Piston Slap: Reading the Light Bulb Filaments

Sajeev Mehta
by Sajeev Mehta
piston slap reading the light bulb filaments

TTAC commentator Celebrity208 writes:


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.

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2 of 37 comments
  • -Nate -Nate on Jan 30, 2013

    They do occasionally check ~ Over a year after the crazy a$$hat armo in the rinky-dink Taxi co's cab ran me over on my Moto and nearly killed me , his fly by night lawyer showed up with an accident investigator in tow to look at what was left of my mangled Moto ~ he actually took the time to look at the smashed bulb and said ' yep , both filiments were lit ' . I knew this as I was waiting for a red light when the jerkwad ran me over @ 50 + MPH and never touched his brakes.... Not like it matters , I'm crippled for life(crushed vertibrae) and have to wear a back brace and use a fracking _cane_ for the rest of may days.... nearly five years later and I still can't walk upright and have chronic pain you can't imagine . -Nate -Nate

  • Robert Gordon Robert Gordon on Jan 31, 2013

    I am police trained in crash reconstruction and can confirm absolutely that the condition of the filament is an important tool when dealing with major collisions. A filament will deform differently when shocked whilst they are on compared to if they are off.

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