Originally Posted by semtex

Actually, maybe it does make sense. If the cop is going West at 60mph, and his radar unit zeroes itself at that speed, then any object moving away from him in the rear at 60mph would automatically be adjusted to show as zero. That would automatically filter out stationary objects. Any object moving away at more than 60mph would register as a positive speed. So an object moving away at, say, 100mph would show as doing 40mph. An object moving away at 120mph would show as doing 60mph, and so on.