The friction optimum before lockup
A tire generates maximum friction at approximately 10-15% slip, a region called the adhesion or friction peak. Slip is the difference between where the tire is rolling versus where the wheel is actually moving: zero slip means pure rolling, 100% slip means the wheel is locked and skidding. At the friction peak, the tire's molecules are breaking and reforming contact with the road surface at an optimal rate, creating maximum grip.
Below the friction peak, grip increases with slip. Above it, grip degrades: a locked wheel at 100% slip produces only 60-70% of peak friction because the rubber is sliding rather than gripping, generating heat instead of traction. This is why locked wheels skid off the road or into objects, while properly braked wheels with some slip maintain better directional control.
How ABS prevents losing a third of your braking
Anti-lock braking systems (ABS) pump the brakes rapidly, keeping wheel slip in the 10-20% zone where grip is highest. This modulation happens too fast for a human driver to replicate manually, and it allows the vehicle to decelerate at the maximum possible rate while maintaining traction for steering. Without ABS, panic braking on slippery surfaces often locks the wheels, reducing grip by 25-35% and making the car impossible to steer.
The practical result is that ABS-equipped vehicles stop shorter than vehicles with locked wheels in emergency stops on wet or snowy roads. The traction gained from staying on the friction peak, combined with maintained steering control, reduces accident severity. This is why ABS has been mandatory on new vehicles in most developed countries since the 1990s, despite the slightly longer stopping distance on dry pavement where a skilled driver might manually modulate brake pressure.