from http://blogs.motorbiker.org/blogs.nsf/dx/06092005104801MWECC4.htm "Here's a question I get often enough. It's a known fact that in France when you see cars and motorcycles on the opposite lane flashing their lights, they are signaling an upcoming radar speed trap. QUESTION: is this legal ?? Will you get arrested for warning others about the radar. ANSWER: No !! This is very legal, since you are preventing other motorcyclists (and cars drivers) from committing a misdemeanor !! Same if you see someone leaving a store without paying for an item, and you tell that person not to do it. So fire away !! When you see a radar somewhere, alert the others !!"