TTAC commentator Celebrity208 writes:

Sajeev:
I’d always thought that police crash investigators would check the tail light bulbs of a car that was rear ended to determine if its lights were on at the time of the crash. I thought it had something to do with the way the filament was broken/burnt/etc. So my question is two-fold, am I crazy and do they do this, and if so how might LED tail lights remove this piece of forensic evidence regarding correctly operating brake lights at the time of an accident (presuming the fault was contested)?

Sajeev answers:

Hi Clayton! You are not crazy (I hope) but I doubt the Police check the tail lights/brake lights in some sort of CSI operation for car accidents. For two reasons:

1. The wear from cold or hot “restrikes” of a tail light bulb’s filaments probably don’t tell much, other than their remaining lifespan. And once the vehicle crashes, well the evidence could be destroyed. Headlights, however, are a different story.
2. Why bother with this when we have event data recorders?

Here’s a list (unverified for accuracy) of late-model vehicles with EDRs.  Basically any vehicle with an OBD-II computer (1996-present) is capable of recording a metric ton of data as to your driving habits.  Combine this 1990s advancement with the ancient technology of the brake light switch and you’ll know exactly what was going on before the accident.  Why bother looking downstream (light at the back of the car) when you see the source upstream (at the brake pedal assembly)?

Bonus! A Piston Slap Nugget of Wisdom:

This talk of lighting filaments always takes me back to a core truth of automobile ownership: check the filaments on your (Halogen) headlights!

If the chrome plated(?) looking finish on those tiny wires isn’t flawless, replace the bulbs. In pairs!  Life is too short to risk it all on $20-30 worth of new bulbs, as they degradate so slowly that a visual inspection of the chrome plating is the cheapest and easiest way to ensure your nighttime driving safety.  I’ve seen 2-year-old vehicles in dire need for new bulbs!  So it happens, and you better do something about it.

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry.