What can you do if you have conflicting testimony about whether headlights or taillights were on or off? The answer to the dilemma may be in the wreck itself.
When an automotive lamp is incandescent and giving light, the filament is hot and in a plastic state. If the incandescent lamp is subjected to the shock of a typical crash the forces on the lamp will cause filament to stretch.
The photograph shows just such a lamp. Notice the filament coil is no longer tightly wound.
Evidence that a lamp was on is definite.
Evidence that a lamp was off is less definite. The lamp depicted in the next photograph was either not incandescent when it was shocked or it was not shocked at all.
Notice the difference in the filaments. The second is still tight and symentric.