Preliminary Evaluation of the Effectiveness of Antilock Brake Systems for Passenger Cars

Charles J. Kahane, Ph.D.

Abstract

The accident involvement rates of passenger cars equipped with Antilock Brake Systems (ABS) were compared to the rates of counterpart cars without ABS, based on 1990-92 Florida, Pennsylvania and Missouri data, and the 1989-93 Fatal Accident Reporting System. This statistical analysis of the initial years of exposure of the first groups of cars equipped with ABS showed mixed results. Involvements in multivehicle crashes on wet roads were significantly reduced in the cars equipped with ABS: fatal crashes were reduced by 24 percent, and nonfatal crashes by 14 percent. Fatal collisions with pedestrian and bicyclists were down a significant 27 percent with ABS. However, these reductions were offset by a statistically significant increase in the frequency of single vehicle, run-off-road crashes (rollovers or impacts with fixed objects), as compared to cars without ABS. Fatal run-off-road crashes were up by 28 percent, and nonfatal crashes by 19 percent. It is unknown to what extent this increase is a consequence of ABS, or is due to other causes. In particular, it is unknown to what extent, if any, the increase is due to incorrect responses by drivers to their ABS systems, and, if so, whether the effect is likely to persist in the future. The increase may involve all types of ABS or only certain ABS designs.

Executive Summary

Antilock Brake Systems (ABS) are a noteworthy development in motor vehicle technology. Since 1985, they have been voluntarily installed by manufacturers on millions of cars and light trucks. They have been welcomed by consumers and have already become standard equipment in many new cars and most light trucks. The Highway Safety Act of 1991 instructed the National Highway Traffic Safety Administration (NHTSA) to contemplate requiring ABS in all passenger vehicles. NHTSA published an Advance Notice of Proposed Rulemaking (ANPRM) at the end of 1993, discussing ABS and other changes in braking technology. The ANPRM stated that the agency did not yet have sufficient data to estimate the safety benefits of ABS. However, ABS was introduced as standard equipment on a number of high-volume family and economy cars during 1991-92; by mid-1994, State data bases had accumulated a sufficient number of accident cases for this preliminary statistical evaluation of the effects of ABS on police-reported crashes and fatalities of passenger cars.

There are two types of ABS. Four-wheel systems, which are almost the only type installed on passenger cars and are becoming increasingly numerous on light trucks, are the subject of this study. Rear-wheel antilock systems, which were the principal type installed on light trucks through model year 1991, were evaluated by NHTSA in December 1993.

The fundamental safety problem addressed by ABS is that few drivers are able to optimize the pressure they apply on the brake pedal, given a sudden emergency situation or unexpectedly slippery surface. When excessive pedal pressure locks the wheels, the vehicle can yaw out of the driver's control (rear-wheel lockup), or go straight ahead, impossible to steer (front-wheel lockup). On most, but not all, road surfaces, a skidding vehicle needs a longer distance to stop than a vehicle with the brakes applied and wheels still rolling. The objective of ABS is to take over the optimization task from the driver. A four-wheel system is intended to keep all the wheels rolling during panic braking, to prevent yawing, to allow steering throughout the emergency and, on many surfaces, to shorten the stopping distance. The combination of efficient stopping and steering is intended to help the driver avoid mobile and fixed obstacles. The effect, however, is not inevitably for the better. ABS confers the capability to steer a car while slamming on the brakes, but the average driver in a panic situation might not always use this capability to advantage, and might even steer the car into a worse situation than the one which the driver was trying to avoid.

During 1988-91, NHTSA performed two extensive series of stopping tests involving vehicles with four-wheel ABS, on various road surfaces. The tests confirmed that ABS was highly effective in preventing yawing and allowing the driver to steer the car during panic braking. Stopping distances decreased substantially with four-wheel ABS on wet surfaces, but decreased only slightly on dry pavement and increased considerably on gravel.

The statistical analyses of the effectiveness of ABS are based on 1990-92 accident data from Florida, Pennsylvania and Missouri and 1989-93 data from the Fatal Accident Reporting System (FARS). The statistical analyses compare the accident involvements of passenger cars of the first 2 model years with ABS to cars of the same makes, models and subseries, but from the last 2 model years before ABS became standard equipment.

The principal findings and conclusions from the statistical analyses of accident data are the following:

• ABS significantly reduced the involvements of passenger cars in multivehicle crashes on wet roads. ABS reduced police-reported crash involvements by an estimated 14 percent, and fatal involvements by 24 percent. The finding is consistent with the outstanding performance of ABS in stopping tests on wet roads.

• Certain types of collision involvements on wet roads, such as striking another vehicle in the rear, or striking a stopped vehicle, were reduced by 40 percent or more. This benefit, however, was partially offset by an increased likelihood of being struck in the rear by another vehicle. The better your own braking capabilities, the more likely that a following vehicle with average braking capabilities will hit you.

• ABS had little effect on multivehicle crashes on dry roads. The contrast in the results for wet roads and dry roads is consistent with findings in stopping tests, where ABS improved stopping distances and directional control substantially on wet surfaces, but much less so on dry surfaces.

• The risk of fatal collisions with pedestrians and bicyclists was reduced by a statistically significant 27 percent in passenger cars with ABS. Unlike the effects for multivehicle crashes, this reduction was about equally large on wet and dry roads.

• All types of run-off-road crashes - rollovers, side impacts with fixed objects and frontal impacts with fixed objects - increased significantly with ABS. Nonfatal run-off-road crashes increased by an estimated 19 percent, and fatal crashes by 28 percent.

• Rollovers and side impacts with fixed objects - crashes that typically follow a complete loss of directional control - had the highest increases with ABS. Nonfatal crashes increased by 28 percent, and fatal crashes by 40 percent.

• Frontal impacts with fixed objects, where the driver is more likely to have retained at least some directional control prior to impact, increased by about 15-20 percent, both nonfatal and fatal.

• The negative effects of ABS on run-off-road crashes were about the same under wet and dry road conditions.

• The reason for these negative effects is unknown. One possibility is that average drivers may at times steer improperly in panic situations. Because ABS preserves steering control under hard braking, cars may be swerving or heading off the road.

• The observed effects of ABS on snowy or icy roads, while not statistically significant, were all similar to the effects on wet roads - i.e., positive for multivehicle collisions, negative for run-off-road crashes.

• The overall, net effect of ABS on police-reported crashes (including multivehicle, pedestrian and run-off-road crashes) was close to zero.

• The overall, net effect of ABS on fatal crashes was close to zero.

This report is not the first statistical evaluation of ABS for passenger cars. In late 1993, the Highway Loss Data Institute published an analysis of the effect of ABS on collision and property-damage-liability claims. They found that ABS had little effect on the overall, insurance-reported accident rates of cars. This report's findings on the overall, net effect of ABS corroborate the earlier study. However, this report also shows that ABS is not ineffectual. The net benefit is close to zero, because significant reductions in pedestrian impacts and wet-road multivehicle crashes are nullified by significant increases in run-off-road crashes.

These preliminary results need to be viewed with caution for several reasons. The principal observed effects, both positive and negative, although statistically significant and consistent from State to State, are quite high compared to what is usually seen in evaluations of crash avoidance measures. The increase in run-off-road crashes is surprising in view of the good performance of ABS in stopping tests. Further study is needed, such as interviews with drivers of ABS-equipped cars that were involved in run-off-road crashes, or stopping tests that involve combinations of hard braking and abrupt steering, before the increase can be unequivocally attributed to the ABS system and/or the driver's interaction with ABS. The FARS samples in this report were sometimes too small for conclusive results. The data comprise the initial years of exposure of the first groups of cars equipped with ABS; results could change as drivers gain more experience with the ABS in these cars, or for later cars with different ABS systems. The results of this report apply only to passenger cars and should definitely not be extended to light trucks equipped with four-wheel ABS. The effectiveness of four-wheel ABS for light trucks will be estimated when sufficient accident data become available.