# Which acceleration threshold for blinking brake light?

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.

• 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
• 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. 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