must be 16 to 20 inches off the ground and limit the amount of damage that can be sustained in a low-speed crash. The idea is to ensure that the bumpers on colliding cars engage, absorbing most of the energy of the impact and keeping damage away from expensive-to-repair parts - like fenders, grilles, headlights, and taillights. Car bumper rules don't apply to light trucks. In fact, it's still legal to sell these vehicles without any bumpers at all. Federal regulators' longstanding thinking ...