Hello Jim,

Since you said this is a road with very little traffic can't you just
rig up two IR sensing paths? Like on garage doors? It's unlikely that
there'd be two cars passing each other in opposite direction.

Then all you'd need would be a ramp generator that gets started by
whichever IR sensor goes first. It is reset by the other sensor. If the
reset occurs and the ramp voltage didn't reach a set threshold that
means whatever triggered it was going to fast.

Regards, Joerg

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