Electronic Stability Program (ESP) is the system that sets things right when a truck is about to lose its grip on the road and either skid or roll over. ESP counteracts the forces that tend to drive the truck off the road, through interaction between the truck's Electronic Brake System, engine management system and retarder, and the trailer's braking system.
The system consists of three sensors on the tractor that measure the yaw angle, lateral acceleration and steering wheel position. A central unit continuously computes the incoming readings and when these readings do not match, the brakes are activated individually on one or more wheels as necessary. At the same time, engine torque is reduced to lower road speed until equilibrium is regained.