An anti-lock braking system (ABS), as its name implies, prevents the wheels of a vehicle from locking under braking in order to assist a driver to maintain steering control of a vehicle in emergency braking situations. In addition to providing an additional layer of control, ABS can shorten braking distances on both dry and slippery surfaces by eliminating the skidding heavy braking can instigate.
Recent developments in electronics technology have enabled the anti-lock brake system to also vary brake force from front to rear, as well as between individual wheels. These improvements have allowed the ABS system in today’s cars to interact with traction control and stability control systems as well.
Prior to the advent of anti-lock brakes, drivers were advised to pay careful attention to the feel of the car under braking and to “pump the brakes” if they felt the wheels locking while the car was still in motion. This technique is called “threshold braking” as the driver was attempting to anticipate the threshold of the imminent lock up and prevent it by releasing the brake pedal at just the right moment. It sounds complicated, but experienced drivers could usually master the technique.
The National Highway Traffic Safety Administration describes the functioning of ABS as follows: "ABS works with your regular braking system by automatically pumping the brakes. In vehicles not equipped with ABS, the driver has to manually pump the brakes to prevent wheel lockup. In vehicles equipped with ABS, your foot (can) remain firmly planted on the brake pedal, while ABS pumps the brakes for you while you concentrate on steering to safety."
ABS was actually developed in 1929 for use in aircraft. With the speeds pilots were trying to arrest during the landing of an airplane, threshold braking was just too difficult to pull off consistently. A number of mechanical iterations of ABS were tried in the ensuing years, but most proved too expensive or too unreliable. The first known application of a computerized system to an automobile was for the 1971 Chrysler Imperial. Chrysler called it “Sure Brake”.