Antilock braking systems (A.B.S.) are designed to help prevent collisions. When you apply the brakes hard in a conventional car without ABS, the wheels jam, and the vehicle skids. When wheels jam, braking distance is increased, and there is a loss of steering control. If the wheels on one side are on a dry surface, and on ice on the other side, the car may start spinning like a top.
Shorter braking distance and better steering control on slippery surface are the main advantages of antilock systems. ABS does not make much of a difference on dry pavement but do provide more stability when applying brakes in emergency situations.
The driver must never "pump" the brakes though, because the main function of ABS in fact is to "pump" those brakes up to 15 times per second. Instead you must apply the brakes firmly and steadily, otherwise if you release the brake pedal the system will be deactivated. Many drivers have a tendency to release the pedal when ABS is active because they feel it somewhat pulsating through their foot. But this pulsation is normal, and it is caused by the system's pumping action.
If the antilock system experiences some malfunction an amber coloured ABS warning light comes on on the dash, and the brakes reverse to the traditional mode.
THE VARIOUS SYSTEMS
There are different ABS systems but they all have the same basic working principle. Each system uses sensors to measure the wheels rotation speed. So when you brake, if one of the wheels is about to stop turning, the sensor will send a signal to a computer which will activate a pump that will relieve the pressure and prevent the wheel from locking, making it brake smoothly.
The real difference between those systems is how they are configured.
In the independent four-way system each wheel that tends to lock will be controlled by the pump. In the three-way system the front wheels are controled separately, but in the rear if one wheel becomes locked then the pressure is relieved in both of them. Finally there is the rear wheel antilock (RWAL) which is mostly installed on pickup trucks since they are usually lighter in the rear than other vehicles.
BRAKING DISTANCES AND CONTROL
A number of factors can affect braking distance: road conditions, tire wear, brake conditions, and driver's reaction time. Because there is a high friction ratio between tire and road on rough and dry pavement, there won't be much of a difference in braking distance whether using ABS or not. But on ice, unless you are an expert driver who can pump the brakes as efficiently as an ABS system, the antilock will greatly improve braking. Also when the front wheels lock on a slippery surface the vehicle will keep going in a straight line even if you try to swing the wheels, thus losing control. But the ABS system allows the wheels to keep turning. It is possible to maneuver to avoid a collision even on the ice.
Results from studies by the Highway Loss Data Institute (HLDI) in 1994, were not conclusive in showing a reduced accident rate among vehicles equipped with ABS brakes compared to those equipped with traditional brakes. Other studies performed by General Motors have shown that ABS cars are subject to fewer front collisions but suffer more rear hits.
So remember never to "pump" ABS brakes. Get some practice. Go to a deserted parking lot in icy conditions and experiment with your brake system. You will become more familiar with your vehicle's reaction and that will allow you to maneuver more safely in an emergency situation.