A tungsten lamp filament can tell a crash investigator a great deal post collision if you look carefully. Was it on or was it off? The outcome may mean that the headlights were on or that the signal light was never used. This information may be important to the outcome of the investigation.
A lamp that is on has a filament that is very hot and pliable. Subject it to a significant shock, such as the forces of a collision, it will stretch out inside the glass envelope, possibly even breaking. If it breaks, the broken ends will be pulled thin as the hot tungsten stretches.
If the glass around it breaks, little bits of glass will be seen melted to the filament if the power is withdrawn before the filament burns in the air. If not, the filament will be black and it's surroundings coated with the yellow-white powder of oxidized tungsten.
A lamp that is off has a filament that is cold and brittle. Subject it to shock and it will break leaving intact coils inside the glass envelope. The ends of the filament will be sharp and flat. This could also mean that the lamp had cooled below 400 degrees F prior to the shock.
I questioned this in the article Left Turn Surprise but with the new LED lights which is almost all turn signal bulbs today is there any way to exam them to see if they were working? I understand the filament in the old bulbs but do wonder about the LED.
I left collision reconstruction before LED, but after GDL (Gaseous Discharge) headlights. Then the lamps with filaments might tell a story and GDL's did not.
I have not heard of any way to determine post crash on/off for either GDL or LED lamps.