They are illegal in some states. Don't forget, you don't just need radar...............
You need a lazer detector, too
AND modern lazers/ radar can stay shut off if the cop wants to, and turned on quickly to get a read. Too late for you to slow down!!!
Even today's lasers have what is called splash. The idea is to pick it up before you get into it's range, while the cop is timing someone else on the road.
One of the best radar detectors ever, is completely free. It's called tail lights.
Especially useful when cresting hills. If a car or a group of cars tap their brakes while cresting a hill in front of you, it's usually a pretty good sign that there's something there that is out of the ordinary. Most times that something is a cop.
Same goes for a motorist driving in the opposite direction, flashing his lights. That's usually a pretty good indicator that there's something ahead to be aware of. Slowing down for a moment cost you nothing, and usually means you'll arrive at your location at exactly the same time, anyway.
Another good indicator is truck traffic. Big rigs usually push the speed limits, if you notice the trucks around you slowing down, especially as a group, it's a good chance the got the mail...
These things aren't new to anyone, but, I'm still amazed at the number of cars I see going speeding by when I slow down because a group of cars a quarter mile or an eight of a mile ahead of us all hit their brakes at the same time.
To be honest, I'm also tickled with I see one of those speeders-by stopped a little further down the road simply because of his failure to pay attention and
"read the road".