Anti-lock braking system: Effectiveness

On high-traction surfaces such as bitumen, or concrete some ABS-equipped cars are able to attain braking distances better (i.e. shorter) than those that would be easily possible without the benefit of ABS. Even an alert, skilled driver without ABS would not be able, even through the use of techniques like threshold braking, to match or improve on the performance of a typical driver with an ABS-equipped vehicle. ABS reduces chances of crashing, and/or the severity of impact. The recommended technique for non-expert drivers in an ABS-equipped car, in a typical full-braking emergency, is to press the brake pedal as firmly as possible and, where appropriate, to steer around obstructions. In such situations, ABS will significantly reduce the chances of a skid and subsequent loss of control.

In gravel and deep snow, ABS tends to increase braking distances. On these surfaces, locked wheels dig in and stop the vehicle more quickly. ABS prevents this from occurring. Some ABS calibrations reduce this problem by slowing the cycling time, thus letting the wheels repeatedly briefly lock and unlock. The primary benefit of ABS on such surfaces is to increase the ability of the driver to maintain control of the car rather than go into a skid – though loss of control remains more likely on soft surfaces like gravel or slippery surfaces like snow or ice. On a very slippery surface such as sheet ice or gravel it is possible to lock multiple wheels at once, and this can defeat ABS (which relies on detecting individual wheels skidding). Availability of ABS relieves most drivers from learning threshold braking.

But part of the answer is that on HEAVY snow, locked wheels can be useful because they gather up a “wedge” of snow which helps to slow the vehicle. ABS allows this wedge to clear every times the wheels are unlocked. The same can apply on sand in some conditions. Can’t explain it for ice though.

Note, however, that this somewhat simplistic test compares ABS with locked wheels. A good driver with a car with a decently designed braking system, designed to minimize the chances of accidentally locking the brakes during a “panic stop”, would fare better under these conditions.

A June 1999 NHTSA study found that ABS increased stopping distances on loose gravel by an average of 22 percent.

Other tests shows results that differ from those above when braking on ice. An independent test, with a 1989 Dodge Omni, a small economy car, and a 1995 Pontiac Grand Am equipped with ABS (Mid Sized family Vehicle) The Pontiac matched or had shorter stopping distances on the glare ice, despite being heavier. However, since the vehicles, brakes and tires were different, this is not a completely valid comparison.

When activated, some earlier ABS systems caused the brake pedal to pulse noticeably. As most drivers rarely or never brake hard enough to cause brake lockup, and a significant number rarely bother to read the car’s manual, this may not be discovered until an emergency. When drivers do encounter an emergency that causes them to brake hard and thus encounter this pulsing for the first time, many are believed to reduce pedal pressure and thus lengthen braking distances, contributing to a higher level of accidents than the superior emergency stopping capabilities of ABS would otherwise promise. Some manufacturers have therefore implemented Mercedes-Benz’s “brake assist” system that determines that the driver is attempting a “panic stop” and the system automatically increases braking force where not enough pressure is applied. Nevertheless, ABS significantly improves safety and control for drivers in most on-road situations.