Abstract: An external vehicle airbag system (10), a method of its use and a principal vehicle (10) on which it is mounted involves mounting an airbag (16) at a bottom side (24) of the principal vehicle at a position spaced substantially laterally inwardly from a lateral-side periphery (27) of the principal vehicle. Also involved is a sensor (20) for being mounted on the principal vehicle at a position more laterally outwardly, in a direction toward the lateral-side periphery, than is the airbag. The sensor senses a collision of the principal vehicle with a colliding vehicle and, in response thereto, inflates the airbag below the principal vehicle, along the bottom thereof toward the lateral-side periphery, to thereby absorb energy from the colliding vehicle and prevent under-riding. In one embodiment, the sensor is positioned approximately at the lateral-side periphery and it can be mounted on a rigid under-ride guard.