Loop detectors on the free way
The loops in the freeway are buried coils of wire. They are
connected to a cabinet on the side of the road (not pictured).
Inside of the cabinet is a device which measures the change
in inductance of the coil. By doing this it can count how many
vehicles went over the coil and how long they were on the coil, namely, volume and occupancy. Speed is roughly equal to the ratio of volume to occupancy, multiplied by the average vehicle length, but it's not always true in general. For more details, refer to
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