What you said is correct only sometimes. Often, the police are hitting each car that comes by with radar, thus your device will detect them lighting up the cars in front. If traffic is low, you are in more danger of being hit without warning. Or occasionally there is an officer who understands this and he only lights up the cards he thinks are really speeding. In this case he has to watch you, so you might see him before he decides to light you up. All in all, the ordinary radar detectors do work and provide a worthwhile addition to your vehicle. I don't even speed that much, but I know mine has saved me a few tickets.