What are the two cables in some roads?

A large wheel with lowish pressure (lorry) will have more contact with the ground than a smaller wheel (mini etc). The lorry will appear to be going faster due to the shorter amount of time between the wheel releasing the first cable and pressing the second one (due to the increased contact area).

NOT used for speed ;)

I imagine the timing function is simply initiated by the time point 0 matching when the resistance in the wire 1 spikes and time t is stopped at the point where wire 2 sees the sam condition. It doesnt really matter when or even if the 1st wire is released before wire 2 sees pressure.

you have t from that and you know d frim when you fitted the wires to the ground...
 
1 tube across the road is a traffic counter

2 tubes across the road is for speed measurement.

3 tubes across the road is usually a speed trap. There is normally a video camera in a van tied up to them. They are normally used like this in built up areas where camera vans cant be used.
 
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