analysis of four-wheel ABS for LTVs..................................................................... 60 

3.3.1 Results............................................................................................................. 61 

4. Discussion.......................................................................................................................... 66 

4.1 Summary and comparison of effectiveness estimates .............................................. 66 

4.2 Why is the effect still negative in run-off-road crashes? .......................................... 68 

4.3 Combined effect of four-wheel ABS and ESC......................................................... 69 

4.4 Long-term benefits and costs of four-wheel ABS .................................................... 71 

References................................................................................................................................... 74 

iii 
LIST OF ABBREVIATIONS 

ABS Antilock brake system 

AIS Abbreviated Injury Scale 

ANPRM Advance Notice of Proposed Rulemaking 

BAC Blood alcohol concentration, measured in grams per deciliter (g/dL) 

BMW Bayerische Motoren Werke 

CY Calendar Year 

df Degrees of Freedom 

DOT United States Department of Transportation 

ESC Electronic Stability Control 

FARS Fatality Analysis Reporting System, a census of fatal crashes in the United 

States since 1975 

FMVSS Federal Motor Vehicle Safety Standard 

GDP Gross Domestic Product 

GES General Estimates System of NASS 

GM General Motors 

GVWR Gross vehicle weight rating, specified by the manufacturer, equals the 

vehicle’s curb weight plus maximum recommended loading 

IIHS Insurance Institute for Highway Safety 

ISTEA Intermodal Surface Transportation Efficiency Act of 1991 

LTV Light Trucks and Vans, includes pickup trucks, SUVs, minivans, and full-size 

vans 

MY Model year 

NASS National Automotive Sampling System, a probability sample of policereported crashes in the United States since 1979, investigated in detail 

NHTSA National Highway Traffic Safety Administration 

iv 
NPRM Notice of Proposed Rulemaking 

OMB Office of Management and Budget of the United States Government 

Pct Pt Percentage Point 

PSU Primary Sampling Unit 

SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy 

for Users, 2005 

SAS Statistical and database management software produced by SAS Institute, Inc. 

SUV Sport utility vehicle 

VIN Vehicle Identification Number 

VMT Vehicle miles of travel 

VW Volkswagen 

v 
EXECUTIVE SUMMARY 

Antilock brake systems (ABS) have close to a zero net effect on fatal crash involvements. Runoff-road crashes significantly increase, offset by significant reductions in collisions with 

pedestrians and collisions with other vehicles on wet roads. But ABS is quite effective in 

nonfatal crashes, reducing the overall crash-involvement rate by 6 percent in passenger cars and 

by 8 percent in LTVs (light trucks – including pickup trucks and SUVs – and vans). In a few 

years all new vehicles will be equipped with electronic stability control and will almost certainly 

also be equipped with ABS. The combination of ESC and ABS will prevent a large proportion 

of fatal and nonfatal crashes. 

The fundamental safety problem addressed by four-wheel ABS is that, in an emergency 

situation, the average driver brakes too hard, locking the wheels, which causes the vehicle to lose 

directional control. If the front wheels lock, the vehicle will continue in a straight path, but the 

driver will be unable to steer it. If the rear wheels lock, the vehicle can spin out and lose control. 

ABS senses if any of the four wheels is about to lock, and if so, it quickly releases the brakes on 

that wheel. Cycles of releasing, holding, and reapplying brakes are repeated many times per 

second. As long as the driver maintains firm pressure on the brake pedal, ABS automatically 

provides optimum braking force short of lockup. ABS enables the driver to steer while braking, 

prevents yawing due to rear-wheel lockup, and on many surfaces reduces stopping distances 

relative to a skidding vehicle. 

Federal Motor Vehicle Safety Standard No. 126 will require all new passenger vehicles to be 

equipped with ESC, a remarkably effective crash-avoidance technology, after September 1, 

2011. All ESC systems to date and for the foreseeable future incorporate ABS technology. The 

ESC standard will apparently soon place ABS on every new car and LTV sold in the United 

States. 

Numerous stopping tests by expert drivers at test tracks showed that four-wheel ABS is 

successful, especially on wet pavements, in improving overall vehicle stability during braking, 

preserving the ability to steer, and reducing stopping distances on many surfaces. But previous 

statistical evaluations of ABS have had ambiguous results. Analyses of data from the early 

1990s showed significant increases in fatal run-off-road crashes with ABS, on the order of 28 

percent. The increase was baffling, given the success of ABS on the test track. However, at that 

time, many drivers did not yet know how to use ABS correctly. During the mid-1990s, the 

safety community worked hard to inform the public about the correct use of ABS (“Don’t let up 

on the brakes”; “Stomp, stay, and steer”). A second generation of analyses circa 2000 showed 

much smaller increases in run-off-road crashes that were no longer statistically significant. But 

they were based on just two or three years of data and left uncertainty about the overall effect of 

ABS. 

It is time for a new evaluation of ABS, including the crash data that has accumulated over the 

past decade and in view of the fact that all new cars and LTVs will likely soon be equipped with 

ABS along with ESC. The analyses are based on data for calendar years 1995 to 2007 from the 

Fatality Analysis Reporting System and the General Estimates System (GES) of the National 

Automotive Sampling System; 1995 was the launch of major public-information programs about 

vi 
ABS. The goal is to estimate the long-term effect of ABS. Data up to 2007 comprises most of 

the later service life of the makes and models that were originally equipped with ABS. It also 

allows inclusion of models that received ABS more recently, and even some models that once 

had standard ABS and subsequently just optional ABS. 

The following table displays the principal statistical findings of this evaluation – namely, the 

estimated percentage reductions in crash involvements for cars or LTVs equipped with fourwheel ABS relative to comparable cars or LTVs not equipped with any ABS. Positive estimates 

(reductions in crash involvements with ABS) are shown in black, negative estimates in red. 

Statistically significant estimates (at the one-sided .05 level) are printed bold, and they are 

shaded light blue if positive, yellow if negative. 

ESTIMATED CRASH REDUCTION (%) BY FOUR-WHEEL ABS, 1995-2007 

 

 Wet, Snowy,

All Roads or Icy Roads 

Cars LTVs Cars LTVs 

FATAL CRASH INVOLVEMENTS 

All fatal involvements 1 - 1 - 1 - 6 

All run-off-road crashes - 9 - 6 - 34 - 10 

Side impacts with fixed objects - 30 none - 85 - 4

 First-event rollovers - 11 - 10 - 52 - 31 

All other run-off-road crashes - 3 - 5 - 17 - 3 

Pedestrian/bicyclist/animal 13 14 none - 14 

Culpable involvements w other veh 4 - 1 12 - 6 

ALL CRASH INVOLVEMENTS 

vii 

 

All crash involvements 6 8 16 14 

All run-off-road crashes - 1 11 - 13 3 

Side impacts with fixed objects - 20 - 9 - 43 - 15

 First-event rollovers 3 17 - 12 6

All other run-off-road crashes 5 15 - 3 9

Pedestrian/bicyclist/animal - 8 - 42 - 8 - 10 

Culpable involvements w other veh 17 20 37 36 
 

	 The long-term overall effect of ABS on fatal crash involvements is close to zero in both 

cars and LTVs. The observed effect in cars is a 1-percent reduction (90% confidence 

bounds range from a 2% increase to a 4% reduction). The observed effect in LTVs is a 

1-percent increase (confidence bounds: -6% to 4%). 

	 But the overall effect of ABS on all crash involvements, including nonfatal involvements 

is beneficial and statistically significant in both cars and LTVs. The observed reductions 

are 6 percent in cars (confidence bounds: 4% to 8%) and 8 percent in LTVs (confidence 

bounds: 3% to 11%). 

	 The overall reduction of nonfatal-injury crashes is approximately the same as the 

reduction of all crashes.1 

	 Fatal run-off-road crashes increase with ABS by a statistically significant 9 percent in 

cars (confidence bounds: 3% to 15% increase). The long-term effect is substantially 

smaller than in the early years of ABS (28% increase), but it is still a significant increase. 

The observed effect in LTVs is a non-significant 6-percent increase (confidence bounds: 

-16% to 3%). 

	 On wet, snowy, or icy roads, where ABS is most likely to activate, the increase in fatal 

run-off-road crashes is a statistically significant 34 percent in passenger cars (confidence 

bounds: 20% to 50% increase). On these roads, all three types of fatal run-off-road 

crashes increase significantly for cars and so do fatal rollovers of LTVs. 

	 On dry roads, the increase in fatal run-off-road crashes is a non-significant 4 percent in 

passenger cars. 

	 Side impacts with fixed objects generally increase even more with ABS than other types 

of run-off-road crashes (except for LTV fatalities). Fatal and nonfatal crashes both 

increase significantly for passenger cars. 

	 The statistical analyses continue to show persistent, significant increases in run-off-road 

crashes with ABS, especially on wet roads. They remain at odds with the impressive 

performance of ABS on the test track, especially on wet roads. They do not tally with the 

benefits of ABS observed in other types of crashes. We are still unable to provide a 

convincing explanation or empirical evidence (other than the crash statistics themselves) 

for the increase in run-off-road crashes. 

	 Fatal collisions with pedestrians, bicyclists, or animals decrease significantly, overall, 

with ABS. The observed reductions are 13 percent in cars (confidence bounds: 5% to 

20%) and 14 percent in LTVs (confidence bounds: 3% to 25%). But the observed effects 

on wet, snowy, or icy roads are not positive. 

	 Culpable fatal involvements with other vehicles on wet, snowy, or icy roads were 

reduced by a statistically significant 12 percent in passenger cars (confidence bounds: 3% 

to 20%). 

1

 Not shown in the table on the preceding page. 

viii 
 

	 Culpable involvements with other vehicles on all roads decreased significantly in cars 

and LTVs. The observed reductions are 17 percent in cars (confidence bounds: 13% to 

22%) and 20 percent in LTVs (confidence bounds: 12% to 28%). 

	 The effects on culpable involvements with other vehicles on wet, snowy or icy roads are 

a remarkable 37 percent reduction in cars (confidence bounds: 29% to 45%) and 36 

percent in LTVs (confidence bounds: 24% to 46%). ABS is highly effective here. 

Although the preceding analyses show a significant 9-percent increase for ABS on run-off-road 

crashes of passenger cars, the increase is small relative to the likely benefits of ESC. NHTSA’s 

2007 evaluation of ESC, based on statistical analyses through calendar year 2004, found a 36-

percent reduction in fatal run-off-road crashes. Thus, the combined effect of ESC and ABS is an 

estimated 30-percent reduction of fatal crashes.2 

The following table estimates the combined effects of four-wheel ABS and ESC. The estimates 

for ABS are from this evaluation; the ESC numbers are from NHTSA’s 2007 evaluation and 

estimate the effect on ESC on a vehicle that had previously been equipped with ABS. (A caveat: 

these are still somewhat early estimates of ESC effectiveness; NHTSA plans follow-up statistical 

analyses to estimate the long-term effectiveness of ESC.) 

	 The combined effects are all substantial reductions. 

	 Negative effects of ABS on run-off-road crashes – whatever may be causing them – are 

possibly remedied and in any case dwarfed by the likely benefits of ESC. 

	 The combined effects on culpable involvements with other vehicles, where ABS is 

especially effective, are large. 

2

 Dang (2007); 1 - [(1 + .09) x (1 - .36)] = 30% reduction. 

ix 
ESTIMATED CRASH REDUCTION (%) FOR FOUR-WHEEL ABS PLUS ESC 

Four-Wheel ABS ESC Combined 

FATAL CRASH INVOLVEMENTS 

Passenger cars 

All fatal involvements 1 14 15 

All run-off-road crashes – 9 36 30 

Culpable involvements w other veh 4 19 22 

LTVs 

 All fatal involvements – 1 28 27 

All run-off-road crashes – 6 70 68 

Culpable involvements w other veh – 1 34 33 

ALL CRASH INVOLVEMENTS 

Passenger cars 

 All crash involvements 6 8 14 

All run-off-road crashes – 1 45 44 

Culpable involvements w other veh 17 13 28 

LTVs 

All crash involvements 8 10 17 

All run-off-road crashes 11 72 75 

Culpable involvements w other veh 20 16 3 

x 
 

CHAPTER 1 

HISTORY OF ABS AND PREVIOUS EFFECTIVENESS FINDINGS 

1.1 Rationale for ABS 

The fundamental safety problem addressed by antilock braking systems is that few drivers are 

able to modulate pressure on the brake pedal optimally in a sudden emergency situation or on an 

unexpectedly slippery surface. The average driver will either brake timidly, lengthening 

stopping distance, or brake too hard, locking the wheels. If excess pedal pressure locks only the 

front wheels, the vehicle will continue in a straight path, but the driver will be unable to steer it 

and avoid obstacles. If it locks the rear wheels, the vehicle can spin out and lose control. 

Four-wheel ABS senses if any of the four wheels is about to lock, and if so, it quickly releases 

the brakes on that wheel and lets it start rolling again. Cycles of releasing, holding and 

reapplying brakes are repeated many times per second. As long as the driver maintains firm 

pressure on the brake pedal, ABS will automatically modulate the pressure at the wheels at a 

level close to the optimum braking force short of lockup. ABS will enable the driver to steer 

while braking, prevent yawing due to rear-wheel lockup, and on many surfaces reduce stopping 

distances relative to a skidding vehicle.3 

Light trucks and vans – including pickup trucks, sport utility vehicles, minivans, and full-size 

vans with a GVWR less than 10,000 pounds – are especially prone to rear-wheel lockup when 

they are not heavily loaded, and especially crash-prone once they yaw out of control. Many 

LTVs, including all domestic pickup trucks, were equipped with rear-wheel-only ABS for 

several model years before they received four-wheel ABS. This system senses if any of the rear 

wheels is about to lock and if so, it quickly releases the brakes on that wheel and lets it start 

rolling again. Rear-wheel ABS, however, does not prevent front-wheel lockup or assure steering 

control during braking. If the front wheels lock while the rear wheels turn, the truck will usually 

just slow down in a straight line, without yawing. 

The preceding are good reasons why drivers might want ABS on their vehicles. But now a 

compelling rationale has developed for equipping all new vehicles with ABS. Federal Motor 

Vehicle Safety Standard No. 126 will require all new passenger vehicles to be equipped with 

electronic stability control after September 1, 2011. ESC is a remarkably effective crashavoidance technology.4

 However, all ESC systems to date and for the foreseeable future 

incorporate ABS technology. The ESC standard will apparently soon place ABS on every new 

car and LTV sold in the United States. 

3

 Kahane, C. J. (2004). Lives Saved by the Federal Motor Vehicle Safety Standards and Other Vehicle Safety 

Technologies, 1960-2002, NHTSA Technical Report. DOT HS 809 833, pp. 25-31. Washington, DC: National
 

Highway Traffic Safety Administration. 

4

 Dang (2007). 

1
 
 

 

1.2 Stopping-test findings 

NHTSA carried out two extensive series of stopping tests at its East Liberty, Ohio, facility 

between 1988 and 1991. The agency tested 14 vehicles with four-wheel ABS (12 cars and 2 

LTVs) and 3 pickup trucks with rear-wheel ABS.5 The tests included a variety of road surfaces, 

straight-line stops at various speeds, and maneuvers requiring steering plus braking. Each 

vehicle was tested with the ABS enabled and disabled and with the vehicle empty and fully 

loaded. The road surfaces included dry concrete, three types of wet asphalt or concrete (different 

levels of smoothness), two slippery surfaces (wet Jennite and wet epoxy), and gravel. Wet 

Jennite (roadway sealant) has a much lower sliding than rolling coefficient of friction, making it 

likely to show an exceptional improvement in stopping distance with ABS. Wet epoxy has 

coefficients of friction similar to ice, although it is not intended as a surrogate for ice: unlike wet 

Jennite, the sliding coefficient of friction is only slightly lower than the peak-friction coefficient. 

The objectives of the tests were to study the effect of ABS on general directional stability, 

vehicle response to steering input, and stopping distances. 

Table 1-1 shows that four-wheel ABS maintained excellent directional stability during straightline stops on homogeneous surfaces. 

TABLE 1-1: EFFECT OF FOUR-WHEEL ABS ON VEHICLE YAWING 

IN STRAIGHT-LINE SPIKE STOPS, BY TYPE OF ROAD SURFACE 

 Number of Tests, by Angle of Yaw 

Road ABS 

Surface Status No Yaw < 10o 10-45o > 45o 

Dry concrete Enabled 46 

Disabled 40 6 

Wet asphalt/concrete Enabled 276 

Disabled 170 99 7 

Wet Jennite Enabled 88 

Disabled 16 37 24 11 

Wet epoxy Enabled 42 

Disabled 10 22 5 5 

Gravel Enabled 42 

Disabled 17 21 3 

5

 Hiltner, E., Arehart, C., & Radlinski, R. (1991). Light Vehicle ABS Performance Evaluation. NHTSA Technical 

Report. DOT HS 807 813. Washington, DC: National Highway Traffic Safety Administration; Arehart, C., 

Radlinski, R., & Hiltner, E. (1991). Light Vehicle ABS Performance Evaluation – Phase II, NHTSA Technical 

Report. DOT HS 807 924. Washington, DC: National Highway Traffic Safety Administration; Kahane, C. J. (1993). 

Preliminary Evaluation of the Effectiveness of Rear-Wheel Antilock Brake Systems for Light Trucks, pp. 9-14. 

NHTSA Docket No. 70-27-GR-026. Washington, DC: National Highway Traffic Safety Administration. 

2
 
In a total of 494 tests, on different road surfaces, with ABS enabled, there was not a single case 

of yawing. With ABS disabled, some of the vehicles yawed on every surface. There was more 

yawing on the slippery surfaces. On dry concrete, only 6 of 46 tests with ABS disabled involved 

yawing, and always less than 10 degrees. On wet Jennite, 72 of 88 tests resulted in yawing, 11 

of them more than 45 degrees. 

Table 1-2 shows that rear-wheel ABS substantially, but not completely, reduced the yaw of 

pickup trucks. Even with the rear wheels rolling, front-wheel lockup can lead to moderate 

amounts of yaw. With the ABS enabled, the amount of yaw was always less than 10 degrees on 

wet asphalt/concrete and less than 45 degrees on wet Jennite, while there was yawing in excess 

of 45 degrees on both surfaces with the ABS disabled. On dry concrete and gravel, however, the 

tests did not show an advantage for rear-wheel ABS. 

TABLE 1-2: EFFECT OF REAR-WHEEL ABS ON VEHICLE YAWING 

IN STRAIGHT-LINE SPIKE STOPS, BY TYPE OF ROAD SURFACE 

 Number of Tests, by Angle of Yaw 

Road ABS 

Surface Status No Yaw < 10o

 10-45o > 45o 

Dry concrete Enabled 12 

Disabled 12 

Wet asphalt/concrete Enabled 57 15 

Disabled 39 26 2 5 

Wet Jennite Enabled 1 17 6 

Disabled 0 11 10 3 

Wet epoxy Enabled 1 5 

Disabled 0 6 

Gravel Enabled 5 7 

Disabled 6 6 

NHTSA also tested stops on heterogeneous surfaces that were more slippery under one side of 

the vehicle than the other (so-called "split-mu" surfaces) – e.g., wet epoxy under the left wheels 

and wet concrete under the right wheels. They resemble a roadway with slippery patches. Fourwheel and rear-wheel ABS were both highly effective in preventing or minimizing yaw in panic 

stops, whereas the yaw was often 180 degrees or more when the systems were disabled. 

For a test of combined braking and steering, the vehicles with four-wheel ABS were subjected to 

emergency stops in a curve or lane-change maneuver on wet asphalt or Jennite. In all cases, the 

vehicles successfully negotiated the maneuvers during panic braking with the ABS enabled. 

3
 
Vehicles with rear-wheel ABS experienced front-wheel lockup during panic braking and could 

not be steered around the curve or to another lane. 

The effect on stopping distance in straight-line panic stops is not uniformly beneficial for fourwheel ABS and, in fact, somewhat negative for rear-wheel ABS. Table 1-3 shows the median 

percentage reduction of stopping distance, by road surface type, for a test with ABS enabled 

relative to the corresponding test with ABS disabled. Four-wheel ABS reduced stopping 

distances by only 5 percent on dry concrete, but had a substantially larger effect on wet asphalt 

or concrete (14 percent on the average). Because wet Jennite has a much higher rolling 

resistance than sliding resistance, the reduction in stopping distance for ABS is 43 percent. 

Jennite is not extensively used to pave real highways, but there are certain conditions where 

actual pavements can approach the characteristics of Jennite (wet, highly worn, dirty and/or 

oily). The much smaller reduction on wet epoxy (10%) suggests that the exceptional result on 

Jennite is due to the characteristics of that material and is not true for all slippery materials (e.g., 

ice). Finally, four-wheel ABS lengthens stopping distances on gravel by 28 percent: A car with 

the wheels locked causes gravel to build up in front of the wheels, resulting in a plowing effect 

that slows the vehicle quicker than ABS (although steering control is lost and the car may yaw). 

Surfaces with other loose material, such as snow, would have a similar effect. 

TABLE 1-3: EFFECT OF ABS ON STOPPING DISTANCE 

IN STRAIGHT-LINE SPIKE STOPS, BY TYPE OF ROAD SURFACE 

Median Percent Reduction of Stopping Distance 

ABS Enabled Versus ABS Disabled 

Road Surface Four-Wheel ABS Rear-Wheel ABS 

Dry concrete 5 – 6 

Wet asphalt/concrete 14 – 6 

Wet Jennite 43 7 

Wet epoxy 10 – 12 

Gravel – 28 – 18 

Rear-wheel ABS lengthens stopping distances on all surfaces except wet Jennite; the increase is 

6 percent on dry or wet concrete/asphalt. In general, a truck with four wheels sliding stops a 

little sooner than a truck with the front wheels sliding and the rear wheels rolling. However, 

keeping the rear wheels rolling greatly improves directional stability. 

In summary, NHTSA's tests show that four-wheel ABS is effective, especially on wet 

pavements, in improving overall vehicle stability during braking, preserving the ability to steer, 

and reducing stopping distances. Rear-wheel ABS, on the other hand, did not reduce stopping 

distance or preserve steering control during braking; benefits were limited to improved 

directional stability. 

4
 
 

 

Nevertheless, the test results do not take into account phenomena that may limit the utility of 

ABS in actual crashes. The tests were performed by expert drivers who knew they had to apply 

firm pressure on the brakes in combination with relatively delicate evasive steering. The average 

driver in a panic situation might have difficulty combining firm braking with delicate steering, 

and might be too timid with the pedal or too bold with the steering wheel. The stopping tests 

also did not include maneuvers where vehicles were sliding even before the brakes were applied. 

1.3 Market and regulatory history 

Modern four-wheel ABS was first offered as standard equipment in 1985 on some lines of 

BMW, Lincoln and Mercedes and in 1986 on Chevrolet Corvette. Consumers liked the 

technology. Availability of ABS increased gradually from 1987 to 1990 and dramatically in 

1991 and 1992, when it became standard on the majority of GM cars. By the mid-1990s, ABS 

was standard or a popular option on a wide variety of cars comprising the principal 

manufacturers. Figure 1-1 shows that from 1994 onwards at least 55 percent of new passenger 

cars have been equipped with ABS each model year.6 

FIGURE 1-1: PERCENT OF CARS WITH ABS, BY MODEL YEAR 

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

Percent 

1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 

Model Year 

In general, ABS has usually been standard on the larger and the more expensive cars, optional 

and not too frequently sold on small economy cars, and standard or a popular option on the midsize cars with high sales. Market share dropped from 1999 to 2003 as several makes and models 

6 Ward’s Automotive Yearbooks, 1987-2008. Detroit: Ward’s Communications; tables of factory-installed equipment 

on vehicles of the preceding model year. 

5
 
 

 

 

dropped standard ABS and made it optional, but rebounded after 2005 as other models with high 

sales made ABS standard after their latest redesign. 

Rear-wheel ABS first appeared as standard equipment in 1987 Ford F-Series pickup trucks, 

Bronco, and Bronco II. By 1988 to 1990, it was standard in most domestic pickup trucks and 

SUVs as well as many vans. Four-wheel ABS installations for LTVs began in 1989 on some 

Chevrolet Astro and GMC Safari minivans, Jeep Cherokee, and Jeep Wagoneer. Figure 1-2 

shows that from 1995 onwards, 90 percent of new LTVs were equipped with either rear-wheel 

(pink region) or four-wheel (blue region) ABS. However, during model years 1992 to 2002, 

four-wheel systems rapidly superseded rear-wheel ABS. By 2004, only four-wheel ABS was 

available on new vehicles. 

FIGURE 1-2: PERCENT OF LTVs WITH REAR-WHEEL AND FOUR-WHEEL ABS, BY MODEL YEAR 

Percent 100 

90 

80 

70 

60 

50 

40 

30 

20 

10 

0 

1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 

Model Year 

ABS has not to date been explicitly required for passenger vehicles with GVWR less than 10,000 

pounds. But there was some movement in that direction in the early 1990s, obviously spurred by 

the outstanding performance of ABS on the test track. The Intermodal Surface Transportation 

Efficiency Act of 1991, Section 2507 instructed NHTSA to initiate rulemaking, by December 31, 

1993, to consider the need for additional brake-performance standards for light vehicles, 

including ABS.7

 The agency published an Advance Notice of Proposed Rulemaking on January 

4, 1994, asking for information about the effectiveness and potential benefits of ABS 

technologies.8 

7

 Public Law 102-240, December 18, 1991. 

8 Federal Register 59 (January 4, 1994): 281. 

6
 
By 1994, production vehicles with rear-wheel and four-wheel ABS had accumulated enough onthe-road experience for preliminary statistical evaluations to see if the effect of ABS on actual 

crashes matched the promise it showed on the test track. These studies, which will be discussed 

in the next section, failed to show significant net benefits for ABS. Based on these analyses, as 

well as other responses to the 1994 ANPRM, NHTSA issued a second ANPRM on July 12, 

1996, deferring indefinitely any ABS requirement for light vehicles.9 

1.4 Initial effectiveness analyses 

Because rear-wheel ABS for LTVs began to appear in large numbers several years before fourwheel ABS for cars or LTVs, the first statistical analysis was NHTSA’s December 1993 

evaluation of rear-wheel ABS.10 The basic approach in this study, as well as most of the others, 

is to compare the ratio of response-group to control-group crashes in vehicles with ABS to the 

corresponding ratio in vehicles without ABS. The response groups comprise types of crash 

involvements that ABS might prevent if it improves directional stability, steering control, or 

stopping distance (or increase if it worsens any of these): run-off-road crashes, involvements as 

the frontally impacting vehicle in collisions with other vehicles, and collisions with pedestrians. 

The control group consists of crash involvements that are unaffected by ABS, such as being hit 

while standing still. 

Because rear-wheel ABS has little effect on steering control and perhaps even a negative effect 

on stopping distance, the most likely expected benefit would be a reduction in run-off-road 

crashes, such as rollovers and collisions with fixed objects. Indeed, analyses of crash data from 

Florida, Michigan, and Pennsylvania showed statistically significant reductions of nonfatal 

rollovers averaging 30 percent and side impacts with fixed objects averaging 20 percent.11 There 

was little or no effect on nonfatal involvements in collisions with other vehicles.12 

However, corresponding analyses of FARS data did not show reductions of fatal run-off-road 

crashes.13 FARS analyses of fatal involvements in multi-vehicle collisions showed a mix of zero 

and negative effectiveness.14 Pedestrian crashes were the only type with a significant fatality 

reduction, on the order of 10 percent, but it was not clear why rear-wheel ABS should be 

effective there.15 

NHTSA issued a second FARS analysis of rear-wheel ABS in 1995, based on logistic 

regressions rather than contingency table analyses, the method of the previous study.16 This 

report associated rear-wheel ABS with a statistically significant increase of fatal involvements as 

the striking vehicle in a multi-vehicle collision. It did not find a reduction of fatal run-off-road 

 

9 Federal Register 61 (July 12, 1996): 36698. 10 Kahane (1993). 

11 Ibid., pp. 19-44 and 108. 12 Ibid., pp. 100-109. 13 Ibid., pp. 42-49 and 100-109.
 14 Ibid., pp. 78-85. 15 Ibid., pp. 93-99 and 108. 16 Hertz, E., Hilton, J., & Johnson, D. M. (1995a). An Analysis of the Crash Experience of Light Trucks Equipped 

with Antilock Braking Systems. NHTSA Technical Report. DOT HS 808 278. Washington, DC: National Highway 

Traffic Safety Administration.
 

7
 
 

 

crashes. Pedestrian crashes were not considered in that study. By 1995, four-wheel ABS was 

rapidly superseding rear-wheel ABS in LTVs, and the effectiveness of the rear-wheel systems 

became a moot point. 

The first statistical analysis of four-wheel ABS for passenger cars was published by the Highway 

Loss Data Institute in January 1994.17 It is based on the frequency and cost of collision damage 

claims per 1000 insured vehicle years in MY 1992 models with standard ABS versus the same 

models in MY 1991 without ABS. HLDI found that ABS had little or no effect. However, the 

data mix all types of crashes and contain a large percentage of low-speed collisions that are 

unlikely to be affected by ABS. The results did not preclude the possibilities that ABS might 

have an effect in crashes of higher severity or might be beneficial in certain crash types and 

harmful in others. 

A May 1994 study of the effects of ABS in Sweden was the first publication indicating that ABS 

was associated with a statistically significant shift from colliding with other vehicles to running 

off the road. The paper did not fully resolve whether this happened because ABS reduced 

collisions with other vehicles, increased run-off-road crashes, or both. But it first revealed a 

pattern that would reappear in almost every subsequent analysis. Furthermore, it showed this 

pattern in a different country, with a substantially different mix of makes and models on the 

road.18 

NHTSA’s preliminary evaluation of four-wheel ABS for passenger cars is dated December 

1994.19 The analysis is based on FARS data from 1989 to 1993. In multi-vehicle crashes on dry 

roads, the ratio of involvements as a frontally impacting car to involvements as a car that was 

struck in the rear or side, or while standing still, was nearly the same with or without ABS. That 

suggests ABS has, at most, a small effect on multi-vehicle crashes on dry roads. For the 

remaining analyses, multi-vehicle crash involvements on dry roads (regardless of impact type) 

serve as the control group.20 Multi-vehicle crash involvements on wet roads were reduced by a 

statistically significant 24 percent with ABS:21 

FARS 1989-93 Wet Dry Ratio of 

MULTI-VEHICLE CRASHES Roads Roads Wet/Dry 

Last 2 model years without ABS 246 1,021 .241 

First 2 model years with ABS 158 858 .184 

Fatal collisions with pedestrians, bicyclists, trains, or animals, on all road surfaces, were reduced 

by a statistically significant 27 percent.22 

17 Collision and Property Damage Liability Losses of Passenger Cars With and Without Antilock Brakes. (1994). 

Insurance Special Report No. A-41., Arlington, VA: Highway Loss Data Institute. 

18 Kullgren, A., Lie, A., & Tingvall, C. (1994). The Effectiveness of ABS in Real Life Accidents, Proceedings of the
 

Fourteenth International Technical Conference on the Enhanced Safety of Vehicles, Paper No. 94-S4-O-07. 

Washington, DC: National Highway Traffic Safety Administration.

19 Kahane (1994). 

20 Ibid., pp. 57-62. 21 Ibid., p. 65.
 22 Ibid., pp. 63-68. 

8
 
 

 

 

 

 

But fatal run-off-road crashes (rollovers and impacts with fixed objects) increased by a 

statistically significant 28 percent on all road surfaces, with ABS:23

 Rollover + Dry-Road

FARS 1989-93 Fixed Object Multi-Vehicle Ratio 

Last 2 model years without ABS 431 1,021 .422 

First 2 model years with ABS 463 858 .540 

The increases were about equally large on dry, wet, and snowy/icy roads. In particular, side 

impacts with fixed objects, a crash type typically preceded by loss of directional control, 

increased by a statistically significant 57 percent. The increase in run-off-road crashes nearly 

offset the reductions in pedestrian and wet-road multi-vehicle crashes, resulting in a near-zero 

net effect on overall fatal-crash risk.24 

The report’s analysis of nonfatal crashes is based on Florida, Missouri, and Pennsylvania data 

from 1990 to 1992.25 Here, the control group consisted of cars struck while standing still or 

moving 5 mph or less. On wet roads, cars with ABS experienced a statistically significant, 28 

percent reduction of frontal impacts, relative to the control group and relative to cars of the same 

makes and models without ABS.26 For all road conditions (wet, dry, snowy, icy), the overall 

reduction in multi-vehicle frontal impacts with ABS was a statistically significant 9 percent. 

Nonfatal run-off-road crashes, however, increased rather than decreased with ABS. Relative to 

the control group, rollovers and fixed-object impacts increased overall by a statistically 

significant 19 percent in the ABS-equipped cars.27 

In May 1995, NHTSA published a second statistical analysis of ABS in passenger cars, based on 

data from 1989 to 1993 from FARS and four States, using logistic regression rather than 

contingency-table analyses. It confirmed the statistically significant increases of rollovers and 

side impacts with fixed objects in the ABS-equipped cars, in both fatal and nonfatal crashes. It 

also confirmed the benefits of ABS in fatal and nonfatal multi-vehicle crashes on wet roads.28 

Furthermore, NHTSA’s initial analysis of four-wheel ABS in LTVs showed similar increases 

(although not statistically significant) of rollovers and side impacts with fixed objects in the 

LTVs with the four-wheel ABS.29 

In 1996, the Insurance Institute for Highway Safety issued a study examining fatal crash 

involvements from CY 1993 to 1995 of selected vehicles for the model year prior to the 

23 Ibid., p. 95.
 24 Ibid., pp. 115-117. 25 Ibid., pp. 10-57. 26 Ibid., p. 53.
 27 Ibid., p. 84-92.
 28 Hertz, E., Hilton, J., & Johnson, D. M. (1995b). An Analysis of the Crash Experience of Passenger Cars Equipped
 

with Antilock Braking Systems. NHTSA Technical Report. DOT HS 808 279. Washington, DC: National Highway 

Traffic Safety Administration.

29 Hertz (1995a). 

9
 
 

introduction of ABS and the model year after ABS became standard equipment.30 IIHS observed 

a 28-percent increase in run-off-road crashes with ABS, identical to the increase observed by 

NHTSA. IIHS also found that vehicles equipped with ABS were more likely to be involved in 

crashes fatal to their own occupants (particularly single-vehicle crash involvements) but less 

likely to be involved in crashes fatal to occupants of non-ABS equipped vehicles or nonoccupants (pedestrians, bicyclists). Overall, ABS had little effect on fatal crash involvements. 

1.5 The safety community’s response 

The various initial effectiveness analyses seemed to agree that: 

	 The overall effect of ABS on fatal crash involvements was close to zero. 

	 Vehicles with four-wheel ABS had significantly higher rates of fatal run-off-road crashes 

than vehicles without ABS. In fact, the overall effect netted out to zero only because this 

increase was offset by a reduction in collisions with other vehicles on wet roads. 

These fairly strong statistical results did not square with intuition. The behavior of ABS on the 

test track did not provide any obvious reason that run-off-road crashes should increase; if 

anything, they suggested there ought to be a benefit. 

With the coordination of NHTSA’s Motor Vehicle Safety Research Advisory Committee, 

government, industry, and the safety community discussed possible reasons for the observed 

increases in run-off-road crashes. These were the principal hypotheses: 

	 Driver inexperience/lack of knowledge about ABS. Drivers might remove their foot 

from the pedal in response to the [unanticipated] noise and vibration of ABS, or try to 

pump the pedal as with conventional brakes. 

	 A misperception of how much ABS reduces stopping distances or enhances control. 

Some drivers may negotiate curves or change lanes more aggressively because they 

believe ABS will enable them to stop in a shorter distance or retain control of their 

vehicle in extreme driving maneuvers. 

	 Longer stopping distances with ABS on the loose surfaces that vehicles encounter after 

they leave the road. 

	 The enhanced steering control while braking with ABS could allow unsafe panic steering 

maneuvers. An inexpert driver in a panic situation might try an abrupt evasive steering 

maneuver while slamming on the brakes. Without ABS, the front wheels lock; the car 

goes straight ahead, essentially ignoring the steering input. With ABS, the vehicle 

responds to the abrupt, instinctive steering input, possibly running off the road and badly 

out of control. 

	 Possible flaws in the performance of the early ABS technologies in certain maneuvers or 

on some roadway surfaces. 

30 Farmer, C. M., Lund, A. K., Trempel, R. E., & Braver, E. R. (1996). Fatal Crashes of Passenger Vehicles Before 

and After Adding Antilock Brake Systems. Arlington, VA: Insurance Institute for Highway Safety. 

10
 
 

	 The increase in run-off-road crashes was not due to ABS. It was due to concurrent 

changes in some makes and models that were equipped with ABS, such as increased 

horsepower or a sportier image. 

NHTSA undertook a multiyear research program to test out these and other hypotheses.31 The 

activities included: 

	 In-depth reviews of cases involving ABS-equipped cars that ran off the road. 

	 Interviews with drivers who complained to the NHTSA's Auto Safety Hotline about the 

performance of their ABS systems. 

	 Discussions with crash investigators and police officers who drive ABS-equipped 

vehicles or who investigate crashes involving ABS-equipped vehicles. 

	 A study, conducted on the driving simulator, of how average drivers respond to imminent 

crash threats. 

	 A “naturalistic” study that tracked the maneuvers of average drivers over extended time 

periods. 

	 Combined braking-and-steering tests to establish the range of maneuvers that can be 

successfully executed without a loss of directional control. 

The research, in fact, did not identify any significant problems with ABS other than owners’ 

initial lack of knowledge and experience with the systems. There was little evidence of any 

behaviors that would cause drivers of vehicles equipped with ABS to run off the road. There 

was also little evidence that drivers became more aggressive when they had ABS. 

Beginning in 1995, NHTSA, the manufacturers, ABS suppliers (who had formed an ABS 

Education Alliance32), and the insurance industry developed media to educate vehicle owners 

about how ABS responds when activated, and how to use it properly. Some of the main points 

were: 

	 Do not pump the brake pedal in cars equipped with ABS. This can defeat the purpose of 

ABS and may reduce braking capability. Don’t let up on the brakes. Stomp, stay, and 

steer. 

	 The ABS system can make noise and vibrate the brake pedal when it is working. Drivers 

should not take their foot off the brake pedal when they hear noise or feel pedal vibration. 

	 Drivers should be aware that extreme steering maneuvers executed while using ABS 

brakes could steer the car off the road. 

	 ABS can significantly lengthen stopping distances on loose surfaces such as gravel or 

soft snow. Drivers should slow down and allow extra distance between vehicles under 

those conditions. 

31 Kahane (1994), pp. v-viii. 

32 http://www.abs-education.org. 

11
 
 

 

	 Many drivers think the main purpose of ABS is to reduce stopping distances. This is a 

serious misconception. ABS will only reduce stopping distances significantly in some 

special road conditions, but may increase distances in others. 

	 The principal goals of ABS are to prevent skidding and loss-of-control due to lockedwheel braking and to allow a driver to steer the vehicle during hard braking. 

	 If a driver makes a car skid for reasons other than braking, such as going around a curve 

too quickly, ABS will not prevent or relieve the skid. 

	 Drivers of cars equipped with ABS must maintain the same distance behind vehicles they 

follow that they would have kept without ABS. They should not expect to stop more 

quickly because they have ABS. 

	 Drivers of cars equipped with ABS should not drive around curves, change lanes, or 

perform other steering maneuvers any faster or more aggressively than they would have 

without ABS. They should not expect ABS to improve their control in these maneuvers. 

1.6 The second generation of effectiveness analyses 

By the late 1990s, NHTSA and the safety community were satisfied that consumers understood 

how to use ABS properly and had become familiar with ABS. None of the other hypotheses for 

the increase in run-off-road crashes was confirmed by empirical evidence. It was time to analyze 

new data and see if the effect was still there. Three reports published in 2000-02 suggest that the 

increase in run-off-road crashes may have been largely or entirely temporary. 

NHTSA’s follow-up analysis in 2000 used the same logistic regression models as its 1995 

reports, but applied them to FARS and State data from calendar years 1995 and 1996 rather than 

1989 to 1993.33 Makes and models included those with standard ABS considered in the previous 

studies, supplemented by models that offered optional ABS. The observed effects of ABS in 

passenger-car rollovers and side impacts with fixed objects were still negative, but less than half 

as large as in the 1995 study. They were no longer statistically significant. The observed effect 

in frontal impacts with fixed objects had crossed from a non-significant negative to a nonsignificant positive. There were non-significant reductions in collisions with pedestrians and in 

frontal collisions with other vehicles. Four-wheel ABS did not have a statistically significant 

effect on any type of fatal crash, except for an increase in LTV rollovers. Several types of 

nonfatal crash involvements were significantly reduced with ABS: rollovers, frontal impacts with 

fixed objects, and frontal impacts into other vehicles. The report did not estimate the overall 

effect of ABS on fatal or on nonfatal crashes. 

The IIHS follow-up study of 2001 compared the crash experience of models in the first model 

year after receiving four-wheel ABS as standard equipment to the experience of the same models 

in the last model year before ABS.34 Whereas run-off-road crashes increased significantly with 

33 Hertz, E. (2000). Analysis of the Crash Experience of Vehicles Equipped with All Wheel Antilock 

Braking Systems (ABS) – A Second Update Including Vehicles with Optional ABS, NHTSA Technical Report. DOT 

HS 809 144. Washington, DC: National Highway Traffic Safety Administration.. 

34 Farmer, C. M. (2001). New Evidence Concerning Fatal Crashes of Passenger Vehicles Before and After Adding 

Antilock Braking Systems, Accident Analysis and Prevention, Vol. 33, pp. 361-369. 

12
 
 

ABS during CY 1993 to 1995 these same MY models no longer experienced a significant 

increase in run-off-road crashes during CY 1996 to 1998. However, this study did not find a 

statistically significant overall effect for ABS on fatal crashes in 1996-98; it did not analyze 

nonfatal crashes. 

Harless and Hoffer confirmed the IIHS results and, moreover, presented evidence that the 

increase in run-off-road crashes during the CY 1993 to 1995 was virtually confined to drinking 

drivers.35 It was especially prevalent in young, drinking drivers or in drinking drivers with a 

history of high-risk driving behavior. Fatal crash involvements of drinking drivers of ABSequipped vehicles were 64 percent higher than expected during 1993 to 1995, given the number 

of fatal crash involvements of drinking drivers of non-ABS vehicles of the same makes and 

models. The number of involvements of non-drinking drivers of ABS-equipped vehicles was 11 

percent lower than expected during those years. But in CY 1996 to 1998, there was no observed 

increase in crashes with ABS even for the drinking drivers. Harless and Hoffer suggested that 

drinking drivers were at first especially unaware and unable to use ABS properly, but eventually 

improved at least their knowledge of ABS, if not their driving skills. 

These studies apparently satisfied the safety community that the initial increase in run-off-road 

crashes with ABS, if it was “real” at all, must have been largely due to owners’ inexperience and 

lack of knowledge about the systems. Thanks to the efforts to educate the public about ABS, and 

as owners simply accumulated years of experience with their vehicles, the increase in run-offroad crashes ostensibly faded away. Subsequent improvements in ABS technology perhaps also 

helped, such as better response and feedback to the driver or the “brake assist” feature on some 

vehicles, which amplifies input from the driver. 

The consensus appeared to be that no further regulatory action was needed: 

	 Because there was no significant overall fatality reduction, there was little motivation to 

reverse the July 12, 1996, decision to shelve any ABS requirement. 

	 Because the adverse effect on run-off-road crashes was becoming a thing of the past, 

there was no need to consider regulation addressing that issue. 

	 To the extent that ABS was technology that many customers desired because it enhanced 

vehicle performance – even if it had no quantifiable safety benefits, and if it did help 

reduce nonfatal crashes, all the better – it was satisfactory to just let market forces 

establish the demand for ABS. 

However, a possible flaw in the logic is that the observed effects on run-off-road crashes were 

sill negative. They were just no longer statistically significant – in part, because they were based 

on relatively small numbers of cases. One study was based on two calendar years of data (1995 

to 1996); the others were based on three years (1996 to 1998), but they limited the data to the last 

model year before ABS and the first model year with ABS. While the observed effects were not 

as negative as in the earlier studies, there was still a fair amount of uncertainty about them. 

35 Harless, D. W., & Hoffer, G. E. (2002). The Antilock Braking System Anomaly: A Drinking Driver Problem? 

Accident Analysis and Prevention, Vol. 34, pp. 333-341. 

13
 
 

 

Another flaw is that none of the analyses adjusted for the effect of frontal air bags. It so 

happened that many models of cars and LTVs were equipped with frontal air bags in the same 

model year or within a year of when they received four-wheel ABS. Air bags save lives in 

frontal impacts, including frontal run-off-road crashes. Unless the FARS analyses adjust in some 

way for the effect of air bags, they spuriously attribute a reduction of frontal fatal crash 

involvements to ABS, when that reduction is in fact due to the air bags changing the crashes 

from fatal to nonfatal. 

The logic could also be flawed under the “best case” assumption: that the negative effect on fatal 

run-off-road crashes had really ended. If so, wouldn’t the positive effects in other types of fatal 

and nonfatal crashes still be there, resulting in a potentially significant net benefit for ABS? 

1.7 Developments since 2002 

Electronic stability control is surely the most exciting development in crash-avoidance 

technology during the past decade. ESC monitors the speed of each wheel, the steering wheel 

angle, the overall yaw rate, and the lateral acceleration of the vehicle. It compares a driver’s 

intended course with the vehicle’s actual movement and detects when a driver is about to lose 

control. It intervenes in split seconds by automatically applying the brakes to any one or to 

several individual wheels – or by reducing engine torque – to provide stability and help the 

driver stay on course. It may then slow down the vehicle to a speed more appropriate for 

conditions. ESC can prevent an impending crash even before a driver is aware of the danger. It 

can activate even if a driver does nothing, unlike ABS, which cannot activate until a driver has 

applied the brakes. 

Mercedes-Benz first offered ESC on selected cars in 1997; Cadillac and BMW in 1998. By 2000 

it was standard on a fair number of luxury cars. Among SUVs, ESC was standard on Mercedes 

in 1999 and on selected Toyota and Lexus models in 2000 or 2001. By 2004, vehicles had 

accumulated enough on-the-road experience for preliminary statistical analyses of crash data, 

indicating large reductions of fatal rollovers and other single-vehicle crashes.36 The Safe, 

Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users of 2005 

(SAFETEA-LU), Section 10301 asked NHTSA to publish, by October 1, 2006, a Notice of 

Proposed Rulemaking on stability-enhancing technologies and a final rule by April 1, 2009.37 

The agency published a NPRM on September 18, 2006, and a final rule on April 6, 2007, 

establishing FMVSS No. 126, which will require ESC on all new cars and LTVs by September 

1, 2011, with a phase-in period beginning September 1, 2008.38 NHTSA’s latest statistical 

analysis (2007) found that ESC significantly reduces fatal run-off-road crashes by an estimated 

36 percent in passenger cars and by 70 percent in LTVs, relative to vehicles of the same makes 

and models equipped with four-wheel ABS but not ESC.39 In MY 2007, two model years before 

36 Dang, J. N. (2004). Preliminary Results Analyzing the Effectiveness of Electronic Stability Control (ESC)
 

Systems., NHTSA Evaluation Note. DOT HS 809 790. Washington, DC: National Highway Traffic Safety
 

Administration.. 

37 Public Law 109-59, August 10, 2005. 38 Federal Register 71. September 18, 2006: 54712; 72, April 6, 2007: 17310. 39 Dang (2007), p. vii.
 

14
 
 

 

 

the beginning of the phase-in period, 22 percent of the new cars and 53 percent of the new LTVs 

sold in the United States were already equipped with ESC.40 

All ESC-equipped vehicles to date are also equipped with four-wheel ABS. It is not merely a 

marketing issue (ESC initially being offered on the more expensive models that have long been 

ABS-equipped), but an intrinsic tie between the technologies. The functional capabilities 

inherent to ABS – namely, monitoring the speed of each wheel and automatically modulating the 

brake at individual wheels – are a subset of the functional capabilities of ESC. To date, ESC 

hardware has included within it the components needed to perform ABS functions. For the 

foreseeable future, FMVSS No. 126 will be a regulation that de facto puts ABS into all new cars 

and LTVs. 

1.8 Goals of the evaluation 

It is time for a new evaluation of ABS, as discussed in NHTSA’s 2008-2012 evaluation plan.41 

The agency evaluates the effectiveness of its existing regulations, as required by The 

Government Performance and Results Act of 199342 and Executive Order 1286643 and it also 

evaluates the effectiveness of major safety technologies that are available in production vehicles, 

even if not required by a regulation. ABS is certainly a major technology, and it will soon be in 

all new vehicles, even if it is not explicitly mandated by a regulation. Moreover, NHTSA may 

re-evaluate a technology if its effectiveness is suspected of changing over time, and will do so 

until the long-term effect is identified.44 

As of April 2009, crash data are available through CY 2007. Databases are now many times 

larger than the ones available for the first- and second-generation effectiveness studies. Main 

estimates will be statistically significant unless they are within a few percentage points of zero. 

The databases will not get much larger in the future, because the vehicles of the early 1990s that 

play a critical role in the analyses have been retired or are approaching the end of their service 

lives. 

The principal evaluation questions are: 

	 What is the long-term overall effect of ABS on fatal crashes? 

	 What is the long-term effect of four-wheel ABS on fatal run-off-road crashes? 

o	 If it is negative, how does it compare to the effect of ESC? FMVSS No. 126 will 

require all vehicles to be equipped with ESC, and it is almost certain they will 

40 Ward’s Automotive Yearbook 2008. 41 Allen, K., Dang, J. N., Doyle, C. T., Kahane, C. J., Roth, J. R., & Walz, M. C. (2008). Evaluation Program Plan, 

2008-2012 pp. i and 9. NHTSA Technical Report. DOT HS 810 983. Washington, DC: National Highway Traffic 

Safety Administration. 

42 Public Law 103-62, August 3, 1993. 43 Federal Register 58, October 4, 1993. 44 Kahane, C.J., & Hertz, E. (1998). The Long-Term Effectiveness of Center High Mounted Stop Lamps in Passenger 

Cars and Light Trucks. NHTSA Technical Report. DOT HS 808 696. Washington, DC: National Highway Traffic 

Safety Administration. 

15
 
 

also be equipped with ABS. How does the risk of fatal run-off-road crashes of 

vehicles with ESC and ABS compare to the risk in vehicles with neither? 

o	 If the effect of ABS on fatal run-off-road crashes is close to zero, does ABS now 

have enough positive effects in other types of crashes to result in a significant 

overall fatality reduction? 

	 What is the overall effect of ABS on nonfatal crashes? Even if the net effect of ABS 

on fatal crashes is close to zero, does ABS prevent enough nonfatal injuries and 

property damage to endorse ABS technology for its safety benefits? 

16
 
 

 

 

CHAPTER 2 

EFFECT OF ABS IN FATAL CRASHES: ANALYSES OF 1995-2007 FARS DATA 

2.0 Summary 

ABS has close to a zero net effect on fatal crash involvements. The observed net effects are a 1-

percent reduction of fatal crashes for passenger cars and a 1-percent increase for LTVs. Neither 

is statistically significant. But ABS is not without effect. Run-off-road crashes significantly 

increase while collisions with pedestrians are significantly reduced, as are collisions with other 

vehicles on wet roads. However, the mix of these collision types among fatal crashes is such that 

the added harm and the benefits cancel each other. 

2.1 Analysis for passenger cars 

2.1.1 FARS calendar-year range 

FARS is a census of fatal crashes in the United States since 1975. As of April 2009, the FARS 

database is complete through calendar year 2007. Because the goal is to study the “long-term” 

effect of ABS and not its “initial” effects, only calendar years 1995-2007 are included in the 

analysis. CY 1995 has been selected as the starting point for the data because it was also the 

starting point in Hertz’s 2000 report, the first to indicate that the initially deleterious effect of 

ABS on run-off-road crashes had become non-significant.45 Furthermore, statistical analyses46 

indicating relative increases in run-off-road crashes with ABS had appeared in 1994 and by 1995 

the industry was responding with consumer information on the correct use of ABS. 

The basic analysis approach is to count the number of fatal crash involvements of various types 

that might conceivably be influenced by ABS and compute their ratios to the number of control 

group involvements that are unlikely to be affected by ABS. The ratios are compared for cars 

with ABS and cars of the same makes and models without ABS – in the same calendar years. 

2.1.2 Control-group involvements versus response group 

The control group and the response group are similar to the ones in NHTSA’s evaluation of 

electronic stability control.47 An ideal control group would be cars that had been stopped or 

parked before they were hit by another vehicle. Obviously, ABS in those cars could not have 

made a difference if they were standing still even before the sequence of events that resulted in a 

collision. But there are too few fatal crashes involving stationary cars to obtain statistically 

meaningful results from analyses employing this control group. It is necessary to supplement the 

control group with other crash types where the possible influence of ABS is quite small, even if 

45 Hertz, E. (2000). Analysis of the Crash Experience of Vehicles Equipped with All Wheel Antilock Braking Systems 

(ABS) – A Second Update Including Vehicles with Optional ABS. NHTSA Technical Report. DOT HS 809 144. 

Washington, DC: National Highway Traffic Safety Administration.

46 Kullgren, A., Lie, A., & Tingvall, C. (1994). The Effectiveness of ABS in Real Life Accidents, Proceedings of the
 

14th International Technical Conference on the Enhanced Safety of Vehicles, pp. 1568-73. Washington, DC: 

National Highway Traffic Safety Administration,; Kahane (1994). 

47 Dang (2007). 

17
 
it cannot be completely ruled out. Involvements of cars that were moving less than 10 mph, 

backing up, parking, or leaving a parking space may be included, because ABS is unlikely to 

affect braking performance substantially at those low speeds. Cars struck in the rear while 

moving on any surface and cars involved as non-culpable parties in collisions with other vehicles 

on dry surfaces are also included. FARS does not provide detailed information on the braking 

performance of these cars. But in all likelihood these are not cars that hit somebody else because 

they went out of control during braking or could not stop in time (two things that ABS might 

prevent, depending on the surface), for in most cases they were simply proceeding as intended 

and did nothing to precipitate a collision. 

The FARS codes for control group crashes include the following involvements in collisions with 

other vehicles (VE_FORMS ≥ 2): 

	 Hit while stopped/parked: VEH_MAN = 4, 7 or TRAV_SP = 0 

	 Backing/parking/low-speed: VEH_MAN = 3, 6, 8, 15 or TRAV_SP = 1-10 

	 Struck in rear: IMPACT1 = 5, 6, 7 

	 Non-culpable involvements on dry roads: SUR_COND = 1; none of the “driver 

contributing factors” DR_CF1, DR_CF2, DR_CF3, or DR_CF4 has a value that suggests 

this driver was culpable, namely codes 3, 6, 8, 26, 27, 28, 30, 31, 33, 35, 36, 38, 39, 44, 

46, 47, 48, 50, 51, 57, 58, 79, or 87 

All crash involvements that are not in the control group are in the response group. On roads that 

are not dry, even non-culpable involvements can be in the response group, because ABS is likely 

to benefit drivers in many situations and we do not want to “miss” a possible benefit just because 

FARS classified a driver as non-culpable. In other words, on roads that are not dry, the control 

group is limited to the first three of the four categories listed above. Effectiveness of ABS 

(percent crash reduction relative to the control group) is computed for the response group as a 

whole, and also for the following subgroups – on all roads, and on roads that are not dry: 

	 Run-off-road crashes: VE_FORMS = 1, but excluding HARM_EV = 2, 5, 8, 9, 10, 11, 

14, 16, or 49 (collisions with non-motorists, trains, or parked cars, plus various noncollisions that usually happen on, not off the road) 

o	 First-event rollovers: ROLLOVER = 1 or HARM_EV = 1 

o	 Side impacts with fixed objects: IMPACT1 = 2, 3, 4, 8, 9, or 10 

o	 All other run-off-road crashes 

	 Collisions with pedestrians, bicyclists, other non-motorists, or animals: VE_FORMS = 1 

and HARM_EV = 8, 9, 11 or 49 

	 Culpable involvements in collisions with other vehicles, including any frontal impact into 

the rear of another vehicle: at least one of the “driver contributing factors” DR_CF1, 

DR_CF2, DR_CF3, or DR_CF4 has a value that suggests this driver was culpable, 

namely codes 3, 6, 8, 26, 27, 28, 30, 31, 33, 35, 36, 38, 39, 44, 46, 47, 48, 50, 51, 57, 58, 

79, or 87; or MAN_COLL = 1 and IMPACT1 = 1, 11, or 12 

18
 
 

 

2.1.3 Cars without ABS versus cars with ABS 

ABS can be standard equipment on a make and model in a given model year, or an option, or not 

available at all – or it may be standard on some sub-series of that model, optional on others, and 

unavailable on yet others. For the “cleanest” analysis, the models included should ideally change 

from not offering ABS at all in one model year to having it as standard equipment the next year. 

However, not that many models received ABS that way. Many of those that did were General 

Motors cars, not a representative cross-section of the entire fleet. Therefore, makes and models 

will also be included even if they switched from something more than 0 percent ABS and/or to 

something less than 100 percent ABS, as long as the percentage-point gain in ABS is substantial. 

The analyses will include an adjustment factor to translate the observed effect to the 

effectiveness of changing from 0 to 100 percent ABS. 

Ward’s Automotive Yearbooks began in MY 1986 to specify the percentage of cars of a make, 

model, and model year equipped with ABS. The few installations before 1986 are listed in 

NHTSA’s 1994 evaluation report (based on information furnished by the manufacturers or from 

news articles).48 

The cars without ABS should be as similar as possible to the cars with ABS, so as to avoid biases 

due to different types of driving or exposure. They should be the same makes and models. They 

should be of similar vehicle age. Their crash experience should be from the same calendar years. 

Those goals are accomplished by limiting the database to specific makes and models that shifted 

from a low ABS installation rate in one year to a high rate in the next year, or vice-versa – and 

further limiting the data to the last two model years before the shift versus the first two model 

years after the shift. However, a fair number of models had a “transitional” model year between 

low and high ABS-installation rates. So as not to lose these models from the analysis, they are 

included, skipping the transition year and keeping the two years before and after it. 

Table 2-1 lists the 60 models contributing data to the analyses, the range of model years included 

in each case, and the proportion of cars equipped with ABS before and after the shift. The 

identification of makes and models in the FARS data is based on decoding the VIN. Since 1991, 

NHTSA staff has maintained a series of VIN analysis programs for use in evaluations. The 

programs are available to the public. 

A unique advantage of performing the evaluation in 2009 rather than 10 years earlier is that a fair 

number of models recently shifted from high to low installation rates – either by dropping 

standard ABS and making it optional, or because fewer consumers purchased the option. These 

models are shown in red on Table 2-1. They help “balance” the analysis because, for these 

models, the newer cars are the ones with less ABS. The analysis actually captures some highsales GM models twice: when ABS became standard in the 1990s and when it became an option 

after 2000. 

48 Ward’s Automotive Yearbooks,; Kahane (1994), Appendix A. 

19
 
 

TABLE 2-1: PASSENGER CARS RECEIVING OR LOSING ABS 

MAKES, MODELS AND MODEL YEARS INCLUDED IN THE ANALYSIS 

(Bold model names: 1 model year gap between “pre” and “post” or elsewhere) 

Limited or Standard or 

No ABS High % ABS 

% % Pct Pt 

Make and Model MY ABS MY ABS Change 

Chrysler Sebring Coupe 1998-99 28% 1995-96 100% 72% 

Chrysler Cirrus/Sebring Sedan 2001-02 22% 1998-99 100% 78% 

Dodge Avenger 1998-99 12% 1995-96 58% 46% 

Ford Crown Victoria 1990-91 0% 1993-94 44% 44% 

Lincoln Town Car 1988-89 0% 1991-92 100% 100% 

Lincoln Continental 1983-84 0% 1986-87 100% 100% 

Mercury Grand Marquis 1990-91 0% 1993-94 50% 50% 

Mercury Sable 1991-92 23% 1994-95 79% 56% 

Buick LeSabre 1990-91 3% 1993-94 100% 97% 

Buick Estate/Roadmaster wagon 1989-90 0% 1991-92 100% 100% 

Buick Electra 1989-90 12% 1991-92 100% 88% 

Buick Riviera 1989-90 17% 1991-92 100% 83% 

Buick Century 1992-93 0% 1994-95 100% 100% 

Buick Century 2003-04 32% 2001-02 100% 68% 

Buick Skylark 1990-91 0% 1992-93 100% 100% 

Buick Regal 1990-91 4% 1993-94 79% 75% 

Cadillac DeVille excl Fleetwood 1989-90 0% 1991-92 100% 100% 

Cadillac Fleetwood FWD 1987-88 0% 1989-90 100% 100% 

Cadillac Fleetwood Brougham 1988-89 0% 1991-92 100% 100% 

Cadillac Eldorado 1988-89 13% 1991-92 100% 87% 

Chevrolet Caprice Sedan 1989-90 0% 1991-92 100% 100% 

Chevrolet Caprice wagon 1989-90 0% 1991-92 100% 100% 

Chevrolet Corvette 1984-85 0% 1986-87 100% 100% 

Chevrolet Cavalier 1990-91 0% 1992-93 100% 100% 

Chevrolet Cavalier 2003-04 34% 2001-02 100% 66% 

Chevrolet Corsica/Beretta 1990-91 0% 1992-93 100% 100% 

Chevrolet Lumina 1990-91 0% 1992-93 62% 62% 

Chevrolet Malibu/Classic 2004-05 13% 2001-02 100% 87% 

Oldsmobile Delta 88 1990-91 5% 1993-94 100% 95% 

Oldsmobile Custom Cruiser 1989-90 0% 1991-92 100% 100% 

Oldsmobile 98 1989-90 9% 1991-92 100% 91% 

Oldsmobile Toronado 1988, 1990 11% 1991-92 100% 89% 

Oldsmobile Ciera 1992-93 0% 1994-95 100% 100% 

Oldsmobile Cutlass Supreme 1991-92 17% 1994-95 100% 83% 

Oldsmobile Calais/Achieva 1990-91 1% 1992-93 100% 99% 

Oldsmobile Alero 2003-04 25% 2001-02 100% 75% 

20
 
 

TABLE 2-1 (continued): PASSENGER CARS RECEIVING OR LOSING ABS 

MAKES, MODELS AND MODEL YEARS INCLUDED IN THE ANALYSIS 

(Bold model names: 1 model year gap between “pre” and “post” or elsewhere) 

Limited or Standard or 

No ABS High % ABS 

% % Pct Pt 

Make and Model MY ABS MY ABS Change 

Pontiac Sunbird 1990-91 0% 1992-93 100% 100% 

Pontiac Sunfire 2003-04 20% 2001-02 100% 80% 

Pontiac Grand Am 1990-91 1% 1992-93 100% 99% 

Pontiac Grand Am 2004-05 27% 2001-02 100% 73% 

Saturn L sedan 2001-02 21% 2004-05 100% 79% 

VW Jetta 1997-98 12% 1999-00* 100% 88% 

VW Golf 1997-98 33% 1999-00* 100% 67% 

VW Passat 1993-94 18% 1995-96 78% 60% 

BMW 300 1984-85 0% 1986-87 100% 100% 

BMW 500 1983-84 0% 1986-87 100% 100% 

BMW 600 1983-84 0% 1985-86 100% 100% 

BMW 700 1983-84 0% 1985-86 100% 100% 

Nissan Maxima 1998-99 35% 2001-02 100% 65% 

Honda Accord 1990-91 3% 1993-94 49% 46% 

Jaguar XJ sedan 1986-87 0% 1988-89 100% 100% 

Jaguar XJ-S coupe 1987-88 0% 1989-90 100% 100% 

Mercedes basic sedan 1984-85 0% 1986-87 100% 100% 

Mercedes SDL/SEL/SEC/SD/SE 1983-84 0% 1985-86 100% 100% 

Saab 900 1988-89 0% 1990-91 100% 100% 

Saab 9000 1986-87 0% 1988-89 100% 100% 

Subaru Legacy/Outback 1994-95 35% 1996-97 95% 60% 

Toyota Camry 1991-92 17% 1994-95 82% 65% 

Toyota Avalon 1995-96 26% 1997-98 100% 74% 

Volvo 240 1989-90 0% 1991-92 100% 100% 

Grand Average 7% 88% 81% 

*After redesign 

21
 
 

 

As stated above, crash experience for the low- and high-ABS cars should be from the same 

calendar years. Many of the models in Table 2-1 have model-year ranges that end before 1995, 

in some cases well before 1995. For those models, all FARS cases from CY 1995 to 2007 are 

included. For the other models, FARS data are included beginning only with the last model year 

in the range. For example, the model year ranges for Chrysler Sebring Coupe are 1995 to 1996 

and 1998 to 1999; only FARS data from calendar years 1999 to 2007 are included, because those 

are the only calendar years where the full model-year range of vehicles is represented. 

Three sporty models with high sales – Chevrolet Camaro, Pontiac Firebird, and Ford Mustang – 

are omitted from the study. Their drivers have high rates of run-off-road crashes. That might 

confound the analyses of the possible effect of ABS on those crashes. 

For the 60 makes and models combined, 6.8 percent of the cars in the low-ABS model-year 

range were equipped with ABS; 88.2 percent of the cars in the high-ABS model years had ABS. 

That is an 81.4 percentage-point gain in the share of cars equipped with ABS. These percentages 

are a weighted average of Ward’s percent of ABS installation for each make, model, and model 

year. The weight factors are the sums of registration years for each make, model, and model 

year over the calendar years that FARS data are included in the study. Registration years are 

based on R.L. Polk’s National Vehicle Population Profile. 

The analysis file includes 38,251 FARS cases of cars involved in crashes, ample data for 

statistical analyses. By contrast, NHTSA’s 1994 evaluation was based on 3,703 FARS cases. 

2.1.4 Adjusting the case counts for air-bag effectiveness 

The great influx of ABS in high-sales makes and models was concentrated in the early 1990s, as 

shown in Table 2-1. It so happened that most cars also received frontal air bags in the early 

1990s, often in the same year as ABS. Among the 60 makes and models included in Table 2-1: 

	 During the low-ABS model years, 73 percent of the cars had no air bags, 15 percent 

driver-only air bags, and 12 percent dual air bags. 

	 Whereas during the high-ABS model years, 28 percent had no air bags, 35 percent driveronly, and 37 percent dual. 

Air bags save lives in frontal impacts. If the analysis failed to adjust for the effect of air bags, it 

would spuriously attribute a reduction of frontal fatal crash involvements to ABS, when that 

reduction is in fact due to the air bags changing the crashes from fatal to nonfatal. NHTSA’s 

statistical analyses of FARS estimate that frontal air bags reduce fatality risk in frontal crashes, 

but substantially more so when the principal impact point, IMPACT2 = 12 (front-center or frontdistributed) than when it is 11 or 1 (front-corner). Air bags are also slightly more effective for 

adult passengers than for drivers, and for unbelted than for belted occupants.49 

49 Kahane (2004), pp. 108-113 and 311-312. 

22
 
Estimated Fatality Reduction (%) By Air Bags in Frontals 

Belted Occupants Unbelted Occupants 

IMPACT2 = 12 

Drivers 25 33 

RF passengers age 13+ 28 36 

IMPACT2 = 11 or 1 

Drivers 13 17 

RF passengers age 13+ 15 19 

FARS case counts in the analyses will be weighted upward, when appropriate, by dividing by 

one minus fatality reduction. For example, in a crash involvement with IMPACT2 = 12 in which 

the belted driver of the case vehicle is the only fatality in the entire crash (FATALS = 1), that 

case receives a weight factor of 1/(1 - .25) = 1.333. The rationale is that if 1,000 such cases 

actually existed on FARS, there would have been 1,333 such cases if the cars had not been 

equipped with air bags, because the air bags saved 333 lives (25% of 1,333). As a result those 

crash involvements became nonfatal and never appeared on FARS. But they would have 

appeared on FARS if those cars had been equipped only with ABS and not air bags. 

Cases are not weighted upward – i.e., have a weight factor of just 1.000 when: 

	 The case car is not equipped with air bags 

	 The crash is fatal to anybody other than the driver and right-front passenger of the case 

car – e.g., a back-seat occupant, a pedestrian, or an occupant of another vehicle – because 

that crash would continue to be in FARS whether or not the air bags saved the driver or 

right-front passenger 

	 The crash is fatal only to the right-front passenger, but the car only has a driver air bag, or 

the passenger is less than 13 years old 

	 IMPACT2 is not 1, 11, or 12, or 

	 The crash is a first-event rollover or other non-collision (HARM_EV = 1-6) 

The vast majority of cases have a weight factor of 1.000. Overall, the 38,251 actual FARS cases 

amount to a weighted total of 39,625. The case “counts” in the basic analysis of the overall 

effect of ABS will add up to 39,625. 

One downside of weighting the cases is that chi-square (χ

2

) statistics for contingency tables of 

weighted counts cannot be given their customary interpretation of significance. That would have 

required the tables to be filled with unweighted data derived from a simple random sample. The 

next section explains the procedures for measuring sampling error and testing significance. 

23
 
 

2.1.5 Results 

Table 2-2 shows the actual and weighted case counts for each analysis of the effect of ABS in 

passenger cars, the effectiveness estimates, the significance-test results and the confidence 

bounds. Here is how the statistics are derived for the basic estimate of the effect of ABS in all 

non-control-group crashes on all roads. 

The 2x2 contingency table of actual case counts for the entire database is: 

Cars with Cars with 

7% ABS 88% ABS 

Control group involvements 5,404 5,654 

All non-control-group involvements 13,478 13,715 

The observed effect is: 

1 – [(13,715/5,654) / (13,478/5,404)] = 2.74 percent reduction 

However, the calculation based on actual case counts exaggerates the benefits of ABS because it 

does not adjust for the effect of air bags being introduced more or less simultaneously with ABS. 

The 2x2 table of case counts weighted upwards for the effect of air bags is: 

Cars with Cars with 

7% ABS 88% ABS 

Control group involvements 5,467 5,808 

All non-control-group involvements 13,809 14,542 

Weighting the cases increases each cell count in the contingency table. However, the increase is 

proportionally larger for cars with ABS because they have more air bags. It is also larger for 

response-group involvements, because a larger portion of them is frontal (many control-group 

cases are rear impacts). That shrinks effectiveness. The observed effect is now only: 

1 – [(14,452/5,808) / (13,809/5,467)] = 0.87 percent reduction 

That estimate is rounded to 1 percent and shown in the fifth column of the “all non-control-group 

crashes” row of Table 2-2. But that is the effect of increasing the market share of ABS from 6.8 

percent to 88.2 percent. The effect of increasing all the way from zero ABS to 100 percent ABS 

would be higher: 

{1 – [(14,452/5,808) / (13,809/5,467)]} / {.882 – .068 x [(14,452/5,808) / (13,809/5,467)]} 

= 1.08 percent reduction 

24
 
Table 2-2: Passenger Cars, Distribution of Fatal Crash Involvements with and Without 4-Wheel ABS, FA

Actual FARS Air-Bag- Fatal Crash t 

Case Counts Weighted N Reduction (%) for 

H0

Cars w Cars w Cars w Cars w 88% vs. 100% vs. Red

7% ABS 88% ABS 7% ABS 88% ABS 7% ABS no ABS = 0 

Control group involvements 5,404 5,654 5,467 5,808 ALL ROADS All non-control-group involvements 13,478 13,715 13,809 14,542 1 1 .4

All crash involvements 1 Run-off-road crashes 4,631 5,024 4,784 5,435 -7 -9 2.6

Side impacts w fixed objects 748 978 751 988 -24 -30 4.1

 First-event rollovers 705 815 705 815 -9 -11 1.4

Other run-off-road crashes 3,178 3,231 3,328 3,631 -3 -3 .8

Pedestrian/bicyclist/animal 1,683 1,593 1,684 1,596 11 13 2.9

Culpable involvements w other veh 5,975 5,939 6,130 6,306 3 4 1.6

WET, SNOWY, OR ICY ROADS All non-control-group involvements 3,144 3,273 3,207 3,434 -1 -1 .2

All crash involvements -1 

Run-off-road crashes 738 954 762 1,031 -27 -34 4.9

Side impacts w fixed objects 160 280 161 282 -65 -85 5.2

 First-event rollovers 73 109 73 109 -41 -52 1.9

Other run-off-road crashes 505 565 528 640 -14 -17 2.2

Pedestrian/bicyclist/animal 230 244 231 244 none none .0

Culpable involvements w other veh 1,224 1,141 1,249 1,198 10 12 2.4

25
That estimate, although somewhat higher, still rounds to 1 percent, and it is shown in the sixth 

column of Table 2-2, the column headed by “100% versus no ABS.” It is the basic point 

estimate of the effect of ABS. 

Statistical significance of the fatality reductions cannot be tested with simple 2x2 chi-squares of 

the cell counts, because they are weighted counts, not actual numbers of cases. Some FARS 

cases have been weighted more than 1 to adjust for the effect of air bags. Instead, the FARS data 

are split up into 10 systematic random subsamples. The last digit of the case identification 

number ST_CASE are useful for splitting FARS into systematic random subsamples. Thanks to 

the ample data available, effectiveness can be calculated separately in each subsample and 

significance tested by observing the variation of the estimate across the 10 subsamples. 

However, the variance estimate obtained by running through the procedure just once could be 

too high or too low by chance, depending on what cases happened to get into the 10 subsamples. 

A second iteration of the same procedure, but with FARS split up into subsamples in a different 

way, might generate a lower or higher estimate. Numerous iterations, each with a different 

splitting of FARS into subsamples, will generate a range of variance estimates, and the median 

of these estimates will be used. Specifically, the last two digits of ST_CASE were used to 

subdivide FARS into 100 groups (numbered 0 to 99). The numbers 0 to 99 were randomly reordered by a SAS random-number generator and listed in the new order. The FARS cases whose 

last two ST_CASE digits were among the first 10 on the new list became subsample 1, the next 

10 became subsample 2, and so on. After these 10 subsamples were created, the numbers 0 to 99 

were randomly reordered anew and another set of 10 subsamples was created. In all, the 

procedure was repeated 11 times and it created 11 sets of 10 subsamples each. 

It is easier to work with the effect of increasing the market share of ABS from 6.8 percent to 88.2 

percent than with the effect of increasing all the way from zero ABS to 100 percent ABS. The 

former can be expressed as a log-odds-ratio, which tends to have a normal distribution. If the 

former effect is significant, the latter will be, too. 

Based on the weighted cell counts, the effect of increasing the market share of ABS from 6.8 

percent to 88.2, estimated for the entire FARS database, may be expressed as a log-odds ratio: 

log r = log [(14,452/5,808) / (13,809/5,467)] = -.0088 

On the first of the 11 times that FARS was split into 10 subgroups, subgroup 1 happened to 

consist of cases with ST_CASE ending in 01, 05, 06, 21, 23, 27, 38, 61, 65, or 81. For these 

cases, the corresponding log-odds ratio is: 

log r = log [(1,522/598) / (1,432/566)] = +.0058 

For subgroup 2, on the other hand, log r = -.0767; for subgroup 3, log r = -.0018; and so on. The 

standard deviation of ten estimates for the 10 mutually exclusive subgroups, each based on a 

different tenth of the FARS data is .0558. The standard deviation S1 of the corresponding 

estimate of log r in the full dataset (10 times as many cases) is .0558 / √10 = .0176. This is the 

estimated standard deviation of log r, for the full dataset, based on our first run-through of 

splitting FARS into 10 systematic random subsamples. 

26
 
 

 

On the second iteration, the estimate of the standard deviation S2, derived from the 10 

subsamples created on that iteration, was slightly higher, .0228. On the third iteration, 

S3 = .0182. Over all 11 iterations, the estimates Sj ranged from .0092 to .0319. However, the 

median of these 11 estimates is S = .0198, which was the value obtained on the eighth iteration. 

(In fact, except for the low and high estimates generated on the 4th and 9th iterations, the 

remaining Sj are all quite close to the median.) 

For the full dataset, log r was -.0088. Because t = |log r| / S = .0088 / .0198 = .45 is less than 

1.833 (the 95th percentile of a t-distribution with 9 degrees of freedom50), the observed fatalcrash reduction for ABS is not statistically significant at the one-sided .05 level. 

The 90 percent confidence bounds for the effect of increasing all the way from zero ABS to 100 

percent ABS are: 

{1 – [1 - exp (log r + 1.833 x S)]} / {.882 – .068 x [1 - exp (log r + 1.833 x S)]} 

= {1 – [1 - exp (–.0088 + 1.833 x .0198)]} / {.882 – .068 x [1 - exp (–.0088 + 1.833 x .0198)]} 

= from a 3-percent increase to a 5 percent reduction 

The seventh column of Table 2-2 shows the t value for the effectiveness estimate of ABS. The 

two right columns show the lower and upper 90 percent confidence bounds for the effect of ABS 

– i.e., the shift from no ABS to 100% ABS. 

Now let us discuss the point and interval estimates for all the crash types in Table 2-2. Positive 

estimates (reductions in fatal crashes with ABS, relative to the control group) are shown in black, 

negative estimates in red. Statistically significant point estimates are printed bold, and they are 

shaded light-blue if positive, yellow if negative. Non-significant estimates are not printed bold, 

and have a negative lower confidence bound and a positive upper bound. 

The effect of ABS on all crashes, including the control-group crashes where we assume it has no 

effect, is derived from the preceding estimate. Without ABS, there were 5,467 weighted controlgroup involvements and 13,809 response group involvements. The point estimate for all 

response-group crashes is a 1.08 percent reduction. The point estimate for all crashes is: 

[13,809 / (5,467 + 13,809)] x 1.08 = 0.77 percent reduction 

Its 90 percent confidence bounds, similarly derived, are from -2 to +4 percent: a negligible 

overall effect. 

But ABS does have statistically significant effects in some types of crashes. Run-off-road 

crashes increase by a statistically significant 9 percent. The t value is 2.65, significant at the onesided .05 level (because it exceeds 1.833) and, for that matter, even at the two-sided .05 level 

(because it exceeds 2.262, the 97.5th percentile of t with 9 df). Whereas the effect has diminished 

since calendar years 1989-1993 (a 28% increase in run-off-road crashes, according to NHTSA’s 

50 Because each sj is calculated by computing the variance of 10 observations (log r for each of 10 subgroups), it is 

appropriate to use a t test with 9 df to test if log r is significantly different from zero. 

27
 
 

initial evaluation51), it continues to be negative and statistically significant. The observed effect 

might have diminished over time because more drivers knew how to use ABS properly (in part 

thanks to public information campaigns), or because the technology was refined, or by chance 

alone – but a residual significant effect still remains. 

Furthermore, all three subsets of run-off-road crashes were observed to increase with ABS. The 

largest increase and the only statistically significant one is side impacts with fixed objects, 

increasing by 30 percent (t = 4.15; confidence bounds, 16% to 46% increase). Rollovers 

increased by a non-significant 11 percent, other run-off-road crashes (mostly frontal impacts 

with fixed objects), by 3 percent. 

By contrast, ABS is associated with reductions of crashes that generally do not involve running 

off the road. Fatal collisions with pedestrians, bicyclists or animals decrease by a statistically 

significant 13 percent (t = 2.99). Most of these collisions involve hitting a non-motorist or 

animal on the road, rather than first running off the road and then hitting a non-motorist. 

Culpable involvements in collisions with other vehicles decreased by 4 percent, but the reduction 

falls short of statistical significance (t = 1.61). 

Almost every one of the effects is magnified on wet, snowy, or icy roads, where ABS is much 

more likely to activate. The net effect on these roads is negligible, but the increase in run-offroad crashes is a statistically significant 34 percent. Moreover, the increase is statistically 

significant in all three subsets of run-off-road crashes, reaching 85 percent in side impacts with 

fixed objects. The harm in run-off-road crashes is offset by benefits in collisions with other 

vehicles, including a significant 12 percent reduction of culpable involvements in these 

collisions. The only inconsistent result is a zero effect on collisions with pedestrians, bicyclists, 

and animals; it raises a question about the significant benefit seen on all (i.e., dry) roads. 

On dry roads, the overall increase in run-off-road crashes is a fairly negligible 4 percent; it is not 

statistically significant (t = 1.07). However, side impacts with fixed objects increased by a 

statistically significant 16 percent (t = 2.35). The reduction in fatal collisions with pedestrians, 

bicyclists, and animals on dry roads is a statistically significant 15 percent (t = 3.21). It is not 

clear why ABS should be so effective in pedestrian crashes on dry roads. Perhaps, as drivers 

become used to having ABS, they have become more willing to take immediate, decisive action 

if a pedestrian suddenly materializes in front of them: stomp and stay on the brakes, and steer. 

2.1.6 Comparative results for four cohorts of cars 

A relatively small number of high-sales General Motors models that received ABS as standard 

equipment in the early 1990s predominated the databases of the early evaluations of ABS (Buick 

Park Avenue; Cadillac DeVille; Chevrolet Caprice, Cavalier, and Corsica; Pontiac Grand Am; 

and similar cars). The observed increases of run-off-road crashes in those studies might be 

critiqued as somehow attributable to those specific models. For example, a change in the models 

unrelated to ABS that made them appear sportier or with greater appeal in rural markets could 

have increased run-off-road crashes relative to other types. Or it might have been the specific 

51 Kahane (1994), pp. xi and 95. 

28
 
 

ABS technology in those models. (That is in addition to the more generic critique that drivers 

were initially not aware of how to use ABS.) 

By 2009, a much more representative list of models, as shown in Table 2-1, is available, 

especially if the models with standard ABS are supplemented by those sold with a high 

percentage of optional ABS. The models in Table 2-1 can be grouped into four cohorts and the 

effect of ABS estimated separately in each one: 

	 GM models that had a high percentage of ABS by 1993. 

	 Non-GM models that had a high percentage of ABS by 1993. Ford products and Honda 

Accord predominate. 

	 Models that received a high share of ABS after 1993. Includes high-sales models such as 

VW Jetta, Nissan Maxima, Subaru Legacy, Toyota Camry, and Toyota Avalon. 

	 Models that dropped standard ABS, or where optional ABS lost a substantial share of the 

market: Chrysler and GM cars, including many of the GM models in the first cohort, but 

10 model years later. 

Table 2-3 shows the point estimates for the effect of ABS in each cohort of cars for five 

“bellwether” types of crashes: 

TABLE 2-3
 

OBSERVED PERCENT FATALITY REDUCTION FOR ABS 

BY CAR COHORT AND CRASH TYPE 

GM Non-GM Added 

Crash Type 

ABS by 

1993 

ABS by 

1993 

ABS 

1994+ 

Dropped 

ABS52 

Run-off-road - 9 - 7 none - 13 

Run-off-road, wet-snowy-icy - 28 - 39 - 21 - 22 

Side impact with fixed object - 24 - 34 - 14 - 33 

Pedestrian/bicyclist/animal	 18 9 8 - 6 

Culpable w other veh, wet-snowy-icy 14 7 13 - 18 

The observed effect on run-off-road crashes is never positive and it is negative in three of the 

four cohorts. Run-off-road crashes in wet, snowy, or icy conditions and side impacts with fixed 

objects in any conditions increased substantially and consistently with ABS in every cohort. The 

effect is not a quirk of the early 1990s or the particular ABS technology in use at that time. 

52 In this cohort, effect for each response group was measured relative to vehicle registration years rather than 

control-group involvements. There were unexpectedly few control-group involvements in the ABS-equipped cars. 

29
 
 

Indeed, GM models apparently increased run-off-road crashes when they acquired ABS in the 

early 1990s and then reduced run-off-road crashes when they dropped ABS after 2000. 

Conversely, pedestrian crashes decreased with ABS in three of the four cohorts, as did culpable 

involvements with other vehicles on wet, snowy, or icy roads. 

2.1.7 Drinking versus non-drinking drivers 

Harless and Hoffer presented analyses in 2002 asserting that the increase in run-off-road crashes 

with ABS was primarily confined to drinking drivers, and that non-drinking drivers experienced 

little or no increase in run-off-road crashes.53 Their analyses further showed the increase for 

drinking drivers was limited to calendar years 1993 to 1995, when the GM cars that had received 

standard ABS were 2 to 4 years old, and was non-significant in later years. The database for this 

report makes it possible to compare statistics for drinking and non-drinking drivers from 1995 

through 2007, and also to look at cars other than the GM models of the early 1990s. 

For our analysis, let us define “non-drinking drivers” to be those whose BAC was reported to be 

.00, plus those whose BACs were not reported but whom police reported as not drinking 

(DRINKING = 0). “Drinking drivers” are those with BACs reported and at least .02 g/dL, plus 

those with unreported BACs but police-reported drinking (DRINKING = 1). All other drivers, 

including those with BACs = .01 are excluded. If entirely separate analyses are performed for 

non-drinking and drinking drivers, including control groups drawn only from these drivers, the 

increase in run-off-road crashes indeed seems at first glance limited to the drinking drivers (54% 

increase), with even a 1 percent reduction for the non-drinkers: 

With Separate Cars with Cars with Reduction 

Control Groups 7% ABS 88% ABS for ABS (%) 

Non-Drinking Drivers 

Control group 4,058 4,456 

Run-off-road 2,055 2,234 1 

Drinking Drivers 

Control group 272 205 

Run-off-road 2,014 2,336 - 54 

While this is a logically defensible analysis, the cell counts in the table are not reasonable. The 

numbers of run-off-road crashes, both with and without ABS, are similar for the non-drinkers 

and the drinkers, but the control group is almost 20 times as large for the non-drinkers. Even that 

is reasonable, because drinking drivers are at much higher risk and are usually culpable for the 

crashes they get into. What is not plausible is that there are 272 control-group crashes without 

53 Harless, D. W.,& Hoffer, G. E. (2002, May). The antilock braking system anomaly: a drinking driver problem? 

Accident Analysis and Prevention, Vol. 34, , pp. 333-341. 

30
 
ABS and only 205 with ABS. If control-group involvements are a surrogate for “exposure,” this 

says that the cars without ABS have far more exposure for drinking drivers than cars with ABS – 

even though for non-drinking drivers the exposure is about the same. In other words, drivers 

drink much less when their cars are equipped with ABS. But that is unlikely. The cars with 

ABS are the same makes and models as the cars without ABS, observed in the same calendar 

years, with just two years difference in vehicle age (mostly newer, sometimes older). There is 

every reason to believe drinking would be about the same. The reduction of control group 

crashes from 272 to 205 is most likely a chance event. It is something that could occasionally 

happen by chance with numbers this small. It would almost certainly not have happened if the 

numbers had been in the thousands. It is the drop from 272 to 205 that is “driving” the results. 

If, instead, the same control group is used for the non-drinking and drinking drivers, namely the 

weighted counts for all cases as shown in Table 2-2, the effects on run-off-road crashes are much 

closer: 

With Joint Cars with Cars with Reduction 

Control Group 7% ABS 88% ABS for ABS (%) 

Non-Drinking Drivers 

Control group (from Table 2-2) 5,467 5,808 

Run-off-road 2,055 2,234 - 2 

Drinking Drivers 

Control group (from Table 2-2) 5,467 5,808 

Run-off-road 2,014 2,336 - 9 

A similar result is obtained if run-off-road crashes are analyzed relative to exposure in vehicle 

registration years, rather than relative to a control group: 

31
 
Relative to Cars with Cars with Reduction 

Vehicle Registration Years 7% ABS 88% ABS for ABS (%) 

Non-Drinking Drivers 

Vehicle Years 86,635,000 90,374,000 

Run-off-road 2,055 2,234 - 4 

Drinking Drivers 

Vehicle Years 86,635,000 90,374,000 

Run-off-road 2,014 2,336 - 11 

Whereas the last two analyses greatly shrink the absolute gap between non-drinking and drinking 

drivers, the results are ambiguous. They are statistically compatible with the hypothesis that the 

effect is the same for drinkers and non-drinkers, but also compatible with Harless and Hoffer’s 

hypothesis that the effect is zero for non-drinkers. A clearer picture emerges when the analysis 

is limited to run-off-road crashes on wet, snowy, or icy roads, where ABS is much more likely to 

activate and its negative effect is larger: 

Cars with Cars with Reduction 

7% ABS 88% ABS for ABS (%) 

Non-Drinking Drivers 

Control group (from Table 2-2) 5,467 5,808 

Run-off-road, wet-snowy-icy 369 486 - 23 

Drinking Drivers 

Control group (from Table 2-2) 5,467 5,808 

Run-off-road, wet-snowy-icy 296 397 - 26 

The observed effect is almost identical for non-drinking drivers (23% increase) and drinking 

drivers (26% increase). The negative effect on run-off-road crashes does not appear to be limited 

to drinking drivers and it has persisted over time. Frankly, this result is more convincing than 

the conclusion of Harless and Hoffer. While it could be argued that drinking drivers are 

especially befuddled by ABS and unlikely to use wisely the capability to steer while braking, a 

probably stronger case can be made that a large proportion of the run-off-road crashes of 

drinking drivers do not involve braking at all and could not have been influenced by ABS. 

32
 
 Air-Bag-Weighted N

Cars w Cars w Fatal Crash 

7% ABS 88% ABS Reduction (%) 

Control group involvements 5,467 5,808 

All non-control-group involvements on: 

Dry roads 10,502 11,024 1 

Wet roads 2,630 2,797 none 

Snowy or slushy roads 304 346 - 7 

Icy roads 273 291 none 

2.1.8 Net effect by specific road-surface condition 

Table 2-4 indicates that the net effect of ABS is about the same on dry, wet, snowy, and icy 

roads. The observed net effect is close to zero in each road condition. 

TABLE 2-4 

NET EFFECT OF ABS, BY ROAD-SURFACE CONDITION 

2.2 Analysis of rear-wheel ABS for LTVs 

Almost all GM, Ford, and Dodge pickup trucks and most of their SUVs received ABS for the 

rear wheels only as standard equipment in the late 1980s or by 1990; many vans and foreignbased-nameplate LTVs shortly thereafter. Four-wheel ABS usually did not arrive on LTVs until 

at least several model years later. The analysis of rear-wheel ABS for LTVs closely parallels the 

procedures described in Section 2.1, addressing four-wheel ABS for passenger cars. Here, too, 

the analysis is based on FARS data for calendar years 1995 to 2007. The control group and 

response group of crash involvements are defined exactly as in Section 2.1.2. The control group 

consists of LTVs that were stopped, parked, backing, moving slower than 10 mph, parking, or 

leaving a parking place when they were hit; or were struck in the rear; or were non-culpable 

parties to a collision involving two or more vehicles. 

2.2.1 LTVs without ABS versus LTVs with rear-wheel ABS 

Most LTV models changed from not offering ABS at all in one model year to having rear-wheel 

ABS as standard equipment the next year. Rear-wheel ABS was rarely an option. The analysis 

will include one model that shifted from optional to standard ABS and an adjustment factor to 

translate the observed effect to the effectiveness of changing from 0 to 100 percent ABS. 

33
 
 

 

 

NHTSA’s 1993 evaluation report (based on information furnished by the manufacturers or from 

news articles) and Ward’s Automotive Yearbooks document what LTVs had rear-wheel ABS.54 

Table 2-5 lists the 31 models contributing data to the analyses, the range of model years included 

in each case, and the proportion of LTVs equipped with rear-wheel ABS before and after the 

shift (0% and 100%, respectively, except for the Nissan pickup). The identification of makes 

and models in the FARS data is based on decoding the VIN. Unlike the database for passenger 

cars, there are no LTV models that dropped from standard to no ABS or with a “transitional” 

model year between low and high installation rates. But four models will be included for only a 

single year without ABS (1988) and a single year with rear-wheel ABS (1989), because they 

already began to offer four-wheel ABS in the next year (1990). 

All models in Table 2-5 except the Nissan pickup have model-year ranges that end before 1995 

and all FARS cases from CY 1995 to 2007 will be included for those models. For the Nissan 

pickup, the model year range extends to 1996; only FARS data from 1996 to 2007 will be 

included. For the 31 makes and models combined, 1.2 percent of the LTVs in the low-ABS 

model-year range were equipped with ABS; 100 percent of the LTVs in the high-ABS model 

years had rear-wheel ABS. That is a 98.8 percentage-point gain in the share of LTVs equipped 

with rear-wheel ABS. 

The analysis file includes 26,324 FARS cases of LTVs involved in crashes. By contrast, 

NHTSA’s 1993 evaluation was based on 9,621 FARS cases. 

54 Kahane, C.J. (1993). Preliminary Evaluation of the Effectiveness of Rear-Wheel Antilock Brake Systems for Light 

Trucks. NHTSA Docket No. 70-27-GR-026, p. 15. Washington, DC: National Highway Traffic Safety 

Administration; Ward’s Automotive Yearbooks. 

34
 
 

TABLE 2-5: LTVs RECEIVING REAR-WHEEL ABS 

MAKES, MODELS AND MODEL YEARS INCLUDED IN THE ANALYSIS 

Limited or Standard or High % 

No ABS Rear-Wheel ABS 

% % Pct Pt 

Make and Model MY ABS MY ABS Change 

Dodge Dakota 1987-88 0% 1989-90 100% 100% 

Dodge D/W pickup 1987-88 0% 1989-90 100% 100% 

Dodge Ramcharger 1988-89 0% 1990-91 100% 100% 

Dodge Ram van and wagon 1988-89 0% 1990-91 100% 100% 

Ford Ranger pickup 1987-88 0% 1989-90 100% 100% 

Ford F pickup 1985-86 0% 1987-88 100% 100% 

Ford Bronco 2 1985-86 0% 1987-88 100% 100% 

Ford Bronco 1985-86 0% 1987-88 100% 100% 

Ford Aerostar van and wagon 1988-89 0% 1990-91 100% 100% 

Ford big van 1988-89 0% 1990-91 100% 100% 

Chevrolet S/T pickup 1987-88 0% 1989-90 100% 100% 

Chevrolet C/K pickup 1986-87 0% 1988-89 100% 100% 

Chevrolet S/T Blazer 1988 0% 1989 100% 100% 

Chevrolet V Blazer 1988-89 0% 1990-91 100% 100% 

Chevrolet Suburban 1988-89 0% 1990-91 100% 100% 

Chevrolet Astro wagon/van 1988 0% 1989 100% 100% 

Chevrolet big van 1988-89 0% 1990-91 100% 100% 

Chevrolet Geo Tracker 1989-90 0% 1991-92 100% 100% 

GMC Sonoma pickup 1987-88 0% 1989-90 100% 100% 

GMC C/K pickup 1986-87 0% 1988-89 100% 100% 

GMC S/T Jimmy 1988 0% 1989 100% 100% 

GMC V Jimmy 1988-89 0% 1990-91 100% 100% 

GMC Suburban 1988-89 0% 1990-91 100% 100% 

GMC Safari wagon/van 1988 0% 1989 100% 100% 

GMC big van 1988-89 0% 1990-91 100% 100% 

Nissan pickup 1993-94 27% 1995-96 100% 73% 

Nissan Pathfinder 1989-90 0% 1991-92 100% 100% 

Isuzu pickup 1989-90 0% 1991-92 100% 100% 

Mazda pickup 1989-90 0% 1991-92 100% 100% 

Mazda MPV 1989-90 0% 1991-92 100% 100% 

Suzuki Sidekick 2dr 1989-90 0% 1991-92 100% 100% 

Grand Average 1% 100% 99% 

35
 
 

2.2.2 Results 

Table 2-6 shows the actual case counts for each analysis of the effect of rear-wheel ABS in 

LTVs, the effectiveness estimates, the significance-test results and the confidence bounds. 

Except for the Nissan pickup, accounting for a small portion of the database, none of the models 

in Table 2-5 were equipped with frontal air bags during the model years included in the data. 

Unlike Section 2.1.4, it is unnecessary to weight the cases for the effect of air bags. 

Effectiveness estimates may be derived from the actual case counts. The 2x2 contingency table 

for the entire database is: 

 LTVs with LTVs with 

1% ABS 100% ABS 

Control group involvements 3,577 4,185 

All non-control-group involvements 8,722 9,840 

The observed effect is: 

1 – [(9,840/4,185) / (8,722/3,577)] = 3.57 percent reduction 

That estimate is rounded to 4 percent and shown in the third column of the “all non-controlgroup crashes” row of Table 2-6. But that is the effect of increasing the market share of rearwheel ABS from 1.2 percent to 100 percent. The effect of increasing all the way from zero ABS 

to 100 percent ABS would be just slightly higher: 

{1 – [(9,840/4,185) / (8,722/3,577)]} / {1.000 – .012 x [((9,840/4,185) / (8,722/3,577))]} 

= 3.61 percent reduction 

That estimate still rounds to 4 percent and it is shown in the fourth column of Table 2-6, the 

column headed by “100% versus no ABS.” It is the basic point estimate of the effect of rearwheel ABS. 

The t-tests for statistical significance of the point estimates and the estimation of 90 percent 

confidence bounds are based on the same procedure as for passenger cars (Section 2.1.5). The t 

value for the preceding point estimate is 1.34. Because that is less than 1.833 (the 95th percentile 

of a t-distribution with 9 degrees of freedom55), the observed fatal-crash reduction for rear-wheel 

ABS is not statistically significant at the one-sided .05 level. The 90 percent confidence bounds 

for effectiveness range from a 1-percent increase to an 8 percent reduction in fatal crash 

involvements. The t values and confidence bounds are shown in the last three columns of Table 

2-6. 

55 See Section 2.1.5. 

36
 
Table 2-6: LTVs, Distribution of Fatal Crash Involvements With and Without Rear-Wheel ABS, FARS 

Actual FARS Fatal Crash t 

Case Counts Reduction (%) for 

H0: 

LTVs w LTVs w 100% vs. 100% vs. Red. 

1% ABS 100% ABS 1% ABS no ABS = 0 

Control group involvements 3,577 4,185 

ALL ROADS All non-control-group involvements 8,722 9,840 4 4 1.34 

All crash involvements

 3 

Run-off-road crashes 3,728 4,176 4 4 1.20 

Side impacts w fixed objects 403 436 8 8 .85 

 First-event rollovers 1,209 1,310 7 7 1.76 

Other run-off-road crashes 2,116 2,430 2 2 .43 

Pedestrian/bicyclist/animal 1,266 1,266 14 15 3.35 

Culpable involvements w other vehicle 2,795 3,239 1 1 .32 

WET, SNOWY, OR ICY ROADS All non-control-group involvements 2,045 2,427 -1 -1 .38 All crash involvements -1 Run-off-road crashes 607 664 7 7 1.29 

Side impacts w fixed objects 90 89 15 16 1.01 

 First-event rollovers 139 175 -8 -8 .65 

Other run-off-road crashes 378 400 10 10 1.34 

Pedestrian/bicyclist/animal

 183 

213 1 1 .05 

Culpable involvements w other vehicle 525 638 -4 -4 .68 

37
 

Now let us discuss the point and interval estimates for all the crash types in Table 2-6. Positive 

estimates (reductions in fatal crashes with ABS, relative to the control group) are shown in black, 

negative estimates in red. Statistically significant point estimates are printed bold, and they are 

shaded light-blue if positive, yellow if negative. Non-significant estimates are not printed bold, 

and have a negative lower confidence bound and a positive upper bound. 

The effect of rear-wheel ABS on all crashes, including the control-group crashes where we 

assume it has no effect, is: 

[8,722 / (3,577 + 8,722)] x 3.61 = 2.56 percent reduction 

Its 90 percent confidence bounds, similarly derived, are from -1 to +6 percent. 

The results for specific types of crashes are consistent with the findings of NHTSA’s 1993 

evaluation, which was based on FARS data from 1989 through mid-1992 (no overlap with the 

1995-2007 data in this study). The only significant fatality reduction in this study was a 15 

percent reduction in collisions with pedestrians, bicyclists and animals (t = 3.35; confidence 

bounds, 7% to 23%). The 1993 evaluation claimed a 5 to 15 percent reduction.56 However, the 

reduction does not become stronger on wet, snowy, or icy roads, but drops to 1 percent. 

Effects on run-off-road crashes never achieve statistical significance. The observed effect for all 

run-off-road crashes in all conditions is a 4 percent reduction (t = 1.20; confidence bounds, -2% 

to 11%). But the effects are generally positive: for all three types of run-off-road crashes on all 

roads, and for two out of three types on wet, snowy and icy roads. The 1993 study did not find a 

significant effect, either. Unlike four-wheel ABS, rear-wheel ABS does not empower drivers to 

steer while braking, or to misuse that capability. Rear-wheel ABS could prevent some run-offroad crashes to the extent it prevents rear-wheel lockup and resultant loss of control, but the 

effect in fatal crashes, if any, is not statistically detectable. 

The observed reduction of culpable involvements with other vehicles was 1 percent (confidence 

bounds, -5% to 6%). It indicates the effect is negligible. That clarifies the conflicting results in 

earlier evaluations, with some analyses showing close to zero effect and others a risk increase. 

The reduction in collisions with pedestrians is apparently real. It has persisted and perhaps even 

grown over time. The immediate goal of rear-wheel ABS is to prevent rear-wheel lockup that 

could result in a loss of control and lane departure. It has little effect on stopping distance or 

ability to steer the vehicle while braking. It is not clear why rear-wheel ABS would be directly 

of help if a pedestrian suddenly appears in front of the driver. Perhaps over time drivers have 

become acclimatized to braking immediately and aggressively as they have become less afraid 

that it could result in rear-wheel lockup that would be dangerous for them. That extra braking 

effort may be just enough to avoid hitting some pedestrians. However, by the same logic, a 

reduction in collisions with other vehicles might be expected, but none was observed. 

56 Kahane (1993), p. 5. 

38
 
 

 

2.3 Analysis of four-wheel ABS for LTVs 

Most makes and models of LTVs that were eventually equipped with four-wheel ABS first went 

through an intermediate stage of only rear-wheel ABS, typically for at least several years. 

Relatively few models went directly from no ABS to four-wheel ABS, and even fewer of those 

switched from little or no ABS in one model year to standard or high-percentage optional ABS in 

the next year. That leaves little choice except to estimate the effect of four-wheel ABS in two 

essentially separate steps: as the composite of the effect of switching from no ABS to rear-wheel 

ABS (already estimated in Section 2.2) and the effect of switching from rear-wheel to four-wheel 

ABS. 

The analysis of switching from rear-wheel to four-wheel ABS for LTVs closely parallels the 

procedures addressing four-wheel ABS for passenger cars, described in Section 2.1. Here, too, 

the analysis is based on FARS data for calendar years 1995 to 2007. The control group and 

response group of crash involvements are defined exactly as in Section 2.1.2. The control group 

consists of LTVs that were stopped, parked, backing, moving slower than 10 mph, parking, or 

leaving a parking place when they were hit; or were struck in the rear; or were non-culpable 

parties to a collision involving two or more vehicles. Only the last part of the statistical analysis 

will look different: the significance tests and confidence bounds for the composite effect. 

2.3.1 LTVs with rear-wheel ABS versus LTVs with four-wheel ABS 

ABS might be offered on LTVs in quite a few ways. In a given model year, a make and model 

might offer four-wheel ABS as standard equipment, or rear-wheel ABS, or note at all. Either or 

both types of ABS could be options, and no ABS could also be an option. Different sub-series of 

the same model could have different options or standard equipment. Because the immediate goal 

is to compare rear-wheel and four-wheel ABS, the analysis will exclude any model year where 

some or all of a model run were not equipped with any ABS. Ward’s Automotive Yearbooks 

document the proportions of LTVs that had rear-wheel ABS and that had four-wheel ABS in a 

given model year.57 

The shift from rear-wheel to four-wheel ABS was abrupt in some models but more gradual in 

others, where four-wheel ABS was an option of steadily increasing popularity. For some of 

those models, the analysis may need to skip a “transitional” model year between mostly rearwheel and mostly four-wheel ABS. There are no LTV models that dropped back from fourwheel to rear-wheel ABS. 

Table 2-7 lists the 29 models contributing data to the analyses, the range of model years included 

in each case, and the proportion of LTVs equipped with four-wheel ABS before and after the 

shift (any LTV in Table 2-7 without four-wheel ABS has rear-wheel ABS). The identification of 

makes and models in the FARS data is based on decoding the VIN. 

57 Ward’s Automotive Yearbooks. 

39
 
 

 

TABLE 2-7: LTVs SWITCHING FROM REAR-WHEEL TO FOUR-WHEEL ABS 

MAKES, MODELS AND MODEL YEARS INCLUDED IN THE ANALYSIS 

(All these models have either rear-wheel or four-wheel ABS – none completely without ABS) 

(Bold model names: 1 model year gap between “pre” and “post” or elsewhere) 

Limited or No Standard or High % 

Four-Wheel ABS Four-Wheel ABS 

% % Pct Pt 

Make and Model MY 4-Wheel MY 4-Wheel Change 

Dodge Durango 2002-03 14% 2004-05 100% 86% 

Ford Ranger 1999-2000 32% 2001-02 100% 68% 

Ford F pickup 1997-98 18% 1999-2000 74% 56% 

Ford Explorer 1991-92 0% 1993-94 100% 100% 

Ford Bronco 1991-92 0% 1993-94 100% 100% 

Ford big van/wagon 1997-98 26% 2000-01 100% 74% 

Chevrolet S/T pickup 1993-94 9% 1996-97 100% 100% 

Chevrolet C/K pickup 1993-94 0% 1995-96 100% 100% 

Chevrolet Blazer S/T 2dr 1989-90 5% 1992-93 89% 84% 

Chevrolet Blazer V/K 2dr 1991-92 0% 1993-94 100% 100% 

Chevrolet Suburban 1990-91 0% 1992-93 100% 100% 

Chevrolet Astro wagon 1990-91 12% 1992-93 100% 88% 

Chevrolet Astro cargo van 1991-92 4% 1993-94 100% 96% 

Chevrolet big van/wagon 1991-92 0% 1993-94 100% 100% 

GMC Sonoma pickup 1993-94 0% 1996-97 100% 100% 

GMC Sierra pickup 1993-94 0% 1995-96 100% 100% 

GMC Jimmy S/T 2dr 1989-90 6% 1992-93 91% 85% 

GMC Jimmy V/Yukon 2dr 1991-92 0% 1993-94 100% 100% 

GMC Suburban 1990-91 0% 1992-93 100% 100% 

GMC Safari wagon 1989-90 5% 1992-93 100% 95% 

GMC Safari cargo van 1991-92 7% 1993-94 100% 93% 

GMC big van 1991-92 0% 1993-94 100% 100% 

Nissan Pathfinder 1994-95 0% 1996-97 100% 100% 

Honda Passport 1996-97 20% 1998-99 100% 80% 

Isuzu Trooper 4dr 1994-95 22% 1997-98 100% 78% 

Isuzu Rodeo 1996-97 0% 1998-99 100% 100% 

Mazda Navajo 1991-92 0% 1993-94 100% 100% 

Mazda MPV 1994-95 0% 1996-97 100% 100% 

Toyota 4Runner 4dr 1993-94 0% 1996-97 95% 95% 

Grand Average 8% 96% 88% 

40
 
For the models in Table 2-7 with model-year ranges that end before 1995, all FARS cases from 

CY 1995 to 2007 are included. For the other models, FARS data are included beginning only 

with the last model year in the range. For example, the model year range for Dodge Durango is 

2002 to 2005; only FARS data from calendar years 2005 to 2007 are included, because those are 

the only calendar years where the full model-year range of vehicles will be represented. 

For the 29 makes and models combined, 7.7 percent of the LTVs in the two earlier model years 

were equipped with four-wheel ABS (and 92.3 percent with rear-wheel ABS); 96.3 percent of 

the LTVs in the two later model years had four-wheel ABS (and 3.7 percent had rear-wheel 

ABS). That is an 88.6 percentage-point gain in the share of LTVs equipped with four-wheel 

ABS. 

The analysis file includes 28,952 FARS cases of LTVs involved in crashes. 

2.3.2 Results 

The influx of four-wheel ABS in LTVs was strongest in the mid-1990s, overlapping with the 

introduction of frontal air bags. Among the 29 makes and models included in Table 2-7: 

	 During the rear-wheel-ABS model years, 25 percent of the LTVs had driver-only air bags 

and 6 percent dual air bags. 

	 Whereas during the four-wheel-ABS model years, 53 percent had driver-only air bags 

and 13 percent dual. 

The confounding of four-wheel ABS with air bags is not quite as strong as for passenger cars, 

but it is strong enough that the FARS cases need to be weighted by the same procedure as in 

Section 2.1.4. 

Table 2-8 shows the actual and weighted case counts for each analysis of the effect of four-wheel 

ABS in LTVs, the separate and composite effectiveness estimates, the significance-test results 

and the confidence bounds. Here is how the statistics are derived for the basic estimate of the 

effect of four-wheel ABS, relative to no ABS at all, in all non-control-group crashes on all roads. 

The 2x2 table of actual case counts for the entire database created in Section 2.3.1 is: 

LTVs with LTVs with 

8% 4-wheel 96% 4-wheel 

92% rear-wheel 4% rear-wheel 

Control group involvements 4,585 4,349 

All non-control-group involvements 10,167 9,851 

The observed effect is: 

1 – [(9,851/4,349) / (10,187/4,585)] = 2.15-percent increase 

41
 
Table 2-8: LTVs, Distribution of Fatal Crash Involvements With and Without Four-Wheel ABS, FARS 

Actual FARS Air-Bag- Fatal Crash 

Case Counts Weighted N Reduction (%) 

LTVs w LTVs w LTVs w LTVs w 96/4 Rear-whl 4-wheel 

8% 4-whl 96% 4-whl 8% 4-whl 96% 4-whl vs. vs. vs. 

92% rear 4% rear 92% rear 4% rear 8/92 no ABS no ABS 

Control group involvements 4,585 4,349 4,618 4,406 ALL ROADS All non-control-group involvements 10,167 9,851 10,381 10,332 -4 4 -1 

All crash involvements -1 

Run-off-road crashes 4,320 4,384 4,456 4,659 -10 4 -6 

Side impacts w fixed objects 469 480 473 486 -8 8 none 

 First-event rollovers 1,493 1,655 1,493 1,655 -16 7 -10 

Other run-off-road crashes 2,358 2,249 2,490 2,518 -6 2 -5 

Pedestrian/bicyclist/animal 1,313 1,253 1,314 1,256 none 15 14 Culpable involvements w other veh 3,372 3,182 3,439 3,351 -2 1 -1 

WET, SNOWY, OR ICY ROADS All non-control-group involvements 2,444 2,375 2,488 2,467 -4 -1 -6 

All crash involvements -6 Run-off-road crashes 748 803 774 852 -15 7 -10 

Side impacts w fixed objects 122 139 122 140 -20 16 -4 

 First-event rollovers 209 237 209 237 -19 -8 -31 

Other run-off-road crashes 417 427 443 476 -13 10 -3 

Pedestrian/bicyclist/animal 154 165 154 166 -13 1 -14 Culpable involvements w other veh 613 582 623 607 -2 -4 -6 

42
However, the calculation based on actual case counts understates the increase with four-wheel 

ABS because it does not adjust for the effect of air bags being introduced more or less 

simultaneously with four-wheel ABS. The 2x2 table of case counts weighted upwards for the 

effect of air bags is: 

LTVs with 

8% 4-wheel 

92% rear-wheel 

LTVs with 

96% 4-wheel 

4% rear-wheel 

Control group involvements 

All non-control-group involvements 

4,618 

10,381 

4,406 

10,332 

Weighting the cases increases each cell count in the contingency table, but the lower-right cell 

more so than the others (because it has the highest proportion of frontal crashes in LTVs with air 

bags). The observed effect is now: 

1 – [(10,332/4,406) / (10,381/4,618)] = 4.31-percent increase 

That estimate is rounded to -4 percent and shown in the fifth column of the “all non-controlgroup crashes” row of Table 2-8. But that is the effect of increasing the market share of fourwheel ABS from 7.7 percent to 96.3 percent. The effect of increasing all the way from 100% 

rear-wheel ABS to 100% four-wheel ABS would be higher: 

{1 – [(10,332/4,406) / (10,381/4,618)]} / {.963 – .077 x [(10,332/4,406) / (10,381/4,618)]} 

= 4.89-percent increase 

But that is the estimated effect of shifting from rear-wheel to four-wheel ABS. The effect of 

shifting from no ABS at all to rear-wheel ABS has already been estimated in Section 2.2.2 to be 

a 3.61 percent reduction of crash involvements. It is rounded to 4 and shown in the sixth column 

of Table 2-8. (The sixth column of Table 2-8 recapitulates the fourth column of Table 2-6, the 

principal point estimates for rear-wheel ABS relative to no ABS.) The net effect of shifting from 

no ABS to four-wheel ABS is the composite of the two stepwise effects: 

1 – [(1 – .0361) x (1 + .0489)] = 1.10-percent increase 

It is rounded to -1 percent and shown in the seventh column of Table 2-8. It is the principal 

point estimate for the effect of four-wheel ABS relative to no ABS. 

Significance testing of this effect and estimation of its 90% confidence bounds are based on a 

procedure partly identical and partly somewhat different from Section 2.1.5. What is identical is 

that FARS is split up into 10 systematic random subsamples based on the last two digits of 

ST_CASE – and that procedure is replicated 11 times. In fact, the same ST_CASE numbers 

used in Section 2.1.5 are re-used here. 

43
 
 

 

 

For example, on the first of the 11 times that FARS was split into 10 subgroups, subgroup 1 

happened to consist of cases with ST_CASE ending in 01, 05, 06, 21, 23, 27, 38, 61, 65, or 81. 

Using the FARS database created in Section 2.2.1, but limited to the cases with ST_CASE 

ending in 01, 05, 06, 21, 23, 27, 38, 61, 65, or 81, we estimate the effect E0R,1,1 of shifting from 

no ABS at all to 100% rear-wheel ABS (where 0R means “from no ABS to rear-wheel ABS,” 1 

is subgroup 1, 1 is replication 1 of the procedure). Using the FARS database created in Section 

2.3.1 and again limiting to the cases with ST_CASE ending in 01, 05, 06, 21, 23, 27, 38, 61, 65, 

or 81, we estimate the effect ER4,1,1 of shifting from 100% rear-wheel ABS to 100% four-wheel 

ABS (where R4 means “from rear-wheel to four-wheel ABS”). The composite effect: 

E04,1,1 = 1 – [(1 – E0R,1,1) x (1 – ER4,1,1)] 

is the estimate of shifting from no ABS to 100% four-wheel ABS, based solely on the FARS 

cases with ST_CASE ending in 01, 05, 06, 21, 23, 27, 38, 61, 65, or 81. Similarly, E04,i,j is the 

composite estimate for the ith subgroup in the jth replication of the procedure. 

The composite effectiveness estimates are transformed into log-odds ratios: 

L04,i,j = log(1 – E04,i,j) 

For a particular replication j, let s04,j be the standard deviation of the 10 estimates L04,i,j (where i 

ranges from 1 to 10) for the 10 mutually exclusive subgroups of ST_CASE endings. The 

standard deviation S04,j of the corresponding estimate of effectiveness expressed as a log-odds 

ratio in the full datasets (10 times as many cases) is S04,j = s04,j / √10. Let S04 be the median of 

the 11 estimates S04,j obtained in the 11 replications. 

Specifically, in the analysis of the effect of four-wheel ABS on all non-control-group crash 

involvements, the point estimate was a 1.10-percent increase. Expressed as a log-odds ratio: 

L = log(1 + .0110) = .0109 

S04 = .0365 

t = |L| / S04 = .30 

Because t is less than 1.833 (the 95th percentile of a t-distribution with 9 degrees of freedom58), 

the observed fatal-crash increase for four-wheel ABS is not statistically significant at the onesided .05 level. 

The 90 percent confidence bounds for the effect of increasing all the way from zero ABS to 100 

percent ABS are: 

1 – [1 - exp (L + 1.833 x S04)] = 1 – [1 - exp (.0109 + 1.833 x .0365)] 

= from an 8-percent increase to a 5 percent reduction 

58 Because each sj is calculated by computing the variance of 10 observations (log r for each of 10 subgroups), it is 

appropriate to use a t test with 9 df to test if log r is significantly different from zero. Caveat: the assumption that 

the fairly complex statistic L is t-distributed is more tenuous than the corresponding assumption in Section 2.1.5. 

44
 
 

The eighth column of Table 2-8 shows the t value for the effectiveness estimate of four-wheel 

ABS. The two right columns show the lower and upper 90 percent confidence bounds for the 

effect of four-wheel ABS – i.e., the shift from no ABS at all to 100 percent four-wheel ABS. 

Now let us discuss the point and interval estimates for all the crash types in Table 2-8. Positive 

estimates (reductions in fatal crashes, relative to the control group, with ABS) are shown in 

black, negative estimates in red. Statistically significant point estimates are printed bold, and 

they are shaded light-blue if positive, yellow if negative. Non-significant estimates are not 

printed bold, and have a negative lower confidence bound and a positive upper bound. 

The effect of four-wheel ABS on all crashes, including the control-group crashes where we 

assume it has no effect, is derived from the preceding estimate. Table 2-6 showed that, without 

any ABS, there were 3,577 control-group involvements and 8,722 response group involvements. 

The point estimate for all response-group crashes is a 1.10-percent increase. The effect of fourwheel ABS on all crashes is: 

[8,722 / (3,577 + 8,722)] x 1.10 = 0.78-percent increase 

Its 90 percent confidence bounds, similarly derived, are from -6 to +4 percent. 

Run-off-road crashes increased overall by a non-significant 6 percent (t = 1.27). The initial 4 

percent reduction associated with rear-wheel ABS was somewhat more than canceled after the 

shift from rear-wheel to four-wheel ABS, associated with a 10-percent increase. In fact all eight 

analyses for run-off-road crashes (the four on all roads and the four on wet, snowy, or icy roads) 

showed an increase for four-wheel relative to rear-wheel ABS. That is consistent with the results 

for passenger cars (Table 2-2), where all eight types of run-off-road were observed to increase 

with ABS. But for LTVs, seven of the eight increases are partly or wholly offset by an observed 

improvement for rear-wheel ABS, resulting in a zero or non-significant negative composite 

effect for four-wheel ABS relative to no ABS. Only for rollovers on wet, snowy, or icy roads, 

where the observed effect for rear-wheel ABS was also negative, does the composite effect 

amount to a statistically significant increase at the one-sided .05 level (but not at the two-sided 

.05 level, because t = 2.11 does not exceed 2.262, the 97.5th percentile of t with 9 df). 

Rear-wheel ABS significantly reduced collisions with pedestrians, bicyclists and animals. The 

shift from rear-wheel to four-wheel ABS has little or no additional effect. The composite effect 

is a statistically significant 14 percent reduction (t = 2.27). However, there is no reduction on 

wet, snowy, or icy roads. The observed effect on culpable involvements in collisions with other 

vehicles is negligible. 

2.3.3 Drinking versus non-drinking drivers 

As of April 2009, FARS has enough cases of LTVs with four-wheel ABS to compare its effect 

on run-off-road crashes for drinking and non-drinking drivers, as Harless and Hoffer did for 

passenger cars.59 Our analysis is identical to the procedure for cars in Section 2.1.7, except that 

it compares LTVs with a high proportion of four-wheel ABS to LTVs with primarily rear-wheel 

59 Harless & Hoffer (2002). 

45
 
ABS. When entirely separate analyses are performed for non-drinking and drinking drivers, 

including control groups drawn only from these drivers, the increase in run-off-road crashes is 

almost identical for the two groups (9% and 10%, respectively): 

LTVs with LTVs with Reduction 

With Separate 8% 4-wheel 96% 4-wheel for 4-Wheel 

Control Groups 92% rear-wheel 4% rear-wheel ABS (%) 

Non-Drinking Drivers 

Control group 3,344 3,185 

Run-off-road 1,734 1,799 - 9 

Drinking Drivers 

Control group 178 175 

Run-off-road 1,920 2,073 - 10 

When the same control group is used for the non-drinking and drinking drivers, namely the 

weighted counts for all cases as shown in Table 2-8, the effects on run-off-road crashes are 

likewise about the same: 

LTVs with LTVs with Reduction 

With Joint 8% 4-wheel 96% 4-wheel for 4-Wheel 

Control Group 92% rear-wheel 4% rear-wheel ABS (%) 

Non-Drinking Drivers 

Control group (from Table 2-8) 4,618 4,406 

Run-off-road 1,734 1,799 - 9 

Drinking Drivers 

Control group (from Table 2-8) 4,618 4,406 

Run-off-road 1,920 2,073 - 13 

These results confirm the findings of Section 2.1.7 that the long-term increase in run-off-road 

crashes is not just a drinking-driver problem. 

46
 
 

CHAPTER 3 

EFFECT OF ABS IN ALL CRASHES: ANALYSES OF 1995-2007 GES DATA 

3.0 Summary 

ABS reduces the overall crash-involvement rate by a statistically significant 6 percent in 

passenger cars (90% confidence bounds 4 to 8%) and by a statistically significant 8 percent in 

LTVs (90% confidence bounds 3 to 11%). These are substantial reductions, comparable, for 

example, to the overall effects of electronic stability control.60 For specific crash types, the 

effects of ABS in nonfatal crashes are not unlike the effects found in fatal crashes in Chapter 2: 

an increase in run-off-road crashes but a reduction in collisions with other vehicles, especially on 

wet roads. However, among nonfatal crashes, collisions with other vehicles far outnumber runoff-road crashes; the benefit of ABS in the former greatly overshadows any added harm in the 

latter. 

3.1 Analysis for passenger cars 

3.1.1 GES calendar-year range 

The General Estimates System of the National Automotive Sampling System is a probability 

sample of police-reported crash involvements in the United States61, drawn from 60 primary 

sampling units that are counties or groups of counties. When GES cases are weighted by the 

inverse sampling fractions, the weighted counts may ideally be used to generate unbiased 

estimates of national rates. “Ideally” – if there is no issue of missing data. However, 

information on the vehicle’s make and model is completely missing in 15 percent of the 

weighted cases, mostly concentrated in seven of the 60 PSUs. But 85 percent of the data and 53 

of 60 PSUs is still an acceptably “national” sample. On the other hand, limiting the data to 

vehicle cases with valid VINs and deriving the make and model from the VIN – the customary 

procedure in NHTSA evaluations – would have resulted in an excessive data loss of 28 percent 

of the cases, concentrated in 16 of the 60 PSUs. For the GES analyses, we will derive the make 

and model from the VIN if it is available; if not, we will accept the make and model specified on 

GES (which is usually quite accurate because it is derived from the State’s own VIN decode 

programs). 

As of April 2009, the GES database is complete through calendar year 2007. Only calendar 

years 1995-2007 are included in the analyses here, exactly as in the FARS analyses of Chapter 2. 

The goal is to study the “long-term” effect of ABS and not its “initial” effects. Calendar year 

1995 has been selected as the starting point for the data because it was also the starting point in 

Hertz’s 2000 report, the first to indicate that the initially deleterious effect of ABS on run-off60 Dang (2007), p. viii; the estimated overall reductions in police-reported crash involvements are 8 percent in
 

passenger cars and 10 percent in LTVs, respectively. 

61 The sample frame is the 48 contiguous States plus the District of Columbia. 

47
 
 

 

road crashes had become non-significant, and because consumer information on the correct use 

of ABS began to appear in 1995. 62 

The basic analysis approach is identical to the FARS analyses of Chapter 2: count the number of 

crash involvements of various types that might conceivably be influenced by ABS and compute 

their ratios to the number of control group involvements that are unlikely to be affected by ABS. 

The ratios are compared for cars with ABS and cars of the same makes and models without ABS 

– in the same calendar years. 

3.1.2 Control-group involvements versus response group 

The control group and the response group are essentially identical to those in Chapter 2 – with 

the FARS definitions translated to their equivalents in GES – and similar to the ones in 

NHTSA’s evaluation of electronic stability control.63 The control group consists of cars that had 

been stopped or parked before they were hit by another vehicle; moving less than 10 mph, 

backing up, parking, or leaving a parking space; struck in the rear while moving; or involved as 

non-culpable parties in collisions with other vehicles on dry surfaces. The GES codes for control 

group crashes include the following involvements in collisions with other vehicles (VEH_INVL 

≥ 2): 

	 Hit while stopped/parked: P_CRASH1 = 5, 7 or SPEED = 0 

	 Backing/parking/low-speed: P_CRASH1 = 4, 8, 9, 13 or SPEED = 1-10 

	 Struck in rear: IMPACT = 4, 13, 14 

	 Non-culpable involvements on dry roads: SUR_COND = 1; the following four variables 

do not have values that suggests this driver was culpable: 

o	 The “driver distraction” variable DR_DSTRD does not have values 1, 3, 4, 5, 6, 7, 8, 

9, 10, 11, 12, 13, 14, 97, or 98 

o	 The “contributing factor” variable FACTOR does not have values 1, 2, 3, 4, 5, 6, 7, 8, 

9, 10, 11, 12, 13, 14, 15, 97, or 98 

o	 The “critical event” variable P_CRASH2 does not have values 1, 3, 6, 8, 9, 10, or 11 

o	 The “violation” variable VIOLATN does not have values 1, 2, 3, 4, 5, 6, 7, 50, 97, or 

98 

All crash involvements that are not in the control group are in the response group. On roads that 

are not dry, even non-culpable involvements can be in the response group, because ABS is likely 

to benefit drivers in many situations and we do not want to “miss” a possible benefit just because 

GES classified a driver as non-culpable. In other words, on roads that are not dry, the control 

group is limited to the first three of the four categories listed above. Effectiveness of ABS 

(percent crash reduction relative to the control group) is computed for the response group as a 

whole, and also for the following subgroups – on all roads, and on roads that are not dry: 

62 Hertz (2000). 

63 Dang (2007). 

48
 
	 Run-off-road crashes: VEH_INVL = 1, but excluding EVENT1 = 2, 3, 8, 9, 10, 21, 22, 

23, 24, 26, or 27 (collisions with non-motorists, trains, or parked cars, plus various noncollisions that usually happen on, not off the road) 

o	 First-event rollovers: EVENT1 = 1 or (EVENT1 = 38 or 39 and V_EVENT=1) 

o	 Side impacts with fixed objects: IMPACT = 2, 3, 13, or 14 

o	 All other run-off-road crashes 

	 Collisions with pedestrians, bicyclists, other non-motorists, or animals: VEH_INVL = 1 

and EVENT1 = 21, 22, 24, or 27. Unlike FARS, where these are a substantial proportion 

of the fatal crashes and mostly involve pedestrians or bicyclists, in GES they are a small 

proportion of the nonfatal crashes and mostly involve animals. Effectiveness estimates 

will generally not be statistically meaningful and are computed mostly for comparison 

with the FARS results. 

	 Culpable involvements in collisions with other vehicles, including any frontal impact into 

the rear of another vehicle: 

o	 DR_DSTRD = 1, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 97, or 98, or 

o	 FACTOR = 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 97, or 98, or 

o	 P_CRASH2 = 1, 3, 6, 8, 9, 10, or 11, or 

o	 VIOLATN = 1, 2, 3, 4, 5, 6, 7, 50, 97, or 98, or 

o	 MAN_COL = 1 and IMPACT = 1, 11, or 12 (front-to-rear impact) 

3.1.3 Cars without ABS versus cars with ABS 

The selection of vehicle is the same as in the FARS analyses, as described in Section 3.1.2 and it 

includes the 60 makes and models listed in Table 2-1 for the same ranges of model years. For 

each make and model, two model years without ABS or with a low percentage of optional ABS 

are compared to two other model years with standard ABS or a high percentage of optional ABS. 

For most models, the shift is from low to high ABS, but for some (shown in red on Table 2-1) 

the shift is from high to low. These four model years should be consecutive, or with at most a 

one-year gap between the first two and the last two. The analyses include an adjustment factor, 

if necessary, to translate the observed effect of changing from a low percentage to a high 

percentage of ABS to the effectiveness of changing from 0 to 100 percent ABS. As in Section 

3.1.2, the calendar years included for each model range from 1995 or from the last of the four 

model years included for that model (whichever is later) through 2007. 

In the FARS analyses, all vehicles were identified by decoding the VIN. With GES, we will also 

consider vehicle records with a make and model coded but no VIN. That works for most of the 

entries in Table 2-1, because all of the vehicles with a specific make and model code are selected 

for the same model years. But for some entries in Table 2-1 vehicles with the same code may be 

selected for different model years, depending on the sub-series. For example, Cadillac DeVille, 

Fleetwood FWD, and Fleetwood Brougham all have MAKE = 19 and MODEL = 3 but are 

selected for different model years. It takes the VIN to identify which is which. For these 

models, the GES analysis is limited to cases with valid VINs. 

49
 
 

For the 60 makes and models combined, 6.8 percent of the cars in the low-ABS model-year 

range were equipped with ABS; 88.2 percent of the cars in the high-ABS model years had ABS. 

That is an 81.4 percentage-point gain in the share of cars equipped with ABS. 

The analysis file includes 62,998 GES cases of cars involved in crashes. In raw numbers, that is 

substantially more than the 38,251 FARS cases available for the analyses of Section 2.1. But 

GES is not a simple random sample of crashes. It is a cluster sample and its design effect will 

reduce the statistical power of the analyses to the equivalent of simple random sample that is not 

nearly as large. 

Unlike the FARS analyses, no adjustment or weighting of the data is needed to account for the 

fact that air bags were often introduced at more or less the same time as ABS. Air bags are not a 

crash-avoidance technology. When cars receive air bags, their probability of getting into a crash 

and onto GES is not diminished, even if the air bags might reduce some of the occupants’ 

injuries in that crash. 

3.1.4 Results 

Table 3-1 shows the actual and weighted case counts for each analysis of the effect of ABS in 

passenger cars, the effectiveness estimates, the significance-test results and the confidence 

bounds. Here is how the statistics are derived for the basic estimate of the effect of ABS in all 

non-control-group crashes on all roads. The 2x2 contingency table of actual case counts for the 

entire database is: 

Cars with Cars with 

7% ABS 88% ABS 

Control group involvements 14,478 16,489 

All non-control-group involvements 15,613 16,418 

Whereas the preceding table is of some value for indicating how many actual cases are included 

in the analysis, no meaningful effectiveness estimate can be derived from the cell counts. GES is 

a stratified cluster sample. For any kind of unbiased national estimate, each case needs to be 

weighted by the inverse of its national probability of selection. 

50
 
Table 3-1: Passenger Cars, Distribution of Crash Involvements With and Without 4-Wheel ABS, GES 

 Actual GES Weighted Crash t 

Case Counts N Reduction (%) for 

H0

Cars w Cars w Cars w Cars w 88% vs. 100% vs. Red

7% ABS 88% ABS 7% ABS 88% ABS 7% ABS no ABS = 0 

Control group involvements 14,478 16,489 1,802,565 2,114,616 

ALL ROADS All non-control-group involvements 15,613 16,418 1,789,041 1,878,852 10 13 4.9

All crash involvements 6 

Run-off-road crashes 3,125 3,725 319,209 376,924 -1 -1 .

Side impacts w fixed objects 630 847 70,029 95,275 -16 -20 2.1

 First-event rollovers 370 453 31,969 36,642 2 3 .2

Other run-off-road crashes 2,125 2,425 217,212 245,007 4 5 .9

Pedestrian/bicyclist/animal 1,218 1,393 114,830 142,900 - 6 - 8 .7

Culpable involvements w other veh 8,896 8,961 1,062,451 1,069,673 14 17 6.3

WET, SNOWY, OR ICY ROADS All non-control-group involvements 5,046 4,985 607,089 587,453 17 21 5.7

All crash involvements 16 

Run-off-road crashes 918 1,227 107,858 140,209 -11 -13 1.5

Side impacts w fixed objects 228 341 26,935 42,247 -34 -43 2.8

 First-event rollovers 87 130 8,642 11,117 -10 -12 .5

Other run-off-road crashes 603 756 72,281 86,845 -2 -3 .3

Pedestrian/bicyclist/animal 166 175 16,796 21,048 - 7 - 8 .2

Culpable involvements w other veh 1,946 1,657 241,359 195,464 31 37 6.8

51
 

The contingency table of weighted case counts is: 

Cars with 

7% ABS 

Cars with 

88% ABS 

Control group involvements 

All non-control-group involvements 

1,802,565 

1,789,041 

2,114,616 

1,878,852 

The observed effect of ABS is a substantial reduction of crash involvements: 

1 – [(1,878,852/2,114,616) / (1,789,041/1,802,565)] = 10.48 percent reduction 

That estimate is rounded to 10 percent and shown in the fifth column of the “all non-controlgroup crashes” row of Table 3-1. But that is the effect of increasing the market share of ABS 

from 6.8 percent to 88.2 percent. The effect of increasing all the way from zero ABS to 100 

percent ABS would be even higher: 

{1 – [(1,878,852/2,114,616) / (1,789,041/1,802,565)]} / {.882 – .068 x [(1,878,852/2,114,616) / (1,789,041/1,802,565)]} 

= 12.77 percent reduction 

That estimate rounds to 13 percent and it is shown in the sixth column of Table 3-1, the column 

headed by “100% versus no ABS.” It is the basic point estimate of the effect of ABS. 

Statistical significance of the crash-involvement reductions cannot be tested with simple 2x2 chisquares of the cell counts, because they are weighted counts from a stratified cluster sample, not 

actual numbers of cases from a simple random sample. The procedures for testing significance 

and estimating confidence bounds largely parallel the FARS procedures in Section 2.1.5, except 

that they take into account that GES, unlike FARS, is a cluster sample of 60 PSUs. The 60 PSUs 

are randomly split up into 10 subgroups of PSUs of approximately equal size – i.e., each 

subgroup containing close to 10 percent of the weighted cases of vehicles with known VIN 

and/or known codes for make and model. Thanks to the ample data available, effectiveness can 

be calculated separately in each subsample – i.e., each subgroup of PSUs – and significance 

tested by observing the variation of the estimate across the 10 subsamples. 

However, the variance estimate obtained by running through the procedure just once could be 

too high or too low by chance, depending on what PSUs happened to get into the 10 subsamples. 

A second iteration of the same procedure, but with the GES PSUs split up into subgroups in a 

different way, might generate a lower or higher estimate. Numerous iterations, each with a 

different splitting of the GES PSUs into subgroups, will generate a range of variance estimates, 

and the median of these estimates will be used. 

Specifically, each PSU has a two-digit identification number that is the value of the numeric 

variable PSU on the GES SAS file. The PSU identification numbers were randomly re-ordered 

by a SAS random-number generator and listed in the new order. The number of weighted 

vehicle cases (all vehicle types, all crash types – but with known VIN and/or make and model) 

during 1995-2007 was ascertained for each PSU and cumulated down the list. The list was 

52
 
 

 

 

parsed into consecutive (by the new PSU-ordering) groups of PSUs, each containing about 10 

percent of the cases, and these subgroups of PSUs were numbered 0 to 9. The GES cases from 

the PSUs in the first subgroup on the new list became subsample 1, the cases from the PSUs in 

the second subgroup became subsample 2, and so on. After these 10 subsamples were created, 

the 60 PSUs were randomly reordered anew and another set of 10 subsamples was created. In 

all, the procedure was repeated 11 times and it created 11 sets of 10 subsamples each. 

It is easier to work with the effect of increasing the market share of ABS from 6.8 percent to 88.2 

percent than with the effect of increasing all the way from zero ABS to 100 percent ABS. The 

former can be expressed as a log-odds-ratio, which tends to have a normal distribution. If the 

former effect is significant, the latter will be, too. 

Based on the weighted cell counts, the effect of increasing the market share of ABS from 6.8 

percent to 88.2, estimated for the entire GES database, may be expressed as a log-odds ratio: 

log r = log [(1,878,852/2,114,616) / (1,789,041/1,802,565)] = -.1107 

On the first of the 11 times that the GES PSUs were split into 10 subgroups, subgroup 1 

happened to consist of PSUs nos. 7, 21, 24, 25, 41, 43, 75, 77, and 79. For these cases, the 

corresponding log-odds ratio is: 

log r = log [(157,959/156,500) / (245,115/212,741)] = -.1324 

For subgroup 2, on the other hand, log r = -.1066; for subgroup 3, log r = -.1221; and so on. The 

standard deviation of 10 estimates for the 10 mutually exclusive subgroups of PSUs, each 

containing approximately a tenth of the GES data is .0425. The standard deviation S1 of the 

corresponding estimate of log r in the full dataset (10 times as many cases) is .0425 / √10 = 

.0134. This is the estimated standard deviation of log r, for the full dataset, based on our first 

run-through of splitting the GES into 10 subgroups. 

On the second iteration, the estimate of the standard deviation S2, derived from the 10 

subsamples created on that iteration, was slightly higher, .0180. On the third iteration, 

S3 = .0196. Over all 11 iterations, the estimates Sj ranged from .0120 to .0275. However, the 

median of these 11 estimates is S = .0222, which was the value obtained on the fifth iteration.64 

For the full dataset, log r was -.1107. Because t = |log r| / S = .1107 / .0222 = 4.99 is more than 

1.833 (the 95th percentile of a t-distribution with 9 degrees of freedom65), the observed crash 

reduction for ABS is statistically significant at the one-sided .05 level. In fact, because 4.99 is 

more than 3.250 (the 99.5th percentile of a t-distribution with 9 degrees of freedom), the observed 

crash reduction for ABS is statistically significant at the two-sided .01 level. 

The 90 percent confidence bounds for the effect of increasing all the way from zero ABS to 100 

percent ABS are: 

64 Even though we have 62,998 GES cases but only 38,251 FARS cases available for analyses, S is higher for the 

GES analysis (.0222) than for the FARS analysis (.0198). That reflects the design effect due to cluster sampling. 

65 Because each sj is calculated by computing the variance of 10 observations (log r for each of 10 subgroups), it is 

appropriate to use a t test with 9 df to test if log r is significantly different from zero. 

53
 
 

{1 – [1 - exp (log r + 1.833 x S)]} / {.882 – .068 x [1 - exp (log r + 1.833 x S)]} 

= {1 – [1 - exp (–.1107 + 1.833 x .0222)]} / {.882 – .068 x [1 - exp (–.1107 + 1.833 x .0222)]} 

= from an 8 percent to a 17 percent reduction 

The seventh column of Table 3-1 shows the t value for the effectiveness estimate of ABS. The 

two right columns show the lower and upper 90 percent confidence bounds for the effect of ABS 

– i.e., the shift from no ABS to 100% ABS. 

Now let us discuss the point and interval estimates for all the crash types in Table 3-1. Positive 

estimates (reductions in crash involvements with ABS, relative to the control group) are shown 

in black, negative estimates in red. Statistically significant point estimates are printed bold, and 

they are shaded light-blue if positive, yellow if negative. Non-significant estimates are not 

printed bold, and have a negative lower confidence bound and a positive upper bound. 

The effect of ABS on all crashes, including the control-group crashes where we assume it has no 

effect, is derived from the preceding estimate. Without ABS, there were 1,802,565 weighted 

control-group involvements and 1,789,041 response-group involvements. The point estimate for 

all response-group crashes is a 12.77 percent reduction. The point estimate for all crashes is: 

[1,789,041 / (1,802,565 + 1,789,041)] x 12.77 = 6.36 percent reduction 

Its 90 percent confidence bounds, similarly derived, are from 4 to 8 percent. That is indeed a 

substantial reduction of nonfatal crash involvements. For example, the corresponding estimate 

for electronic stability control is an 8-percent reduction.66 

But ABS is not beneficial in every type of crash. Side impacts with fixed objects increase by a 

statistically significant 20 percent on all roads (t = 2.14) and by a statistically significant 43 

percent on wet, snowy, or icy roads (t = 2.81). The two other types of run-off-road crashes show 

a non-significant reduction on all roads and a non-significant increase on wet, snowy, or icy 

roads. All run-off-road crashes are observed to increase by a non-significant 1 percent on all 

roads and a non-significant 13 percent on wet, snowy, or icy roads. In general, these results 

parallel the findings for fatal run-off-road crashes. But they are less important here, because runoff-road crashes are a smaller slice of the pie. 

Collisions with pedestrians, bicyclists, or animals (which at the nonfatal level are mostly 

collisions with animals and proportionately few in number) showed non-significant increases. 

The point estimates are not statistically meaningful, but they provide no corroboration for the 

significant reduction seen at the fatal level. 

The great benefit for ABS is prevention of culpable involvements with other vehicles. On all 

roads, the reduction is a statistically significant 17 percent (t = 6.39; confidence bounds, 13 to 

22%). On wet, snowy, or icy roads, where ABS is much more likely to activate, the reduction is 

a remarkable 37 percent (t = 6.82; confidence bounds, 29 to 45%). Here, ABS does exactly what 

it was designed for: enabling drivers to stop in time and under control, avoiding vehicles that 

66 Dang (2007), p. viii. 

54
 
 

 

encroach into their lane while not skidding into the paths of vehicles in other lanes. The results 

parallel the FARS analysis (a significant reduction on wet, snowy, or icy roads), except that they 

are substantially larger in magnitude and multi-vehicle crashes are a substantially larger slice of 

the pie. The reduction of multi-vehicle involvements far outweighs any increase in run-off-road 

crashes and produces a substantial net overall benefit, including a significant 21 percent 

reduction of all non-control group involvements on wet, snowy, or icy roads (t = 5.76; 

confidence bounds, 15 to 27%) and a 16 percent reduction of all involvements on these roads 

(confidence bounds, 11 to 21%).67 

3.1.5 Comparative results for four cohorts of cars 

Table 3-2 shows quite similar overall effects for ABS in four mutually exclusive cohorts of cars: 

(1) GM models that had a high percentage of ABS by 1993, (2) non-GM models that had a high 

percentage of ABS by 1993, (3) models that received a high share of ABS after 1993, and 

(4) models that dropped standard ABS, or where optional ABS lost substantial market share. 

TABLE 3-2 

OBSERVED EFFECT OF ABS BY CAR COHORT 

(Weighted GES data, 1995-2007) 

Cars with Cars with Reduction 

Low % ABS High % ABS for ABS (%) 

GM models with ABS by 1993 

Control group involvements 790,498 891,325 

Non-control-group 846,501 856,681 10 

Non-GM models with ABS by 1993 

Control group involvements 193,920 273,208 

Non-control-group 190,372 226,770 15 

Models receiving ABS after 1993 

Control group involvements 665,345 750,496 

Non-control-group 587,444 602,495 9 

Models dropping ABS 

Control group involvements 152,802 199,587 

Non-control-group 164,725 192,907 10 

67 Without ABS, there were 189,984 weighted control-group involvements and 607,089 response-group 

involvements on wet, snowy, or icy roads. 

55
 
Response-group crashes decreased in the first three cohorts after the cars received ABS, by 10 

percent, 15 percent, and 9 percent, respectively. Response-group crashes increased in the fourth 

cohort by 10 percent after the cars dropped ABS. The observed overall effect of ABS is robust 

and does not appear to vary greatly depending on the specific ABS technology in the vehicles or 

the specific makes and models involved. 

3.1.6 Net effect by specific road-surface condition 

Table 3-3 compares the net effects of ABS on dry, wet, snowy, and icy roads. The observed net 

effect is 10 percent on dry roads and 19 percent on wet roads. There are not enough cases for 

statistically precise results on snowy or icy roads, but the observed reductions are similar to those 

on dry and wet roads. 

TABLE 3-3 

NET EFFECT OF ABS, BY ROAD-SURFACE CONDITION 

Weighted N of Crash Involvements 

Cars w Cars w Reduction 

7% ABS 88% ABS for ABS (%) 

Control group involvements 1,802,565 2,114,616 

All non-control-group involvements on: 

Dry roads 1,158,091 1,261,393 10 

Wet roads 472,769 448,776 19 

Snowy or slushy roads 70,683 70,265 15 

Icy roads 63,638 68,412 8 

3.1.7 Net effect on injury crashes 

Approximately one-third of the crashes on GES injured at least one of the people involved in the 

crash (occupants of any of the vehicles or non-motorists). When the basic analysis is limited to 

vehicles involved in crashes that injured somebody (possibly but not necessarily an occupant of 

that vehicle), the observed reduction in response-group involvements is 11 percent: 

Cars w Cars w Reduction 

7% ABS 88% ABS for ABS (%) 

Control group involvements 613,540 688,295 

All non-control-group involvements 613,925 613,269 11 

56
 
That is nearly identical to the 10 percent reduction observed in all GES crashes (fifth column of 

Table 3-1). It demonstrates ABS is effective in preventing nonfatal-injury crashes (as opposed to 

fatal crashes), and that the reduction is about the same as for all crashes. 

3.2 Analysis of rear-wheel ABS for LTVs 

Almost all domestic pickup trucks, most domestic SUVs, and many vans and foreign-basednameplate LTVs received ABS for the rear wheels only in the late 1980s or early 1990s. Fourwheel ABS usually did not arrive on LTVs until at least several model years later. The analysis 

of rear-wheel ABS for LTVs closely parallels the procedures described in Section 3.1. It, too, is 

based on GES data for calendar years 1995 to 2007. The control group and response group of 

crash involvements are defined exactly as in Section 3.1.2. The control group consists of LTVs 

that were stopped, parked, backing, moving slower than 10 mph, parking, leaving a parking 

place, were struck in the rear, or were non-culpable parties to a collision involving two or more 

vehicles. 

The 31 makes and models included in the analysis and the range of model years for each model 

are listed in Table 2-5 of the preceding chapter. In the FARS analyses, all vehicles were 

identified by decoding the VIN. With GES, we will also consider vehicle records with a make 

and model coded but no VIN. That works for almost every model in Table 2-5, because all of 

the vehicles with a specific make and model code are selected for the same model years. But for 

the last model in Table 2-5, the Suzuki Sidekick, where the analysis is limited to the VINidentifiable two-door sub-series, the GES analysis is limited to cases with valid VINs. 

All models in Table 2-5 except the Nissan pickup have model-year ranges that end before 1995 

and all GES cases from CY 1995 to 2007 will be included for those models. For the Nissan 

pickup, the model year range extends to 1996; only GES data from 1996 to 2007 will be 

included. For the 31 makes and models combined, 1.2 percent of the LTVs in the low-ABS 

model-year range were equipped with ABS; 100 percent of the LTVs in the high-ABS model 

years had rear-wheel ABS. That is a 98.8 percentage-point gain in the share of LTVs equipped 

with rear-wheel ABS. 

The analysis file includes 29,030 GES cases of LTVs involved in crashes. That is slightly more 

than the 26,324 FARS cases available for the analyses of Section 2.2. But GES being a cluster 

sample will reduce the statistical power of the analyses as compared to the FARS results. 

3.2.1 Results 

Table 3-4 shows the actual case counts for each analysis of the effect of rear-wheel ABS in 

LTVs, the effectiveness estimates, the significance-test results and the confidence bounds. Here 

is how the statistics are derived for the basic estimate of the effect of ABS in all non-controlgroup crashes on all roads. 

57
 
Table 3-4: LTVs, Distribution of Crash Involvements With and Without Rear-Wheel ABS, GES 19

 Actual GES Weighted Crash t 

Case Counts N Reduction (%) for 

H0

LTVs w LTVs w LTVs w LTVs w 100% vs. 100% vs. Red

1% ABS 100% ABS 1% ABS 100% ABS 1% ABS no ABS = 0 

Control group involvements 6,260 7,465 813,267 940,476 

ALL ROADS All non-control-group involvements 6,971 8,334 847,914 1,006,270 1 1 .2

All crash involvements none 

Run-off-road crashes 1,825 2,046 189,089 206,140 9 9 1.6

Side impacts w fixed objects 336 368 41,439 42,133 15 15 1.3

 First-event rollovers 425 416 37,383 37,253 16 17 1.7

Other run-off-road crashes 1,065 1,262 110,267 126,753 4 4 .5

Pedestrian/bicyclist/animal 493 616 48,448 67,617 -17 -17 1.4

Culpable involvements w other veh 3,612 4,384 459,707 551,680 -1 -1 .

WET, SNOWY, OR ICY ROADS All non-control-group involvements 2,298 2,727 296,705 350,198 1 1 .2All crash involvements 1 Run-off-road crashes 590 641 69,812 74,109 11 11 1.

Side impacts w fixed objects 144 156 19,373 19,694 15 15 1.0

 First-event rollovers 112 116 10,454 11,597 7 7 .3

Other run-off-road crashes 334 369 39,885 42,818 10 10 .8

Pedestrian/bicyclist/animal 67 59 8,233 7,974 19 19 .6

Culpable involvements w other veh 844 985 111,512 132,713 none none .0

58
 

 

The 2x2 contingency table of actual case counts for the entire database is: 

LTVs with 

1% ABS 

LTVs with 

100% ABS 

Control group involvements 

All non-control-group involvements 

6,260 

6,971 

7,465 

8,334 

Whereas the preceding table is of some value for indicating how many actual cases are included 

in the analysis, no meaningful effectiveness estimate can be derived from the cell counts. GES is 

a stratified cluster sample. For any kind of unbiased national estimate, each case needs to be 

weighted by the inverse of its national probability of selection. 

The contingency table of weighted case counts is: 

LTVs with 

1% ABS 

LTVs with 

100% ABS 

Control group involvements 813,267 940,476 

All non-control-group involvements 847,914 1,006,270 

The observed effect of ABS is a negligible reduction of crash involvements: 

1 – [(1,006,270/940,476) / (847,914/813,267)] = 0.55 percent reduction 

That estimate is rounded to 1 percent and shown in the fifth column of the “all non-control-group 

crashes” row of Table 3-4. But that is the effect of increasing the market share of rear-wheel 

ABS from 1.2 percent to 100 percent. The effect of increasing all the way from zero ABS to 100 

percent ABS would be marginally higher: 

{1 – [(1,006,270/940,476) / (847,914/813,267)]} / {1 – .012 x [(1,006,270/940,476) / (847,914/813,267)]} 

= 0.56 percent reduction 

That estimate again rounds to 1 percent and it is shown in the sixth column of Table 3-4, the 

column headed by “100% versus no ABS.” It is the basic point estimate of the effect of rearwheel ABS. 

The t-tests for statistical significance of the point estimates and the estimation of 90 percent 

confidence bounds are based on the same procedure as for passenger cars (Section 3.1.4). The t 

value for the preceding point estimate is .20. Because that is less than 1.833 (the 95th percentile 

of a t-distribution with 9 degrees of freedom68), the observed crash reduction for rear-wheel ABS 

is not statistically significant at the one-sided .05 level. The 90 percent confidence bounds for 

68 See Section 3.1.4. 

59
 
effectiveness range from a 5-percent increase to a 5 percent reduction in crash involvements. 

The t values and confidence bounds are shown in the last three columns of Table 3-4. 

Now let us discuss the point and interval estimates for all the crash types in Table 3-4. Positive 

estimates (reductions in fatal crashes with ABS, relative to the control group) are shown in black, 

negative estimates in red. None of the point estimates is statistically significant; if any had been, 

they would have been printed bold and shaded light-blue if positive, yellow if negative. 

The observed effect of rear-wheel ABS on all crashes, including the control-group crashes where 

we assume it has no effect, is: 

[847,914 / (813,267 + 847,914)] x 0.56 = 0.28 percent reduction (rounded to zero%) 

Its 90 percent confidence bounds, similarly derived, are from -2 to +3 percent. 

Each of the eight observed effects on run-off-road crashes is positive: overall, and for each of the 

three subgroups of run-off-road crashes; on all roads as well as on wet, snowy, or icy roads. 

Observed reductions range from 4 to 16 percent. But none are statistically significant. The 

findings are consistent with the mildly positive results in the FARS analysis. 

Results for collisions with animals and non-motorists conflict: a 17-percent increase on all roads 

but a 19 percent reduction on wet, snowy, or icy roads. Based on small numbers of cases, the 

estimates are not statistically meaningful. The observed effects on culpable involvements with 

other vehicles are close to zero. 

3.3 Analysis of four-wheel ABS for LTVs 

Most makes and models of LTVs that were eventually equipped with four-wheel ABS first went 

through an intermediate stage of only rear-wheel ABS, typically for at least several years. As in 

Section 2.3, the effect of four-wheel ABS is estimated in two essentially separate steps: as the 

composite of the effect of switching from no ABS to rear-wheel ABS (already estimated in 

Section 3.2) and the effect of switching from rear-wheel to four-wheel ABS. 

The analysis of switching from rear-wheel to four-wheel ABS for LTVs closely parallels the 

ABS analysis for passenger cars (Section 3.1). It is based on GES data for calendar years 1995 

to 2007. The control group and response group are defined the same way. Significance testing 

and estimation of confidence bounds parallels the FARS analysis (Section 2.3). 

The 29 makes and models included in the analysis and the range of model years for each model 

are listed in Table 2-7 of the preceding chapter. In the FARS analyses, all vehicles were 

identified by decoding the VIN. With GES, we will also consider vehicle records with a make 

and model coded but no VIN. However, when two sub-series with the same model code are 

included for different model years (and the identification of the sub-series is based on the VIN), 

such as the Chevrolet Astro cargo van and wagon, the GES analysis is limited to cases with valid 

VINs. 

60
 
For the models in Table 2-7 with model-year ranges that end before 1995, all GES cases from 

CY 1995 to 2007 are included. For the other models, GES data are included beginning only with 

the last model year in the range. For the 29 makes and models combined, 7.7 percent of the 

LTVs in the two earlier model years were equipped with four-wheel ABS (and 92.3 percent with 

rear-wheel ABS); 96.3 percent of the LTVs in the two later model years had four-wheel ABS 

(and 3.7 percent had rear-wheel ABS). That is an 88.6 percentage-point gain in the share of 

LTVs equipped with four-wheel ABS. 

The analysis file includes 37,142 GES cases of LTVs involved in crashes. That is more than the 

28,952 FARS cases available for the analyses of Section 2.3. But GES being a cluster sample 

will reduce the statistical power of the analyses as compared to the FARS results. 

3.3.1 Results 

Table 3-5 shows the actual and weighted case counts for each analysis of the effect of four-wheel 

ABS in LTVs, the separate and composite effectiveness estimates, the significance-test results 

and the confidence bounds. Here is how the statistics are derived for the basic estimate of the 

effect of four-wheel ABS, relative to no ABS at all, in all non-control-group crashes on all roads. 

The 2x2 contingency table of actual case counts for the entire database is: 

LTVs with 

8% 4-wheel 

92% rear-wheel 

LTVs with 

96% 4-wheel 

4% rear-wheel 

Control group involvements 

All non-control-group involvements 

8,834 

9,593 

9,415 

9,300 

Whereas the preceding table is of some value for indicating how many actual cases are included 

in the analysis, no meaningful effectiveness estimate can be derived from the cell counts. GES is 

a stratified cluster sample. For any kind of unbiased national estimate, each case needs to be 

weighted by the inverse of its national probability of selection. 

The contingency table of weighted case counts is: 

61
 

LTVs with 

8% 4-wheel 

92% rear-wheel 

LTVs with 

96% 4-wheel 

4% rear-wheel 

Control group involvements 1,107,216 1,198,921 

All non-control-group involvements 1,124,731 1,061,334 
Table 3-5: LTVs, Distribution of Crash Involvements With and Without Four-Wheel ABS, GES 19

 Actual GES Weighted Crash 

Case Counts N Reduction (%) 

LTVs w LTVs w LTVs w LTVs w 96/4. Rear-whl 4-wheel 

8% 4-whl 96% 4-whl 8% 4-whl 96% 4-whl vs. vs. vs. 

92% rear 4% rear 92% rear 4% rear 8/92 no ABS no ABS 

Control group involvements 8,834 9,415 1,107,216 1,198,921 

ALL ROADS All non-control-group involvements 9,593 9,300 1,124,731 1,061,334 13 1 15 

All crash involvements 8 

Run-off-road crashes 2,341 2,500 228,394 242,289 2 9 11 

Side impacts w fixed objects 363 488 41,006 55,178 -24 15 -9 

First-event rollovers 554 641 45,711 49,325 none 17 17 

Other run-off-road crashes 1,424 1,371 141,677 137,786 10 4 15 

Pedestrian/bicyclist/animal 666 768 67,372 86,132 -18 -17 -42 Culpable involvements w other veh 5,275 4,838 657,972 579,524 19 -1 20 

WET, SNOWY, OR ICY ROADS All non-control-group involvements 3,048 2,862 370,759 336,386 16 1 19 

All crash involvements 14 

Run-off-road crashes 818 973 92,893 108,351 -8 11 3 

Side impacts w fixed objects 156 246 21,294 30,037 -30 15 -15 

First-event rollovers 185 209 17,261 18,862 -1 7 6 Other run-off-road crashes 477 518 54,337 59,452 -1 10 9 

Pedestrian/bicyclist/animal 73 98 7,563 10,736 -31 19 -10 

Culpable involvements w other veh 1,136 861 149,478 108,492 33 none 36 

62
 

The observed effect of increased installations of four-wheel ABS is: 

1 – [(1,061,334/1,198,921) / (1,124,731/1,107,216)] = 12.85 percent reduction 

That estimate is rounded to 13 percent and shown in the fifth column of the “all non-controlgroup crashes” row of Table 3-5. But that is the effect of increasing the market share of fourwheel ABS from 7.7 percent to 96.3 percent. The effect of increasing all the way from 100 

percent rear-wheel ABS to 100 percent four-wheel ABS would be even more beneficial: 

{1 – [(1,061,334/1,198,921) / (1,124,731/1,107,216)]} / {.963 – .077 x [(1,061,334/1,198,921) / (1,124,731/1,107,216)]} 

= 14.35 percent reduction 

But that is the estimated effect of shifting from rear-wheel to four-wheel ABS. The effect of 

shifting from no ABS at all to rear-wheel ABS has already been estimated in Section 3.2.1 to be 

a negligible 0.56 percent reduction of crash involvements. It is rounded to 1 and shown in the 

sixth column of Table 3-5. (The sixth column of Table 3-5 recapitulates the sixth column of 

Table 3-4, the principal point estimates for rear-wheel ABS relative to no ABS.) The net effect 

of shifting from no ABS to four-wheel ABS is the composite of the two stepwise effects: 

1 – [(1 – .0056) x (1 – .1435)] = 14.83 percent reduction 

It is rounded to 15 percent and shown in the seventh column of Table 3-5. It is the principal 

point estimate for the effect of four-wheel ABS relative to no ABS. 

Significance testing of this effect and estimation of its 90 percent confidence bounds combines 

the computational procedure used on FARS in Section 2.3.2 with the GES subsamples selected 

in the GES analysis for passenger cars (Section 3.1.4). In that analysis, the GES PSUs were 

randomly re-ordered. The list of the PSUs in the new order was split up into 10 subgroups of 

approximately equal size. That procedure was replicated 11 times. In fact, the same subgroups 

of PSUs used in Section 3.1.4 are re-used here. 

For example, on the first of the 11 times that GES PSUs were split into 10 subgroups, subgroup 1 

happened to consist of PSUs nos. 7, 21, 24, 25, 41, 43, 75, 77, and 79. Using the GES database 

created in Section 3.2., but limited to PSUs nos. 7, 21, 24, 25, 41, 43, 75, 77, and 79, we estimate 

the effect E0R,1,1 of shifting from no ABS at all to 100 percent rear-wheel ABS (where 0R means 

“from no ABS to rear-wheel ABS,” 1 is subgroup 1, 1 is replication 1 of the procedure). Using 

the current GES database and again limiting to PSUs nos. 7, 21, 24, 25, 41, 43, 75, 77, and 79, 

we estimate the effect ER4,1,1 of shifting from 100 percent rear-wheel ABS to 100 percent fourwheel ABS (where R4 means “from rear-wheel to four-wheel ABS”). The composite effect is: 

E04,1,1 = 1 – [(1 – E0R,1,1) x (1 – ER4,1,1)] 

is the estimate of shifting from no ABS to 100 percent four-wheel ABS, based solely on the GES 

cases from PSUs nos. 7, 21, 24, 25, 41, 43, 75, 77, and 79. Similarly, E04,i,j is the composite 

estimate for the ith subgroup in the jth replication of the procedure. 

The composite effectiveness estimates are transformed into log-odds ratios: 

63
 
 

 

 

L04,i,j = log(1 – E04,i,j) 

For a particular replication j, let s04,j be the standard deviation of the 10 estimates L04,i,j (where i 

ranges from 1 to 10) for the 10 mutually exclusive subgroups of PSUs. The standard deviation 

S04,j of the corresponding estimate of effectiveness expressed as a log-odds ratio in the full 

datasets (10 times as many cases) is S04,j = s04,j / √10. Let S04 be the median of the 11 estimates 

S04,j obtained in the 11 replications. 

Specifically, in the analysis of the effect of four-wheel ABS on all non-control-group crash 

involvements, the point estimate was a 14.83 percent reduction. Expressed as a log-odds ratio: 

L = log(1 – .1483) = -.1605 

S04 = .0512 

t = |L| / S04 = 3.13 

Because t is greater than 1.833 (the 95th percentile of a t-distribution with 9 degrees of 

freedom69), the observed crash reduction for four-wheel ABS is statistically significant at the 

one-sided .05 level. In fact, because t is greater than 2.821 (the 99th percentile of a t-distribution 

with 9 df), the observed crash reduction for four-wheel ABS is statistically significant at the onesided .01 level. 

The 90 percent confidence bounds for the effect of increasing all the way from zero ABS to 100 

percent ABS are: 

1 – [1 - exp (L + 1.833 x S04)] = 1 – [1 - exp (-.1483 + 1.833 x .0512)] 

= from a 6 percent to a 22 percent reduction 

The eighth column of Table 3-5 shows the t value for the effectiveness estimate of four-wheel 

ABS. The two right columns show the lower and upper 90 percent confidence bounds for the 

effect of four-wheel ABS – i.e., the shift from no ABS at all to 100 percent four-wheel ABS. 

Now let us discuss the point and interval estimates for all the crash types in Table 3-5. Positive 

estimates (reductions in fatal crashes, relative to the control group, with ABS) are shown in 

black, negative estimates in red. Statistically significant point estimates are printed bold, and 

they are shaded light-blue if positive, yellow if negative. Non-significant estimates are not 

printed bold, and have a negative lower confidence bound and a positive upper bound. 

The effect of four-wheel ABS on all crashes, including the control-group crashes where we 

assume it has no effect, is derived from the preceding estimate. Table 3-4 showed that, without 

any ABS, there were 813,267 control-group involvements and 847,914 response group 

involvements. The point estimate for all response-group crashes is a 14.83 percent reduction. 

The effect of four-wheel ABS on all crashes is: 

69 Because each sj is calculated by computing the variance of 10 observations (log r for each of 10 subgroups), it is 

appropriate to use a t test with 9 df to test if log r is significantly different from zero. Caveat: the assumption that 

the fairly complex statistic L is t-distributed is more tenuous than the corresponding assumption in Section 3.1.4. 

64
 
 

[847,914 / (813,267 + 847,914)] x 14.83 = 7.57 percent reduction 

Its 90 percent confidence bounds, similarly derived, are from 3 to 11 percent. 

Run-off-road crashes decreased overall by a non-significant 11 percent (t = 1.52). The initial 9-

percent reduction associated with rear-wheel ABS was augmented by an additional 2 percent 

reduction after the shift from rear-wheel to four-wheel ABS. A non-significant increase in side 

impacts with fixed objects was more than offset by reductions in rollovers and other run-off-road 

crashes. The results on wet, snowy, or icy roads are similar. 

Collisions with animals or non-motorists increased by 42 percent, statistically significant at the 

one-sided but not the two-sided .05 level.70 But on wet, snowy, or icy roads, the observed effect 

was only a non-significant 10-percent increase. It is not clear how statistically meaningful these 

findings are, but they provide no corroboration and perhaps even raise a question about the 

statistically significant reductions observed in fatal collisions with pedestrians, bicyclists, and 

animals. 

The great benefit for ABS, for LTVs just as for passenger cars is prevention of culpable 

involvements with other vehicles. On all roads, the reduction is a statistically significant 20 

percent (t = 4.32; confidence bounds, 12 to 28%). On wet, snowy, or icy roads, where ABS is 

much more likely to activate, the reduction is a remarkable 36 percent (t = 4.79; confidence 

bounds, 24 to 46%). Here, ABS evidently enables drivers to stop in time and under control, 

avoiding vehicles in their own lane while not invading other lanes. The overall reduction of noncontrol group involvements on wet, snowy, or icy roads is a significant 19 percent (t = 3.10; 

confidence bounds, 8 to 28%) and the reduction of all crashes on these roads is 14 percent 

(confidence bounds, 6 to 22%). 

The five statistically significant reductions in Table 3-5, 15, 8, 20, 19, and 36 percent are almost 

identical to the corresponding significant crash reduction for passenger cars in Table 3-1: 13, 6, 

17, 21, and 37 percent. This shows ABS effectiveness is consistent across the two types of 

passenger vehicles, cars and LTVs. 

70 t = 2.11; t must exceed 1.833 for significance at the one-sided .05 level and 2.262 for significance at the two-sided 

.05 level. 

65
 
 

 

CHAPTER 4 

DISCUSSION 

4.1 Summary and comparison of effectiveness estimates 

Table 4-1 summarizes the effectiveness estimates for four-wheel ABS – the findings of Chapters 

2 and 3 – in fatal crashes and in all crashes; for passenger cars versus LTVs; on all roads and on 

wet, snowy, or icy roads; by type of crash. 

TABLE 4-1: ESTIMATED CRASH REDUCTION (%) BY FOUR-WHEEL ABS, 1995-2007 

[Reduction, Increase, Significant Reduction (one-sided p < .05), Significant Increase] 

 Wet, Snowy,

All Roads or Icy Roads 

Cars LTVs Cars LTVs 

FATAL CRASH INVOLVEMENTS 

All fatal involvements 1 - 1 - 1 - 6 

All non-control-group involvements 1 - 1 - 1 - 6 

All run-off-road crashes - 9 - 6 - 34 - 10 

Side impacts with fixed objects - 30 none - 85 - 4

 First-event rollovers - 11 - 10 - 52 - 31 

All other run-off-road crashes - 3 - 5 - 17 - 3 

Pedestrian/bicyclist/animal 13 14 none - 14 

Culpable involvements w other veh 4 - 1 12 - 6 

ALL CRASH INVOLVEMENTS 

All crash involvements 6 8 16 14 

All non-control-group involvements 13 15 21 19 

All run-off-road crashes - 1 11 - 13 3 

Side impacts with fixed objects - 20 - 9 - 43 - 15

 First-event rollovers 3 17 - 12 6 

All other run-off-road crashes 5 15 - 3 9 

Pedestrian/bicyclist/animal - 8 - 42 - 8 - 10 

Culpable involvements w other veh 17 20 37 36 

66
 
Effects are largely consistent across vehicle types, severity levels, and roadway conditions. 

	 The overall effect of ABS on fatal crash involvements was close to zero in both cars 

(observed 1% reduction) and LTVs (observed 1% increase). 

	 The overall effect of ABS on nonfatal crash involvements was beneficial and statistically 

significant in both cars (observed 6% reduction) and LTVs (observed 8% reduction). 

	 The observed overall effect on run-off-road crashes is always negative for cars and 

negative at the fatal level for LTVs. It is negative and significant for car fatalities. It is 

consistently more negative on wet, snowy, or icy roads, where ABS is most likely to 

activate, than on all roads. 

o	 ABS has an observed negative effect on every subgroup of run-off-road crashes for 

passenger cars at the fatal level (all roads and wet-snowy-icy roads) and on wet, 

snowy, or icy roads at the nonfatal level. 

o	 On wet, snowy, or icy roads, every subgroup of fatal run-off-road crashes increased 

significantly for passenger cars. Every subgroup also increased for LTVs, but only 

one of them significantly (rollovers). 

o	 Run-off-road crashes on wet, snowy, or icy roads increased with ABS in each of four 

separate cohorts of cars: GM models that had received ABS by 1993, non-GM cars 

that had received ABS by 1993, models that received ABS after 1993, and models 

that dropped standard ABS (see Table 2-3). 

o	 The net effects on run-off-road crashes are less unfavorable in LTVs because the 

initial shift from no ABS to rear-wheel ABS was generally beneficial. But the 

subsequent shift from rear-wheel to four-wheel ABS usually increased run-off-road 

crashes in LTVs, including every type of fatal run-off-road crash and every type on 

wet, snowy, or icy roads (see Tables 2-8 and 3-5). 

	 The increase was always greater in side impacts with fixed objects than in other types of 

run-off-road crashes, except for LTV fatalities. All four observed effects for passenger 

cars on side impacts with fixed objects were negative and significant. Side impacts with 

fixed objects appear to epitomize the negative effect of ABS on run-off-road crashes. 

o	 Side impacts with fixed objects increased with ABS in each of the four separate 

cohorts of cars. 

	 Results were least consistent for collisions with pedestrians, bicyclists, and animals. The 

overall effect on fatal crashes was consistent between cars (significant 13% reduction) 

and LTVs (significant 14% reduction). But the observed effects on wet, snowy, or icy 

roads and on nonfatal crashes (mostly collisions with animals) were not positive. 

	 The effect on culpable involvements with other vehicles is consistently positive, except 

for LTVs at the fatal level. Five of the eight observed effects are a statistically significant 

benefit. 

o	 The effect on nonfatal culpable involvements with other vehicles is nearly identical 

for cars and LTVs; it is always significant and positive. 

67
 
 

 

	 The effect on nonfatal culpable involvements with other vehicles on wet, snowy or icy 

roads is a remarkable 37 percent reduction in cars and 36 percent in LTVs. Here is where 

ABS is the most effective. 

4.2 Why is the effect still negative in run-off-road crashes? 

It is unknown why run-off-road crashes are still increasing with ABS. But the 9-percent increase 

of fatal run-off-road crashes in passenger cars is statistically significant, and so is the 34-percent 

increase on wet, snowy and icy roads. Table 4-1 also shows four significant increases in side 

impacts with fixed objects and two significant increases in first-event rollovers. 

It is important to note, however, that the increase is not nearly as large as in the early years of 

ABS. NHTSA’s analysis of CY 1989 to 1993 FARS data and the Insurance Institute for 

Highway Safety’s analysis of CY 1993 to 1995 FARS data each showed a 28-percent increase of 

fatal run-off-road crashes for passenger cars with ABS.71 This study shows only a 9-percent 

increase in FARS data from 1995 to 2007; furthermore, the increase during 1995 to 2007 is 

similar in the older and more recent cohorts of cars – i.e., the age or design of the car does not 

appear to be the issue, since relatively new cars with the latest ABS technology are showing 

about the same increases as the older cars with first-generation ABS. That suggests the 

substantially higher increase before 1995 may be related to drivers’ lack of understanding how to 

use ABS. The safety community’s effort to inform the public about ABS and the simple passage 

of time and spread of the technology may have substantially attenuated but not eliminated the 

increase in run-off-road crashes. Maybe the effect is still negative simply because a portion of 

the public still doesn’t know how to use ABS, or has had little experience in applying that 

knowledge because ABS is activated rather infrequently over the life of a vehicle. 

At this time there is little or no empirical support for some of the other hypotheses. Research 

projects conducted after 1995, using a driving simulator or tracking the actual maneuvers of 

volunteer drivers on the road (see Section 1.5), found little evidence that drivers engaged in 

evasive maneuvers that, with ABS, resulted in running off the road. These projects likewise 

found little or no evidence that drivers became more aggressive because their vehicles are 

equipped with ABS. There have been occasional performance issues with some ABS systems on 

unusual road surfaces, but no link between these issues and fatal run-off-road crashes. 

Side impacts with fixed objects have consistently increased more with ABS than other types of 

run-off-road crashes, especially on wet roads (85% fatality increase in cars). A distinctive 

feature of side impacts with fixed objects is that the vehicle was probably yawing rather than 

strictly rolling forward prior to impact.72 ABS can prevent yawing associated with lockup of the 

rear wheels, but it will not necessarily prevent yawing due to steering input that results in the loss 

of tire adhesion. Whereas ABS theoretically preserves steering control while braking, the 

amount of permissible steering input may be quite limited on a wet road when much of the tire 

adhesion is already consumed by braking. Even moderate steering – well short of wild evasive 

maneuvers – may initiate a yaw (oversteering). An electronic stability control (ESC) system 

71 Kahane (1994), p. 95; Farmer et al (1996). 

72 This may not always be true, as impacts to the front of the vehicle near the corner may sometimes be reported as 

side impacts. 

68
 
 

 

would take immediate, automatic action to counteract the yaw, but ABS alone, without ESC, 

usually cannot do so. The persistent increase in run-off-road crashes may to some extent reflect 

that ABS is an incomplete crash-avoidance technology; ESC makes it a more complete system. 

4.3 Combined effect of four-wheel ABS and ESC 

FMVSS No. 126 will require ESC in all new cars and LTVs after September 1, 2011. All ESCequipped vehicles to date have also been equipped with four-wheel ABS. As explained in 

Section 1.7, that is unlikely to change in the future, because ESC essentially incorporates the 

functions and components of ABS. In model year 2005, just before SAFETEA-LU73 triggered 

the ESC rulemaking process, approximately 40 percent of new cars and 10 percent of new LTVs 

were not yet equipped with ABS. In a sense, these models will be receiving a “package” of ABS 

and ESC. 

What will be the combined effect of the technologies on crashes in general and on run-off-road 

crashes in particular? NHTSA’s 2007 evaluation estimated crash reductions for ESC with ABS 

relative to vehicles with ABS alone, based on statistical analyses of crash data through CY 

2004.74 Table 4-2 shows the effect on fatal involvements of ABS alone, the incremental effect of 

ESC in a vehicle equipped with ABS, and the combined effect of ESC plus ABS relative to a 

vehicle equipped with neither. For example, if the observed effective of ABS on all fatal 

involvements of cars is a 1 percent reduction and the effect of ESC is a 14 percent reduction, the 

combined effect is a 1 - [(1 - .01) x (1 - .14)] = 15 percent reduction. 

The combined effect is always a substantial fatality reduction. Even for run-off-road crashes, the 

negative effect of ABS is dwarfed by the benefit of ESC. According to Table 4-2, the 

combination of ABS and ESC will reduce fatal run-off-road crashes by an estimated 30 percent 

in passenger cars and by 68 percent in LTVs. One caveat on Table 4-2 is that these are early 

estimates of ESC effectiveness, based on the crash experience of the models equipped with ESC 

by 2004; NHTSA plans follow-up statistical analyses to estimate the long-term effectiveness of 

ESC, analogous to this report’s numbers for ABS.75 

73 Public Law 109-59, August 10, 2005. 74 Dang (2007), pp. vii and viii. 

75 Allen et al. (2008), p. 16. 

69
 
TABLE 4-2 

COMBINED EFFECT OF FOUR-WHEEL ABS AND ESC ON FATAL CRASHES 

Percent Reduction of Fatal Involvements 

 Four-Wheel ABS ESC Combined 

Passenger cars 

All fatal involvements 1 14 15 

All run-off-road crashes – 9 36 30 

Culpable involvements w other veh 4 19 22 

LTVs 

 All fatal involvements – 1 28 27 

All run-off-road crashes – 6 70 68 

Culpable involvements w other veh – 1 34 33 

Table 4-3 estimates the combined effect of ABS and ESC on all police-reported crashes: an 

overall crash reduction of 14 percent in passenger cars and 17 percent in LTVs. 

TABLE 4-3 

COMBINED EFFECT OF FOUR-WHEEL ABS AND ESC ON ALL CRASHES 

70
 

Percent Reduction of Fatal Involvements 

 Four-Wheel ABS ESC Combined

Passenger cars 

 All crash involvements 6 8 14 

All run-off-road crashes – 1 45 44

Culpable involvements w other veh 17 13 28 

LTVs 

All crash involvements 8 10 17

All run-off-road crashes 11 72 75 

Culpable involvements w other veh 20 16 33 
 

The combination is quite effective in preventing both run-off-road crashes and culpable 

involvements with other vehicles. In fact, the capabilities of ABS and ESC dovetail, for ABS is 

especially effective in the multi-vehicle crashes and ESC in the single-vehicle crashes. 

4.4 Long-term benefits and costs of four-wheel ABS 

This report concludes that four-wheel ABS has essentially zero net effect on fatal crashes, but 

significantly reduces nonfatal crash involvements. We are now ready to tackle the last 

evaluation question set out in Section 1.8, namely: does ABS prevent enough nonfatal injuries 

and property damage to endorse ABS technology for its safety benefits just in the nonfatal 

crashes? For example, NHTSA’s evaluations of center high-mounted stop lamps, side marker 

lamps, and head restraints showed little or no effect on fatalities but significant reductions of 

property damage and/or nonfatal injuries.76 

Because ABS is not officially a regulatory requirement, the question will be addressed by an 

informal order-of-magnitude assessment of benefits and costs rather than a detailed economic 

analysis. According to Table 4-1, ABS reduces the overall risk of being involved in a crash by 6 

percent for passenger cars and 8 percent for LTVs. Given that sales of cars and LTVs are fairly 

similar, let us assume an overall average 7 percent reduction of crash involvements. The net 

effect on fatal crashes is close to zero, but the analysis in Section 3.1.7 showed that the effect on 

crashes involving at least one nonfatal injury was approximately the same as the effect on all 

crashes. 

NHTSA’s analysis of the economic impact of motor vehicle crashes in 2000 estimated that the 

total economic cost was $231 billion in that year.77 Economic costs include medical and 

emergency treatment, lost productivity, insurance administration and legal costs, travel delay, 

and property damage. Of that amount, $60 billion was incurred in property-damage-only 

crashes, the remainder in crashes that resulted in fatal or nonfatal injury to at least one person. 

This remainder was allocated according to the maximum injury level, on the Abbreviated Injury 

Scale (AIS),78 of the people involved in the crashes; $5 billion was allocated to uninjured people 

involved in these crashes (AIS 0), $49 billion to people with minor injuries (AIS 1), and $117 

billion to people with non-minor (AIS 2 to 5) or fatal injuries. 

For this informal analysis, let us conservatively assume that ABS effectiveness is close to zero 

not only for the fatalities but also for the nonfatal injuries that are not minor (AIS 2 to 5). The 

sum of the economic cost of crashes with property damage only ($60 billion), or for people with 

no injury ($5 billion) or minor injury ($49 billion) was $114 billion in 2000. Based on the gross 

domestic product (GDP) deflator computed by the Bureau of Economic Analysis,79 which had 

index value 100 in 2000 and 116.676 in 2006, the economic impact in 2000 would have 

amounted to $133 billion in 2006 dollars. If ABS reduces these crashes by 7 percent, that 

76 Kahane (2004), pp. 34-41 and 53-58. 

77 Blincoe, L., et al. (2002). The Economic Impact of Motor Vehicle Crashes, 2000. NHTSA Technical Report. DOT 

HS 809 446. Washington, DC: National Highway Traffic Safety Administration.

78 The Abbreviated Injury Scale (AIS) - 1990 Revision, (1990). Des Plaines, IL: American Association for 

Automotive Medicine, 

79 Bureau of Economic Analyses, Gross Domestic Product, Percent Change from Preceding Period, 

http://www.bea.gov/national/index.htm, http://www.bea.gov/national/xls/gdplev.xls. 

71
 
 

 

 

 

 

 

amounts to an aggregate economic benefit of $9.3 billion, in 2006 dollars, accruing over the life 

of the vehicle. That aggregate total, however, overstates the net present value because benefits in 

future years are not discounted to present value. 

As stated above, safety benefits can occur at any time during the vehicle's lifetime. The agency 

assumes that the distribution of weighted yearly VMT is an appropriate proxy measure for the 

distribution of crashes over the vehicle's lifetime – and for the distribution of benefits (crashes 

avoided). This measure takes into account both vehicle survival rates and changes over time in 

annual average VMT. The Office of Management and Budget’s guidelines are to examine both 

a 3 percent and a 7 percent discount factor to determine the present value of future benefits, by 

putting them on a comparable basis to costs, which are incurred at the time of purchase. 

Multiplying the percent of a vehicle's total lifetime mileage that occurs in each year by the 

discount factor and summing these percentages over the years of the vehicle's operating life 

results in an average factor for passenger cars and LTVs of 0.8163 at a 3 percent discount rate. 

For the 7 percent discount rate, the factor is 0.6502. At a 3 percent discount rate, the present 

value of the benefit would be $7.6 billion ($9.3*0.8163). At a 7 percent discount rate, the 

present value of the benefit would be $6.0 billion ($9.3*0.6502).80 

In 2006, a NHTSA contractor estimated the purchase-price increase added to passenger cars and 

LTVs by ABS, ESC, and traction control systems, based on detailed inspection and disassembly 

of the brake systems in 11 makes and models of MY 2004 to 2006.81 The purchase-price 

increase includes the materials, labor, tooling, and variable burden needed to produce the parts 

and assemble them into the vehicle, plus mark-ups by the parts suppliers (where applicable), the 

auto manufacturers, and dealers.82 The cost analysis differentiated between the basic price 

increase of just adding ABS to a vehicle and the supplemental cost of also adding ESC and/or 

traction control systems. The basic price increase for ABS averaged $381.85, in 2006 dollars.83 

This may be considered the long-term cost of ABS per vehicle. It is a considerable savings from 

the cost of early ABS systems in five cars of MY 1988 to 1990, which averaged $597.56, in 

2002 dollars.84 It is not unusual for unit costs of safety equipment to drop substantially from 

initial levels, as manufacturers find more efficient ways to produce it. 

Vehicle sales in the United States averaged 17 million per year from 2003 to 2007.85 At $381.85 

per vehicle, that amounts to an annual cost of $6.5 billion for ABS, in 2006 dollars. The 

conservative annual economic benefit of $6.0 to $7.6 billion, computed above, is comparable to 

an annual cost of $6.5 billion. This informal analysis does not include some additional costs 

such as the fuel penalty associated with the additional weight of ABS or the repair and 

80 For derivation of these discount multipliers, see Vehicle Survivability and Travel Mileage Schedules, NHTSA
 

Docket No. NHTSA-2005-22223-2218, 2006. 

81 Ludtke, N. F., Osen, W., Gladstone, R., & Lieberman, W. (2006). Cost and Weight Analysis of the Combined 

System of Electronic Stability Control (ESC), Antilock Braking System (ABS), and Traction Control System (TCS). 

NHTSA Technical Report. DOT HS 810 655. Washington, DC: National Highway Traffic Safety Administration. 

82 Tarbet, M. J. (2004). Cost and Weight Added by the Federal Motor Vehicle Safety Standards for Model Years 

1968-2001 in Passenger Cars and Light Trucks. NHTSA Technical Report. DOT HS 809 834. Washington, DC: 

National Highway Traffic Safety Administration. 83 Ludtke et al., p. iii. 

84 Tarbet, p. 26. 85 Ward’s Automotive Yearbook 2008, p. 248.
 

72
 
maintenance costs for ABS. Nevertheless, it shows that, even without any lives saved by ABS, 

its economic benefits in damage and injuries avoided are at least equivalent to the costs of ABS. 

73
 
REFERENCES 

The Abbreviated Injury Scale (AIS) - 1990 Revision. (1990). Des Plaines, IL: American 

Association for Automotive Medicine. 

Allen, K., Dang, J. N., Doyle, C. T., Kahane, C. J., Roth, J. R., & Walz, M. C. (2008). 

Evaluation Program Plan, 2008-2012. NHTSA Technical Report. DOT HS 810 

983.Washington, DC: National Highway Traffic Safety Administration. 

Arehart, C., Radlinski, R., & Hiltner, E. (1992). Light Vehicle ABS Performance Evaluation – 

Phase II. NHTSA Technical Report. DOT HS 807 924. Washington, DC: National Highway 

Traffic Safety Administration. 

Blincoe, L., Seay, A., Zaloshnja, E., Miller, T., Romano, E., Luchter, S., & Spicer, R. (2002). 

The Economic Impact of Motor Vehicle Crashes, 2000. NHTSA Technical Report. DOT HS 809 

446. Washington, DC: National Highway Traffic Safety Administration. 

Bureau of Economic Analyses. Gross Domestic Product, Percent Change from Preceding 

Period. http://www.bea.gov/national/index.htm, http://www.bea.gov/national/xls/gdplev.xls. 

Collision and Property Damage Liability Losses of Passenger Cars With and Without Antilock 

Brakes. (1994). Insurance Special Report No. A-41. Arlington, VA: Highway Loss Data 

Institute. 

Dang, J. N. (2004). Preliminary Results Analyzing the Effectiveness of Electronic Stability 

Control (ESC) Systems. NHTSA Evaluation Note. DOT HS 809 790. Washington, DC: National 

Highway Traffic Safety Administration. 

Dang, J. N. (2007). Statistical Analysis of the Effectiveness of Electronic Stability Control (ESC) 

Systems – Final Report. NHTSA Technical Report. DOT HS 810 794. Washington, DC: 

National Highway Traffic Safety Administration. 

Farmer, C. M. (2001). “New Evidence Concerning Fatal Crashes of Passenger Vehicles Before 

and After Adding Antilock Braking Systems,” Accident Analysis and Prevention, 33, 361-369. 

Farmer, C. M., Lund, A. K., Trempel, R. E. and Braver, E. R. (1996). Fatal Crashes of 

Passenger Vehicles Before and After Adding Antilock Brake Systems. Arlington, VA: Insurance 

Institute for Highway Safety. 

Federal Register Notices: 

58 (October 4, 1993): 51735, Executive Order 12866 – Regulatory Planning and Review. 

59 (January 4, 1994): 281, ANPRM asking for information about the effectiveness and 

potential benefits of ABS technologies. 

61 (July 12, 1996): 36698, ANPRM deferring indefinitely the ABS requirement. 

74
 
71 (September 18, 2006): 54712, NPRM to require ESC. 

72 (April 6, 2007): 17310, Final Rule requiring ESC. 

Government Performance and Results Act of 1993. Public Law 103-62, August 3, 1993. 

Harless, D. W., & Hoffer, G. E. (2002). The Antilock Braking System Anomaly: A Drinking 

Driver Problem? Accident Analysis and Prevention, 34, 333-341. 

Hertz, E. (2000). Analysis of the Crash Experience of Vehicles Equipped with All Wheel Antilock 

Braking Systems (ABS) – A Second Update Including Vehicles with Optional ABS. NHTSA 

Technical Report. DOT HS 809 144. Washington, DC: National Highway Traffic Safety 

Administration. 

Hertz, E., Hilton, J., & Johnson, D. M. (1995a). An Analysis of the Crash Experience of Light 

Trucks Equipped with Antilock Braking Systems. NHTSA Technical Report. DOT HS 808 278. 

Washington, DC: National Highway Traffic Safety Administration. 

Hertz, E., Hilton, J., & Johnson, D. M. (1995b). An Analysis of the Crash Experience of 

Passenger Cars Equipped with Antilock Braking Systems. NHTSA Technical Report. DOT HS 

808 279. Washington, DC: National Highway Traffic Safety Administration. 

Hiltner, E., Arehart, C., & Radlinski, R. (1991). Light Vehicle ABS Performance Evaluation. 

NHTSA Technical Report. DOT HS 807 813. Washington, DC: National Highway Traffic 

Safety Administration. 

Intermodal Surface Transportation Efficiency Act of 1991. Public Law 102-240, December 18, 

1991. 

Kahane, C. J. (1993). Preliminary Evaluation of the Effectiveness of Rear-Wheel Antilock Brake 

Systems for Light Trucks. NHTSA Docket No. 70-27-GR-026. Washington, DC: National 

Highway Traffic Safety Administration. 

Kahane, C. J. (1994). Preliminary Evaluation of the Effectiveness of Antilock Brake Systems for 

Passenger Cars. NHTSA Technical Report. DOT HS 808 206. Washington, DC: National 

Highway Traffic Safety Administration. 

Kahane, C. J. (2004). Lives Saved by the Federal Motor Vehicle Safety Standards and Other 

Vehicle Safety Technologies, 1960-2002. NHTSA Technical Report. DOT HS 809 833. 

Washington, DC: National Highway Traffic Safety Administration. 

Kahane, C. J., & Hertz, E. (1998) The Long-Term Effectiveness of Center High Mounted Stop 

Lamps in Passenger Cars and Light Trucks. NHTSA Technical Report. DOT HS 808 696, 

Washington, DC: National Highway Traffic Safety Administration. 

Kullgren, A., Lie A., & Tingvall, C. (1994). The Effectiveness of ABS in Real Life Accidents, 

Proceedings of the Fourteenth International Technical Conference on the Enhanced Safety of 

75
 
Vehicles. Paper No. 94-S4-O-07. Washington, DC: National Highway Traffic Safety 

Administration. 

Ludtke, N. F., Osen, W., Gladstone, R., & Lieberman, W. (2006). Cost and Weight Analysis of 

the Combined System of Electronic Stability Control (ESC), Antilock Braking System (ABS), and 

Traction Control System (TCS). NHTSA Technical Report. DOT HS 810 655. Washington, DC: 

National Highway Traffic Safety Administration. 

Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users of 2005. 

Public Law 109-59, August 10, 2005. 

Tarbet, M. J. (2004). Cost and Weight Added by the Federal Motor Vehicle Safety Standards for 

Model Years 1968-2001 in Passenger Cars and Light Trucks. NHTSA Technical Report. DOT 

HS 809 834. Washington, DC: National Highway Traffic Safety Administration. 

Vehicle Survivability and Travel Mileage Schedules. (2006). NHTSA Docket No. NHTSA-2005-

22223-2218, Washington, DC: National Highway Traffic Safety Administration. 

Ward’s Automotive Yearbook. Annual publication. Detroit: Ward’s Communications. 

76
 
DOT HS 811 182

August 2009