In Woodbury, traffic signals usually operate in one of two ways: using vehicle detectors, usually a wire loop buried in the pavement, or via video detection, which uses cameras mounted on mast arms or signal poles.
The vehicle detector loops work like a metal detector, sensing the car's magnetism, not weight. Video detection uses a camera that can recognize an approaching vehicle (Radio Drive/Lake Road and Tamarack Road/Interlachen Parkway use video detection).
In both of the systems, the detectors send a signal to a computer, which then changes the light at the appropriate time in the signal sequence. The signals do have some fixed timings entered, such as minimum and maximum green times, yellow time, or pedestrian timing, but otherwise the signal responds to the traffic that is present.
In the middle of the night, the signal might stay green for hours in the same direction if there is no cross-traffic.