A federal standard specifies a zone on cars for bumper protection extending from 16 to 20 inches off the ground. When two cars collide at low speeds, the bumpers are more likely to engage. Then they have a chance to absorb energy and prevent damage. Because no such bumper requirements apply to SUVs, pickup trucks, or minivans, these vehicles often have flimsier bumpers than cars or no bumper at all. The heights of their bumpers often are different from the mandated heights for car bumpers.

Many advocates believe that federal rules should be changed to make SUVs and cars more compatible. In a crash, the higher bumper on many taller vehicles, such as SUVs and trucks, hits a typical passenger car above the car's bumper line and crumple zone, exerting its force into weaker portions of the smaller vehicle and inflicting greater damage. To address this, many SUVs--especially car-based models--are being designed with lower bumpers. Other models, including the 2003 Ford Expedition and Lincoln Navigator, have had the bumpers lowered when they received a major redesign.

Automakers recently announced plans to design SUVs and pickup trucks in ways that will make them less dangerous to occupants of passenger vehicles. The design changes will be completed by the 2008 model year.