Light bulb filaments from motor vehicles involved in collisions provide valuable objective information on accident causation. For most moderate-to-severe collisions, at least one light bulb of the car will offer useful evidence to the investigator. Changes depicted by one bulb may be so conclusive as to allow considerable insight into the status of vehicle lights and related factors at the time of the collision.

This paper presents the classifications of automobile light bulb filament changes brought about by collision, and the significance of such changes. Filaments removed from vehicles involved in UCLA's full-scale collision experiments provide calibrated references of collision-induced changes in filaments under known "on" or "off" conditions of lights at the time of impact. For the collision investigator, procedural suggestions concerning the status of vehicle lights are also included.