In Reply to: TRAFFIC LIGHTS frequency posted by Ben-WA on June 26, 2000 at 00:06:28:
Back in the '70s they used the "plectron" system to control the
changing of light or runs of multiple lights for emergency vehicles;
I remember as a dispatcher for Thurston Co. Central Fire the engine
would ask for "light #x" which the engine read off a sign mounted on
a telephone pole as they approached the intersection. It was
basically a double tone FM signal much like a pager works. For the
last 20 years or so they have been using the strobe system that
others have commented on above which use a coded string of flashes
that one couldn't possibly duplicate without having the control
circuit programmed with the correct string. It looks like a very
rapid strobe but in fact it is coded in a repeating sequence and
apparently works well even in rain and fog.