Preliminary Evaluation of the Effectiveness of Rear-Wheel Antilock Brake System for Light Trucks

Charles J. Kahane, Ph.D.

Executive Summary

Antilock Brake Systems (ABS) are a promising development for reducing motor vehicle crashes. Since 1985, they have been voluntarily installed by manufacturers on millions of cars and light trucks. They have been welcomed by consumers and are well on their way to becoming standard equipment in most new cars and light trucks. The Highway Safety Act of 1991 instructs the National Highway Traffic Safety Administration (NHTSA) to consider extending this protection to all passenger vehicles, including trucks lighter than 10,000 pounds. This preliminary evaluation of the effectiveness of rear-wheel ABS for light trucks (including pickup trucks, sport-utility vehicles and vans), based on statistical analyses of the accident experience of production vehicles equipped with ABS, is performed in support of NHTSA's regulatory program.

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), or take longer to stop than a vehicle with the wheels still rolling. The objective of ABS is to take over the optimization task from the driver. 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, and rear-wheel antilock (RWAL) system, which were the principal type installed on light trucks through model year 1991. A four-wheel system is intended to keep all the wheels rolling during panic braking, to prevent yawing, allow the driver to steer the vehicle 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. RWAL, on the other hand, is not designed to prevent lockup of the front wheels, preserve steering control, or significantly reduce stopping distances. RWAL was primarily intended to prevent rear-wheel lockup and severe yawing during braking; it was an important first step for light trucks, which have more problems than cars with directional control (run-off-road crashes). Separate analyses need to be done for RWAL on light trucks, four-wheel ABS on passenger cars and four-wheel ABS on light trucks; only the first of these is carried out here.

During 1988-91, NHTSA performed two extensive series of stopping tests involving vehicles with four-wheel ABS or RWAL, on various road surfaces. The tests confirmed that four-wheel 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 on gravel. RWAL greatly reduced the yaw of pickup trucks during straight-line panic stops, but it did not shorten stopping distances; in fact, they became slightly longer.

The statistical analysis of the effectiveness of RWAL for light trucks is based on 1990-91 accident data from Michigan and Florida, 1989-91 data from Pennsylvania and 1989-mid 92 data from the Fatal Accident Reporting System (FARS). RWAL was installed as standard equipment on most domestic Chevrolet, GMC, Ford and Dodge pickup trucks, sport utility vehicles, and vans during 1987-90. The statistical analysis compares trucks of the first 2 model years with RWAL to trucks of the same make-model, of the last 2 model years without RWAL.

The situation is not as simple with passenger cars, precluding a detailed effectiveness analysis at this time. As late as model year 1991, installation of ABS as standard equipment was generally limited to luxury and sporty make-models and the top-of-the-line subseries of medium-priced make-models. Cars of that type may attract a special clientele, and their crash experience may not be directly comparable to basic versions of the same make-model, let alone the average car on the road. When high-volume cars with standard ABS, such as the 1991 Chevrolet Caprice and the 1992 Cavalier, Corsica and Grand Am have accumulated sufficient on-the-road experience (in 1995, or possibly 1994), it will be possible to analyze the effectiveness of four-wheel ABS in passenger cars.

The principal findings and conclusions from the analyses of accident data on light trucks are the following:

• RWAL is quite effective in reducing the risk of nonfatal run-off-road crashes, for almost every type of light truck, under any type of road condition. Nonfatal rollovers were reduced by about 30-40 percent (up to 50 percent for Ford Ranger), side impacts with fixed objects by 15-30 percent and frontal impacts with fixed objects by 5-20 percent. Many run-off-road crashes of light trucks appear to involve a loss of directional control during braking, and RWAL significantly reduces such loss-of-control crashes.

• The accident reductions mostly did not carry over to fatal run-off-road crashes of light trucks. Only the Ford Ranger experienced a significant, 29 percent reduction of fatal rollovers and side impacts with fixed objects. The explanation may be that in most fatal run-off-road crashes, drivers do not brake at all, or lose directional control for reasons unrelated to braking, or apply the brakes under conditions that are too severe for RWAL to prevent a loss of directional control.

• RWAL had little or no effect on the nonfatal multivehicle crashes of light trucks. Since RWAL is primarily designed to prevent catastrophic loss of control during braking, rather than reducing stopping distances or allowing the driver to steer while braking, it is not surprising that RWAL should be effective against run-off-road crashes, rather than multivehicle collisions.

• The current accident data produced conflicting estimates about the effect of RWAL in fatal multivehicle crashes. Some of the analyses showed little or no effect, while others showed significant increases with RWAL.

• The risk of collisions with pedestrians, animals, bicyclists, trains, or on-road objects was significantly reduced in light trucks with rear-wheel ABS. The reduction appears to be about 10-20 percent in nonfatal collisions (mostly with animals) and 5-15 percent in fatal collisions (mostly crashes in which a truck contacts and fatally injures a pedestrian or bicyclist).

These preliminary results need to be viewed with caution for several reasons. The FARS samples in this report were sometimes too small for unambiguous or statistically meaningful results; all estimates of fatality reduction might change as more data become available, allowing more detailed analysis methods. The data cover the initial experience of the first groups of trucks equipped with RWAL; results could change as these trucks get older, or for later trucks with different RWAL systems. The results of this report apply only to light trucks equipped with RWAL and should definitely not be extended to passenger cars or light trucks equipped with four-wheel ABS.