What system prevents wheels from locking during hard braking?

The system that prevents wheels from locking during hard braking is the anti-lock braking system (ABS).

ABS monitors wheel speed and rapidly adjusts hydraulic brake pressure when a wheel is about to stop rotating. By preventing a sustained skid, it helps the driver retain steering control during emergency braking. The driver may feel the brake pedal pulse as the system cycles pressure on and off.

The idea predates modern cars: early anti-skid systems were developed for aircraft and rail vehicles, while electronic automotive systems became practical in the late 20th century. Chrysler offered a four-wheel, computer-operated system on the 1971 Imperial, and Mercedes-Benz introduced a Bosch electronic system as an option in 1978.

ABS is often confused with brake assist, traction control, or electronic stability control. Brake assist increases braking force when it detects panic braking; traction control limits wheelspin during acceleration; stability control can brake individual wheels to help correct a skid. ABS specifically manages wheel lock during braking. On loose gravel or deep snow, it can sometimes lengthen stopping distance while still improving steering control.