Bumpers on cars are not of much use in a collision. To see why, calculate the average force a bumper would have to exert if it brought a 200 kg car (a so-called "compact" model) to a rest in 15 cm when the car had an initial speed of 2 m/s (about 4.5 mph). Bumpers are build with springs which compress to provide a stopping force without (hopefully) denting the metal.