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Anti-lock Braking Systems (ABS)


Video courtesy of Toyota Canada

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Antilock Braking System (ABS) Factsheet

 


What are anti-lock braking systems (ABS)?
When would ABS be useful?
How does ABS work?
How effective is ABS at reducing crashes?
Does ABS have limitations?
How may vehicles have ABS now?


What are anti-lock braking systems (ABS)?

Anti-lock braking systems (ABS) are active safety features designed to help drivers retain steering control by preventing wheels from locking up during an episode of heavy braking. Wheel lock means that the wheels of the vehicle have stopped rotating and as a result the vehicle begins to slide. Whether on wet or dry roads, heavy application of the brakes on a vehicle without ABS can result in wheel lock.

One of the most dangerous aspects of wheel lock is the corresponding loss of steering control. Once the wheels stop rotating, drivers no longer have any control over the direction of the vehicle. Therefore, although the vehicle may still be sliding, drivers are not able to steer to avoid anything that their vehicle may be sliding towards. By preventing wheels from locking in the first place, ABS ensures that drivers will be able to steer after an episode of hard braking.

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When would ABS be useful?

ABS is designed to be useful whenever a driver slams on the brakes and swerves, particularly when road conditions are slippery or wet. In these conditions, a vehicle is more likely to slide or skid if the wheels lock. ABS would benefit drivers in a variety of fairly common situations, including the following:

  • A vehicle suddenly veers in front of you, forcing you to brake and swerve.
  • An animal jumps onto the road and you must brake and steer to avoid it.
  • Bad weather makes the roads slippery, making wheel lock more likely when you must brake to stop.

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How does ABS work?

Not all ABS setups are the same. Some prevent wheel lock on all four wheels, while others prevent only the rear wheels from locking. However, all ABS work by monitoring wheel speed and then, if a potential wheel lock is detected, rapidly applying and releasing the brake to the problematic wheel. ABS functions by effectively “pumping the brake”, using a technique that was taught to drivers before the development of ABS to prevent wheels from locking. The difference is that ABS setups are able to sense potential wheel lock and address the problem faster and with more efficiency than drivers. All ABS are comprised of three major components:

  • wheel speed sensors that monitor wheel rotation speed;
  • hydraulic units that pump the brakes, and;
  • an electronic control unit (ECU)that receives information from the wheel speed sensors and, if necessary, directs the hydraulic units to pump the brakes on one or more of the wheels.

In modern ABS setups, the ECU and the hydraulic units are attached together so that while they have different functions, they are physically one unit. The ECU continually checks for signs of rapid wheel-speed deceleration, an indicator that a wheel is about to lock. If a wheel is about to lock, the ECU directs the hydraulic unit to pump the brake to that wheel until it resumes normal rotation.

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How effective is ABS at reducing crashes?

The primary purpose of ABS is to allow drivers to have directional control over their vehicle after heavy braking. In support of the effectiveness of ABS, it has been associated with:

  • a 35% decrease in frontal collisions on wet roads, and;
  • a 9% decrease in frontal impacts on dry roads.

A decrease in frontal collisions suggests that ABS allows drivers to steer to avoid a collision.

In addition, under controlled test conditions:

  • 58% of drivers without ABS strayed from their intended path after braking;
  • only 24% of drivers with ABS did the same.

One misconception about ABS is that its purpose is to help reduce stopping distance. However, a reduction in stopping distance is not guaranteed and is only a secondary benefit of ABS.

Steering too aggressively can still have severe consequences. On a vehicle without ABS, if a panicking driver steers the wheel sharply out of instinct to avoid a collision, nothing results from this action, as steering control is lost when the wheels lock up. However, the addition of ABS means that those same exaggerated steering commands will have an effect, and could lead to other dangerous situations like road departure, collisions, or rollovers. Drivers with ABS should continue to steer as calmly as possible.

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Does ABS have limitations?

Yes. Like many other safety features, realizing the full benefits of ABS depends largely on whether or not drivers interact appropriately with it. Interacting appropriately with safety features like ABS means continuing to drive safely and attentively. Driver behaviours like speeding, tailgating, or driving while fatigued can negate the beneficial aspects of ABS. For example, tailgating can make it impossible to steer and avoid a vehicle ahead that suddenly slams on its brakes. Even if ABS activates in this case, there simply will not be enough time to avoid a collision due to insufficient distance between the two vehicles. Similarly, by increasing a driver’s reaction time, fatigue or drowsiness can cause drivers to brake too late to safely avoid a collision.

ABS does not compensate for unsafe driving or poor road conditions. In order to ensure the ideal performance of ABS, drivers must continue to use caution and good judgment behind the wheel. When combined with safe driving practices, safety features like ABS have been proven to mitigate and prevent road crashes.

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How may vehicles have ABS now?

Despite the fact that ABS is not required on all new vehicles in Canada, is it still a common feature on today’s roads. Since its production in 1985, the number of vehicles with ABS has risen steadily. The Insurance Institute for Highway Safety in the United States predicts that by 2015, ABS will be installed on all registered vehicles (IIHS 2012).

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