By on January 30, 2013

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 [email protected]com. Spare no details and ask for a speedy resolution if you’re in a hurry.

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37 Comments on “Piston Slap: Reading the Light Bulb Filaments...”

  • avatar

    My ’93 Ford Ranger (bought new) has about 195,000 miles on it. For that entire time, I’ve had two brake light bulbs burn out (the left and the center-high). The right side brake bulb as well as all the turn and backup lights are stock.

    Maybe that’s rare, but it seems like a multiple bulb malfunction is not a likely cause of an accident. Even if all were burnt out, any collision is most likely because the second car was following too close to avoid it. (or the driver was distracted)

    • 0 avatar

      It’s not really about malfunctioning bulbs, but whether the car in front stopped short. You know the types (or maybe you are one);
      “I’ll show that tailgater what for!” *SCREECH*
      There’s also the ones who have their lights off and get hit because they can’t be seen. I was pulled over for this once. Had a fuse blown, driving home from work with my hazards and headlights (my corner lgihts weren’t on, but all four hazards blinked). The cop told me that if I was rear-ended it would be my fault even with the hazards. That was at least by the letter of the law.

  • avatar

    What chrome plating?

    • 0 avatar

      Oops…text changed to reflect the fact that I don’t know what makes the filament look shiny, flawless and seemingly-chrome plated to the untrained eye.

      • 0 avatar

        the filament is tungsten just like any incandescent lamp. Halogen bulbs extend life by filling the glass envelope with a halogen gas, which re-deposits tungsten back onto the filament instead of the inside of the bulb. As they age, the filament loses its “shiny-ness” because the tungsten doesn’t re-deposit uniformly back onto the filament.

      • 0 avatar

        What I notice is a dark coating deposited inside the glass, basically the same thing. Cars that use the headlights as DRL’s accelerate this. Even at reduced voltage, hundreds of extra hours of use still add up.

      • 0 avatar

        Physical vapor deposition works in a similar way. The surface of an element is vaporized in a vacuum chamber and deposited on the target material. Such a process was used to create the “smoked” chrome door handles on the GMT800 pick-ups.

        It is an understandably expensive process, so it only went on the top trim line. I’m afraid I don’t remember the material that was vaporized, but wasn’t tungsten.

        (BTW jz78817, thumbs up)

      • 0 avatar

        ……What I notice is a dark coating deposited inside the glass, basically the same thing. Cars that use the headlights as DRL’s accelerate this. Even at reduced voltage, hundreds of extra hours of use still add up……

        That lower output use actually makes the blackening worse. The process explained by jz78817 does not work as well at a dimmed setting. Thus, lights that run mostly in a dimmed mode blacken more. At home, blackened lights can be cleared up by some extended operation at full brightness…

      • 0 avatar


        “It is an understandably expensive process, so it only went on the top trim line. I’m afraid I don’t remember the material that was vaporized, but wasn’t tungsten.”

        the equipment may be expensive but the process is not. pretty much any “chrome plated” piece of plastic is actually coated with a thin layer of vapor-deposited aluminum. even in $10 car model kits.

  • avatar

    Basically, most insurance companies automatically assign fault to the person who hit you in the back, regardless what they say is the reason for hitting you – unless they got hit and ran into you in turn.

    • 0 avatar

      See above. Fault can still be assigned if your lights are off (blown fuse or shear stupidity). But you are correct, Insurance companies will initiate the claim by assigning fault to the last car in line.

      In PA, liability is first assigned this way by the police, too. “You are responsible for where the front of your car is”. The thought is that a “safe following distance” is required by law, so if you can’t stop in time, you were at a safe distance. But if you leave a two-second gap on I-83, you’re guarenteed to get cut off by literally everyone on the road.

      • 0 avatar

        Cops do assume the last car in line started the chain reaction when it could just easily be the first two. A safe following distance gets drastically reduced when the car in front of you is stopping (crashing) @6 Gs of lateral deceleration. Of course, the best rule is to look several cars ahead and not just a ‘safe’ following distance.

      • 0 avatar

        “But if you leave a two-second gap on I-83, you’re guarenteed to get cut off by literally everyone on the road.”

        So what? You have to tailgate or drive like a jerk because everyone else does? (If yes, then start thinking about YOUR chest and YOUR knees and those of your passengers.)

        After driving a cab for more than 5 years and seeing more stupid driver stunts than I thought possible, I started following at least 2 seconds (3 in bad weather) and it is now so ingrained I don’t even think about it.

        2 second following distance/time was the major thing I insisted on when teaching my children to drive during their required 5,000 miles with me (after do not drink or do drugs and drive or ride with anyone who was).

        We’d all be better off if we left our big me-first egos outside the car when driving.

      • 0 avatar

        “But if you leave a two-second gap on I-83, you’re guarenteed to get cut off by literally everyone on the road.”

        Agree with chug on this. This is ego stuff. In reality, it’s not like you get to your destination that much quicker because 3 cars jumped ahead of you.

        Following distance is a good thing, and actually helps dissipate traffic jams. People slamming the gas and slamming the brakes in stop ‘n’ go “so that no one jumps in” only perpetuates the jam.

        That’s why stop ‘n’ go traffic in the rain moves faster than stop ‘n’ go traffic in good weather — people increase following distance which makes traffic actually flow.

      • 0 avatar


        Absolutely spot on!

        In heavy traffic, I leave enough space to ride out the standing waves without having to play the stop and go game – that might be 15-20 car lengths at times. And even in BOSTON, home of the most aggressive highway drivers in the country, rarely if ever does anyone jump in front of me. If they do, so what? I slack off a little more. Might cost me all of 30 seconds in 5 miles of stop and go, but I rarely have to change gear. My average speed is about the same either way.

  • avatar

    “I doubt the Police check the tail lights/brake lights in some sort of CSI operation for car accidents.”

    They occasionally do, depending on the nature of the crash. Crash reconstructionists will be the more likely people to look at this information.

  • avatar

    The idea stems from aircraft crash reconstruction. A filament light that is on when crashing will break as the filament is almost liquid and the break will show the melted state of the filament. A cold filament is less likely to break and if it does the break will be sharp. Simple and works for cars also.

    • 0 avatar

      I can’t imagine it translates over to cars. It would take an impact force a lot higher than most car crashes to break that filament.

      NTSB air crash investigators also check the instruments for similar evidence. The needles will leave a witness mark on the cluster at impact, showing the last thing indicated. This, I HAVE seen on a car, A Porsche with it’s speedo stuck at 160mph on impact.

  • avatar
    Land Ark

    In my 2 years as an adjuster I probably read 100 accident reports. It is not the officer’s duty to collect evidence of fault but simply record the observable facts. If it is clear that one party has broken the law, they will cite the driver. If it is a serious accident and serious injuries or fatalities are involved, they do more work than usual since a witness to the accident is unable to provide information.
    The report lists who said what and where the vehicles likely impacted and where they came to rest.
    It’s the job of the insurance adjuster to go looking deeper and determine fault. If my insured hit someone and said they had no brakelights, I’d either go look at the car to see if the bulbs were intact or ask the other driver’s adjuster if they go through their own insurance company (all moot if the bulbs broke during impact).
    And the easiest way to tell is while inspecting the car have someone tap the brakes and see if they work. LEDs won’t impact that.
    Remember that the other driver’s broken tail lights do not completely absolve you of fault. You will still be partially at fault. The other adjuster can also argue that the impact caused the bulbs to blow – like when you drop a shop light. The only sure way to prove it is if a 3rd party witness states the car had no brake lights. Good luck getting a witness to stop.

    This brings me to moving your car out of the travel lanes if you are involved in an accident once it is safe to do so. The police are not going to come and take pictures and collect evidence. They are going to yell at you and tell you to move your cars. Be courteous to other drivers and don’t create a traffic jam for no reason.

  • avatar

    I was rear-ended while yielding at yield sign two days ago. My old Toyota pickup looked better than the new one that hit me. His plastic bumpers didn’t fare as well as my black metal one. Anyway the lens cover was shattered and gone. The flashers were functioning, but the officier didn’t look at the brake light.

  • avatar

    I think the OP was referring to broken filaments from smashed tail lights following a rear end collision and the offender claiming the tail lights were NOT functioning before the crash. Of course the driver of the stopped (or slowing) car can deny they weren’t working or burned out at the time.

  • avatar
    DC Bruce

    Funny, by coincidence last night I followed a Prius home into my neighborhood with non-working tailights (both). The brake lights worked and the turn signals worked; and the driver was using here headlights. So it most likely was a blown fuse. I let the driver know of the problem (of which she was unaware), and she seemed appreciative,especially since her car was a sort of black-gray color and not very visible in the dark at anything by close range.

    • 0 avatar

      One of the few good things about the 1971 Opel 1900 Sport Coupe I had was that the light circuits were set up with the left marker lights and right taillight on one circuit and the right marker lights and left taillight on another. This way you wouldn’t lose both taillights if one fuse blew.

      • 0 avatar

        For years, Chrysler products were wired with the dome light and the brake lights on the same circuit. The idea was that if the lights did not come on when the door was opened, the driver was likely to investigate why and get it fixed…

    • 0 avatar

      One of the little niceties with my BMW is that if the computer senses that a light is out, it will use one of the other lights in that general area of the car for that function. So it can flash the brake light, or light up the turn signal as a brake light. The car also has two-stage brake lights – up to a certain deceleration level it lights up only one pair, above that level it lights up a second brighter pair which are also the rear fog lights if the car is so equipped. Nearly all of the lights are LEDs though, so seems unlikely the redundancy will ever be put to use.

    • 0 avatar

      Sounds to me more like a driver who doesn’t/didn’t know that DRLs aren’t “the headlights alwys on”, and so was driving around without them. That would perfectly match what you describe seeing.

    • 0 avatar

      Failed taillights are an issue on mid-decade Prii. Do a search.

  • avatar

    I personally drive with a dash cam. It avoids the I-said vs he-said when you’ve got video and audio evidence. Mine runs on a 12V outlet with a lithium ion battery good for 3 hours; runs on a continous loop onto an SD card (at 2GB/hr); has night time IR capabilities; and has a motion sensor to automatically turn on or off. You can get one for less than $80. Ergonomics are good, but get rid of the suction cup and use a dash mount. It helps if your car has a cigarette outlet in the armrest so you don’t have to constantly plug and unplug things.

    And if you witness something dramatic, you can post it on Youtube and claim it’s from Russia.

  • avatar

    I worked with a car expert once and he explained to me that by looking at the philaments he could tell if the bulb was lit at the moment of crash – something about breaking as a hot filament with the broken edges showing that it was on.

  • avatar

    What are the hour life estimates on the LED the OEMs use for taillights? I worry about average Joe Schmo drivers letting the LEDs burn out one by one waiting weeks, months, or even years before replacing.

    I just have this feeling that 10 years from now I’m going to see a Dart driving around with 2 LEDs lit in the whole rear panel.

    • 0 avatar

      In theory, darned near forever. In practice, who knows? Certainly plenty of Caddies out there with LEDs out on thier 3rd brake light. Genuine GM build quality trumps theory every time! But it is probably not the LEDs themselves, but thier connection to the circuit board that is the issue. Bad soldering job most likely. One thing is for sure, no socket corrosion issues! That has been more of an issue for me over the years than actual bulbs, mostly because people don’t know there is a reason why some bulbs have gold colored bases and some have silver.

  • avatar

    This idea may have come from an old Car and Driver article, or maybe it was written about somewhere else too. The C&D article was about FBI investigators, automotive experts. They weren’t involved in fender benders, more like reconstruction in cases that involved a larger crime warranting the examination of experts. One of their techniques mentioned that they could examine a smashed bulb and determine if the filament was lit when the bulb was smashed or not. I doubt any officer or adjuster knows or cars about such a technique.

    The article also showed how they cast impressions of tire treads, they matched tire models to a giant book of every known tire tread pattern ahhhh the old days that gives you an idea how long ago it was, I think late 80’s early 90s.

  • avatar

    Volunteer fireman here. I have personally pulled the brake light bulbs for a Texas DPS officer investigating a fatality accident. But I agree that this level of investigation probably wouldn’t be done for a fender-bender.

  • avatar

    if u rear end anybody is always your fault, unless u have tons of money then u fight.

    should a car stalled on the fast lane then God help u, anytime u get hit its going to be carried away or almost a basket case. In Van canada several yrs ago, a family van suddenly stalled on the fast lane with smokes coming out, very unlucky some dude was doing everything else except paying attention to whatever infront, he plowed into this stopped caravan, 3 of the young kids eventually passed away, it was very sad.
    The offender was driving a BMW X5 happy go luck only one catch he was single and on a HOV lane that needed one more passenger to make him kosher.
    I was told he absconded somewhere, after a few yrs later i guess his conscious or recurring nightmare rendered him to come back to face the music!
    It was a pretty sad story nobody had wished it happened! Whether right or wong he had took away the young innocent lives.
    God help all of us.

  • avatar

    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 .


  • avatar
    Robert Gordon

    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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