When a passenger vehicle collides with the rear end or side of trucks, trailers or buses not equipped with effective guards (bumpers), it continues to travel beneath the taller chassis of the larger vehicle, i. e., the passenger vehicle "rides under" the larger one. The invasion of the passenger compartment that follows the underriding causes severe head and upper body injuries in the car occupants and consequently results in a very high rate of fatality. Often the car passengers are decapitated, when occurs the so called "guillotine effect".