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Sometimes when waiting for a traffic light, I see the reversing lights on the vehicle in front of me briefly flicker on and then off again. Most often, this is on Mercedes people vans (short buses).

This startles me, since if the vehicle were to be put in reverse by accident, it would hit me when trying to drive away when the light turns green. But it always turns out to be a brief flicker of the reversing lights.

What could be the cause of this?

Is the driver doing something wrong, accidentally putting it in reverse? Is it faulty wiring?

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    Only worry if the backup lights stay on when traffic starts to move, then get ready on the horn.
    – Xen2050
    Commented Dec 1, 2017 at 10:28
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    They're automatic vehicles - the driver has put it in Park, and then needs to move through Reverse to get to Drive. The alternative is to leave it in Drive and put their foot on the brake to hold the car in place, so you get Brake lights instead.
    – PeteCon
    Commented Dec 1, 2017 at 11:08
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    BTW: people who spend the work day driving in the city do this to save their ankles. It gives their braking foot a rest. I know I do when In NY with a truck, and in the majority of taxis I have taken the drivers do this as well.
    – Yorik
    Commented Dec 1, 2017 at 15:49
  • Amazing -- one of those questions every generation has to learn all over again. Maybe, if EVs manage to take over, or even ICE cars with the Mercedes-Benz-style T-shifter (center is neutral/park, up reverse, down drive), this phenomenon will disappear from our world.
    – Carl Witthoft
    Commented Dec 1, 2017 at 17:36
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    @PeteCon That comment should go in the answer box!
    – Tim
    Commented Dec 1, 2017 at 19:46

4 Answers 4

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It's simply because some cars will illuminate the reverse lights when the drive moves the gear selector from D, through R on their way to P when stopping for extended periods.

My Porsche, and my Range Rovers before only showed the reversing light when the gear was actually engaged. Other cars seem to have the light wired to the gear selector position.

It's only a cause of concern if the light stays on, at which point you try and work out whether the car in front is more expensive than the car behind you while leaning on the horn.

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    May I add, putting it in P or N saves you fuel when standing. At least was that way with my VW Passat V6. So there is a reason to do this beyond getting you foot of the brake.
    – Daniel
    Commented Dec 1, 2017 at 11:27
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    Well and I did not say you should. But, I live in flatland, so I could ... BTW: Did I tell you about the woman in the BMW behind me, that took her foot off the break while searching in the glove department while in D? ... ;)
    – Daniel
    Commented Dec 1, 2017 at 11:37
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    @Graham The torque converter itself does not wear in any significant sense during use, as the only wear is friction of oil against its impellers. Torque converter failures are related to impellers that break off, pumps that fail and bearings that go bad. Furthermore, while keeping an idling vehicle in "Drive" does waste some fuel (which is lost through heated transmission oil), the viscosity of the transmission oil mean that the torque converter does not transfer significant power at engine idle, making the loss not very significant.
    – Kenny
    Commented Dec 1, 2017 at 15:05
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    "at which point you try and work out whether the car in front is more expensive than the car behind you" This seems like bad advice. If you do nothing and allow the car to hit you, it will be the person who backed into you at fault and whose insurance will have to pay for everything. If you back up into some one in order to avoid some one else, you may be found at fault.
    – Shufflepants
    Commented Dec 1, 2017 at 15:10
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    Obviously, not so obviously...
    – Tim
    Commented Dec 1, 2017 at 16:03
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As a (former) automotive software engineer, I've spent some time working on car engine controllers.

As @Snow says, the gear lever on an automatic selects whether you're in Park, Reverse, Neutral, Drive or Low gear. The gear selection is arranged in that order, so in the industry you will frequently find this called the PRNDL, to specifically identify the lever (or the sensors monitoring the lever's position) and not any sensor within the transmission itself.

Older cars used a separate switch within the PRNDL or transmission to control the reversing lamp(s). These days, most things are under software control, and there is an increasing drive to reduce sensors for reasons of reliability and cost-saving. On a modern car therefore you will find the reversing lamp driven by software control. The PRNDL position is measured over its travel (usually using a potentiometer, but sometimes a rotary encoder), and using this measurement the reversing lamp is turned on when the PRNDL is in the reverse-gear position. This is just one part of the software finding what gear the car is in, of course, which is used for pedal demand maps and various other settings.

Of course with the PRNDL gear layout, you have to go through Reverse to get to Park. The software therefore uses a timeout mechanism to decide whether to light the reversing lamp, to reduce the effect you describe. The driver needs to have been in Reverse for a short time (maybe half a second, perhaps longer) before the reversing lamp is lit. If the vehicle calibrators have not been conservative with this timeout, it's quite possible that the reversing lamp can flash inadvertently. There can also be a problem with old/worn/sticking gear levers where the user cannot move it through gear positions so easily, meaning that it spends longer than usual in the Reverse position.

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  • I guess you're talking about automatic transmission, as manual transmission cars are virtually nonexistent in the US - but what about manual transmissions? I always thought they're still pushing cogwheels around, and have a separate switch to detect reverse, am I wrong?
    – Guntram Blohm
    Commented Dec 1, 2017 at 17:03
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    @GuntramBlohm You will never see a manual transmission car flicker their reverse lights (assuming no software/hardware faults), because reverse is out of the way of normal gears. I have a manual transmission, and my reverse lights only come on right before I back up.
    – phyrfox
    Commented Dec 1, 2017 at 17:12
  • @GuntramBlohm though, as I said, that assumes no hardware faults. It seems like a lot of cars these days have some sort of faults when they come off the assembly line. My wife and I bought some 2015 cars (one each), and they've both had recalls for rather major potential disasters already.
    – phyrfox
    Commented Dec 1, 2017 at 17:15
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    @phyrfox 2015 cars? That's a lot! O. One each. I see. :)
    – SQB
    Commented Dec 1, 2017 at 18:44
  • @GuntramBlohm Yes, like I said, this is just about the gear lever on an automatic. It will also happen on tiptronic manual/semi-auto gearboxes which use a PRNDL lever, because it's about how you select gears and not how you move the gearbox cogs around. On a regular H-pattern manual gear selector though, as you say, there has to be a separate switch for the reversing light.
    – Graham
    Commented Dec 4, 2017 at 11:13
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It will only happen with automatic gearboxes. as already said, the lever needs to go, on some vehicles, through R to get into D. It's that moment that the reverse lights flick on and off again. If they stay on, prepare for some fun. If it happens in a manual vehicle, the driver has messed up, and gone past the gate, and selected reverse instead of first. If it's a short flash, you're lucky. as they've realised, and found the correct gear!

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    Not disagreeing with you, but can also happen with a fidgety driver with a manual transmission (ask me how I know).
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Dec 1, 2017 at 15:50
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    @Pᴀᴜʟsᴛᴇʀ2 also an old banger with everything worn can take a bit of jiggling to get into first. Going into reverse can sometimes help.
    – Chris H
    Commented Dec 1, 2017 at 16:24
  • @ChrisH - Great point and very true!
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Dec 2, 2017 at 0:58
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Depending on how sensitive the reverse gear detector switch is, in the car in front of you, this may happen with manual transmissions as well. While we learn at driving school to change to the lowest gear and use the motor brake to decelerate, pushing the gear to 1 is actually quite hard while the car is still moving, with many cars.

As a result, I typically just press the clutch and brake, and when the car has come to a stop. switch to gear 1. Sometimes, a few seconds later, I wonder if I'm really in gear 1 - about twice per year I forget and try starting in gear 3, much to the dismay of everybody behind my. So I may switch the gear again just to make sure I'm really in gear 1.

With my current car, even switching the gear from 3 or 2 to 1 while standing triggers the reverse lights for a split second; at least, that's what a colleague told me when he drove behind me and met me on the company parking lot.

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    Where do you learn using the gears to decelerate? Thought that went out decades ago! And with synchromesh boxes - around for many, many years, getting 1st gear should only be a problem with 50 yr. old cars.
    – Tim
    Commented Dec 1, 2017 at 16:00
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    Well, I am old. So is my car, even if it's 10 years, not 50 years, and pushing the gear to 1 is real hard while the car is moving, so I generally avoid that. Anyway, the point is: if you have a manual transmission, and you switch gears after the car comes to a stop, the driver behind you may see a quick flash of your reverse lights.
    – Guntram Blohm
    Commented Dec 1, 2017 at 17:00
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    Bad driving habits, and further if the reverse light is going on either your gearbox is about to die or your shifting patterns are far worse than you think they are.
    – Carl Witthoft
    Commented Dec 1, 2017 at 17:39
  • @Tim I thought that they had speed stops to prevent you engaging first gear when travelling too fast, it's not a meshing problem it's a don't engage first at 70mph it'll break your engine problem!?
    – pbhj
    Commented Dec 2, 2017 at 21:40

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