i had a friend's friend who was working with police planning.
you know all those red light cams? well some are empty. they rotate
the cameras around every month or so. to where i dunno. so some
cameras are a deterrence only.
as for the speed thing, it's more technical, so bear with me. they
use radar waves, cos IR is found in white light, and might confuse
the signal. the speed cam shoots out waves in intervals. it's
actually not how fast the beam comes back, because all
electromagnetic waves travel at same speed. IR waves and radar
waves travel at same speed. so during one beam shoot, the beam
bounce off your vehicle and bounce back, the computer notes the
distance. the next beam does the same thing. after 2 shots, the
camera would know how fast you're going, because it knows how much
you travel in that short interval of time. if you're too fast then
you kenna already. 
hope this helps.
