<Li> Increased the risk of run - off - road crashes by 35 percent . </Li> <P> On high - traction surfaces such as bitumen, or concrete, many (though not all) ABS - equipped cars are able to attain braking distances better (i.e. shorter) than those that would be possible without the benefit of ABS . In real world conditions, even an alert and experienced driver without ABS would find it difficult to match or improve on the performance of a typical driver with a modern Anti-lock Braking System - 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 . </P> <P> In gravel, sand 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 . Some vehicle manufacturers provide an "off - road" button to turn ABS function off . 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 such as gravel or on slippery surfaces such as 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 comparing all four wheels, and detecting individual wheels skidding). Availability of ABS relieves most drivers from learning threshold braking . </P> <P> A June 1999 National Highway Traffic Safety Administration (NHTSA) study found that ABS increased stopping distances on loose gravel by an average of 27.2 percent . </P>

When does anti lock brakes not prevent skidding