That's only if he has his radar on. Cops used to keep the radar turned on so they could rest their finger (i guess), but now they wait till they see YOU, then shoot the radar, which is usually too late for you to slow down.
You could try radar jammers, but most state police use a combination of laser/radar and other technologies (even the good old stop watch method). If you use a radar jammer, and a cop gets a reading of something like 200 mph, he'll just try it again and will get a good reading at some point.