Bumpers have two major safety functions. They diminish height mismatches between vehicles. They also protect pedestrians in the event of a collision. The bumper structure on modern automobiles usually have a plastic cover over a reinforcement bar made up of steel, aluminum, fiberglass composite, or plastic.
Bumpers save other vehicle components by dissipating the impact of kinetic energy generated in a collision. This energy is a function of vehicle mass and velocity squared. In formula form, it is given by which suggests that a vehicle protecting the components at 5 km/ hr must be four times stronger than the bumper that protects at a 2.5 km/hr.