> Well, before Olin clipped my post for the humor value, I'd suggested a
> wheel sensor (without realizing the original poster was looking for a
> stand-alone installation), or some sort of ground motion sensor.  I
have
> no idea what that sensor might be, other than perhaps some variation
on
> an ultrasonic flowmeter.

You could do Doppler microwave underneath the car without too many
problems.
Cosine error would have to be dealt with, but that ends up as a
constant.
Then you just look at how fast you are slowing down, and handle the
brake lights appropriately.

You have the advantage of knowing the speed, as well as the delta speed,
where pretty much everything else discussed of late has significant
complicating issues, or is measuring something indirect, like brake
pressure or pedal position.



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