I would like to modify the third brake light of my car to add blinking when a strong deceleration is detected.

What threshold should I set? given the basic rules of friction, the maximum possible deceleration on flat roads is (friction coefficient) * g. That is about 0.9 g.

I thought about setting 0.6 g, since getting 0.5 g is quite easy to achieve and cannot be regarded as "strong" deceleration, worth a warning like blinking the third brake light.

The sensor will detect only acceleration in the driving direction, no side acceleration and no detection of gravity: depending on the pitch of the car, the effective deceleration needed to trigger the blinking may be slightly higher or lower.

  • 1
    Messing with the third brake light may not be legal depending on where you are, it might be good to look into that before you do it.
    – GdD
    Sep 12, 2016 at 9:12
  • It was not even there in the first place. In any case, in emergency situation it could be useful to notify I'm actually quickly decelerating, it could improve safety. I doubt it would be detected or object of complain. The mechanic doing the periodic check doesn't mind, so it's a risk I take.
    – FarO
    Sep 12, 2016 at 9:29
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    By the way it's legal, at least on stock cars in association with ABS engaged. motor-forum.nl/forum/list_messages/351501/…
    – FarO
    Sep 12, 2016 at 9:40
  • Out of curiosity, what do you mean by the maximum possible deceleration? It seems to me if you have hurricane velocity gusts in your face, your deceleration will increase.
    – anonymous2
    Sep 12, 2016 at 11:55
  • Using the brakes only, on a well maintained road surface, flat. mgf.ultimatemg.com/group2/brakes/big_brakes/brake_myth_1.htm "We know that the maximum deceleration of any tyre is a constant - and is dictated by the coefficient of friction between the tyre and the road surface. Let's say that the maximum rate of retardation our tyres can generate is 0.9G."
    – FarO
    Sep 12, 2016 at 12:44

1 Answer 1


I've seen this implemented in two ways, Citron have a system which engages the hazard warning flashers under heavy braking. I've also seen a BMW which illuminated it's rear fog lights under extreme braking; thus the brake lights became even stronger.

Surely you'd set this at or almost at the maximum braking force the car could generate; i.e. if the maximum the brakes will pull when stamped on is 0.9G, set it to 0.875G. Alternatively, as suggested in the comments, tie it to an ABS trigger although in theory, your best possible stopping distance will be dancing on the threshold for this without actually triggering it.

If it were me, I'd find somewhere private or quiet, attach monitoring and do some stopping tests.

  • Thanks for your contribution. I planned to connect a buzzer anyway, to ensure I get notified when it's activated. I didn't want to set the threshold close to my maximum because I want the car behind me to be alerted early, so that they have time to notice it, think about it (if needed) and brake hard without hitting me. Here people drive VERY close, that's why I want one. My car has no ABS, no luck about using that.
    – FarO
    Sep 13, 2016 at 17:20

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