Q&A: Antilock brakes

March 2013

1 What are antilock brakes?

Antilock brakes are designed to help drivers avoid crashes. Without antilocks, hard braking can cause wheels to lock, sending a vehicle into a skid. Wheel lockup can result in longer stopping distances, loss of steering control and, when road friction is uneven, loss of stability if the vehicle begins to spin.

The main benefit of an antilock braking system (ABS) is that it can reduce these problems on wet and slippery roads. ABS works with a vehicle's normal service brakes to decrease stopping distance and increase the control and stability of the vehicle during hard braking.

The principle behind ABS is that a skidding wheel provides less stopping force and control than a wheel that is rotating. Antilocks prevent wheels from skidding by monitoring the speed of each wheel and automatically pulsing the brake pressure on any wheels where skidding is detected. ABS doesn’t make much difference in stopping distances on dry roads, although it can enhance vehicle stability and allow the driver to maintain steering control during an emergency stop, when conventional brakes might allow wheel lockup and skidding.

2 How does ABS work?

ABS differs among vehicles, but there are some basic similarities. Each system has sensors that monitor the rotational speeds of selected wheels when brakes are applied. When one of these wheels approaches lockup, a control unit reduces brake pressure to that wheel (or set of wheels) just enough to allow rotation again. This typically happens many times per second, resulting in improved control and, on many wet and slippery surfaces, shorter stopping distances.

Differences among antilock braking systems include the following:

• Most passenger vehicles have four-wheel systems with wheel-speed sensors on each wheel. In one type of system, ABS reduces brake pressure to both rear wheels whenever one approaches lockup. Brake pressure to the front wheels of four-wheel systems is controlled independently to maximize stopping power, which is concentrated in the front. In four-wheel independent systems, each wheel is controlled individually, so when any one approaches lockup, brake pressure is reduced to that wheel.
• Some pickups and cargo vans have rear-wheel-only antilock systems to address different braking needs when vehicles are loaded versus unloaded. ABS monitors the rotational speeds of rear wheels only and releases pressure to both when either is about to lock.

Tractor-trailers have separate antilock systems for the tractors and the trailers. Ideally, both the tractor and trailer of a combination rig should have antilock brakes, but putting antilocks on either component should produce improvement compared with conventional brakes. With antilocks on the tractor only, a driver can maintain better steering control even if trailer wheels lock and the trailer swings. If only the trailer has ABS, trailer swing can be reduced even if steering control is lost.

3 Why don't antilocks reduce stopping distances as much on dry roads as wet ones?

Adequate braking is easy to achieve on dry roads with or without antilock brakes. Even if wheels lock, the coefficient of friction between tires and road surface still is relatively high, so a vehicle stops relatively quickly. It is even possible on some surfaces to stop sooner without antilocks than with them, although such instances are rare. They occur, for example, when loosely packed snow or gravel creates a "dam" effect in front of locked wheels, shortening the stopping distance more than antilocks could.

4 Does ABS on passenger vehicles reduce crashes?

Although antilocks perform well on the test track, it’s unclear whether they have made significant reductions in the number of on-the-road crashes. A 1994 Highway Loss Data Institute (HLDI) study Highway Loss Data Institute. 1994. Collision and property damage liability losses of passenger cars with and without antilock brakes. Insurance special report A-41. Arlington, VA. and a subsequent 1995 study Highway Loss Data Institute. 1995. Three years' on-the-road experience with antilock brakes: an update. Insurance special report A-47. Arlington, VA. compared insurance claims for groups of otherwise identical cars with and without antilocks, finding no differences in the frequency or cost of crashes for which insurance claims for vehicle damage are filed. Because ABS should make the most difference on wet and slippery roads, researchers also studied the insurance claims experience in 29 states during winter months. Even here they found no difference in claim frequency for vehicles with and without antilock brakes. A 1997 Institute study Farmer, C.M.; Lund, A.K.; Trempel, R.E.; and Braver, E.R. 1997. Fatal crashes of passenger vehicles before and after adding antilock braking systems. Accident Analysis and Prevention 29(6):745-57. and a 2001 update Farmer, C.M. 2001. New evidence concerning fatal crashes of passenger vehicles before and after adding antilock braking systems. Accident Analysis and Prevention 33(3):361-9. reported no difference in the overall fatal crash involvement of cars with and without antilocks.

According to one federal report, "the overall, net effect of antilock brakes" on both police-reported crashes and fatal crashes "was close to zero." Kahane, C.J. 1994. Preliminary evaluation of the effectiveness of antilock brake systems for passenger cars. Report no. DOT HS-808-206. Washington, DC: National Highway Traffic Safety Administration. A more recent federal report concluded that ABS reduces overall crash involvement risk by 6 percent for cars and 8 percent for pickups and SUVs but has no effect on fatal crash risk. Kahane, C.J. and Dang, J.N. 2009. The long-term effect of ABS for passenger cars and LTVs (DOT-HS-811-182). Washington, DC: National Highway Traffic Safety Administration. Leonard Evans, a researcher with General Motors, reported that antilock-equipped cars were less likely to rear-end other vehicles but more likely to have other vehicles rear-end them. Evans, L. and Gerrish, P. 1996. Antilock brakes and risk of front and rear impact in two-vehicle crashes. Accident Analysis and Prevention 28(3):315-23. The net result was little effect on overall crash risk. In a study conducted for auto manufacturers, Failure Analysis Associates reported a net beneficial effect of antilocks on nonfatal crashes but no effect on fatal crashes. Padmanaban, J. and Lau, E. 1996. Accident experience of passenger vehicles with four-wheel antilock braking systems. Proceedings of the 40th Annual Conference of the Association for the Advancement of Automotive Medicine, 111-25. Des Plaines, IL: Association for the Advancement of Automotive Medicine.

5 Why aren't passenger vehicle antilocks reducing crashes as expected?

No one knows for sure why ABS test performance has not translated into a substantial reduction in real-world crashes. A possible reason is that the average motorist rarely experiences total loss of vehicle control, which antilocks are designed to prevent. There also is evidence that many drivers in the early days of antilock brakes did not know how to use them effectively. A 1994 Institute survey of drivers with antilock-equipped cars found that more than 50 percent in North Carolina and 40 percent in Wisconsin incorrectly thought they should pump the brakes. Williams, A.F. and Wells, J.K. 1994. Driver experience with antilock brake systems. Accident Analysis and Prevention 26(6):807-11.

6 Is motorcycle ABS effective at reducing crashes?

Yes. Results from recent studies by the Institute and HLDI compared crash rates for motorcycles equipped with optional ABS against the same models without the option. The rate of fatal crashes per 10,000 registered vehicle years was 37 percent lower for motorcycles equipped with optional ABS than for those same motorcycles without ABS. In crashes of all severities, the frequency at which collision claims were filed was 22 percent lower for the ABS models. Teoh, E.R. 2011. Effectiveness of antilock braking systems in reducing motorcycle fatal crash rates. Traffic Injury Prevention 12(2):169-73. Highway Loss Data Institute. 2009. Insurance special report: motorcycle antilock braking system (ABS). Highway Loss Data Institute. Arlington, VA. Based on these findings, the Institute in April 2010 petitioned the National Highway Traffic Safety Administration to require manufacturers to equip all new motorcycles with this technology.

Information on antilock brakes for motorcycles is available on the motorcycles topic page.

7 Are antilocks a new idea? Are they widely available?

The idea of antilock brakes has been around for years. They first were used on airplanes in the 1950s. A rear-wheel system was developed for the 1969 Ford Thunderbird, and the 1971 Chrysler Imperial had four-wheel antilocks.

Modern antilocks were first introduced on 1985 models. By the 1987 model year, they were standard or optional on about 30 domestic and foreign car models. Availability soared to 90 models the next year. ABS is a component of electronic stability control (ESC). Thus, the federal requirement for ESC has made antilock brakes standard equipment on all passenger vehicles as of the 2012 model year.

8 Is ABS required on big truck rigs?

In March 1995, the National Highway Traffic Safety Administration issued a rule requiring antilock brakes for heavy trucks, tractors, trailers and buses. All new truck tractors were required to have antilocks after March 1, 1997, and they were mandatory on new air-braked trailers and single-unit trucks and buses after March 1, 1998. New single-unit trucks and buses with hydraulic brakes had to be equipped with antilocks after March 1, 1999. This was not the first antilock standard for U.S. trucks. A federal brake standard took effect in 1975, but its antilock and stopping distance requirements were suspended after litigation in 1978. Antilock brake systems have been required on all new trucks, buses and trailers in Japan and the European Union for several years.

ABS is important for big trucks because of the poor braking capabilities of these vehicles compared with passenger cars. On dry roads, stopping distances for big trucks are much longer than those of passenger cars — 47 percent longer in Institute tests. Insurance Institute for Highway Safety. 1990. Special issue: Antilock brakes for trucks. Status Report 25(5):1-7. On wet and slippery roads, the stopping distance disparity is even worse. Tractor-trailer combinations also have the potential for loss of control and jackknifing, especially onslippery roads. (Jackknifing occurs when the rear wheels of a tractor lock up, allowing the tractor to skid and spin so that it folds into the trailer. This also can happen when trailer wheels lock and cause the trailer to swing around the tractor.) Antilock brakes not only reduce stopping distances on wet and slippery roads, but also help drivers maintain control.

The standard for tractors requires antilock control on the front axle and at least one rear axle. On at least one of the tractor axles, each wheel must be independently controlled by an antilock modulator. This ensures that a wheel provides shorter stopping distances and optimal braking force on all surfaces, especially on roads where one side is slipperier than the other. For semi-trailers, at least one axle must have antilocks. Full trailers must have antilocks for at least one front and one rear axle.

A 2010 report by the National Highway Traffic Safety Administration concluded that ABS on tractors reduced crash involvement by 3 percent. Allen, K. 2010. The effectiveness of ABS in heavy truck tractors and trailers. Report no. DOT HS-811-339. Washington, DC: National Highway Traffic Safety Administration. However, there was no significant effect on fatal crashes.

9 Are anti-jackknifing devices a substitute for truck ABS?

No. Some devices marketed for trucks purportedly would prevent jackknifing in emergency braking situations. One device on the market mechanically limits the amount a trailer can swing around the pin that connects it to the tractor. This kind of device is less effective than antilock brakes. Although it may prevent jackknifing, it does not prevent a truck's wheels from locking up, nor does it provide equivalent handling stability on wet or dry roads.