Speech issues aside...
I think it's not a terribly uncommon opinion (I might even be in the majority on this, but maybe not) to feel that speed limits are generally bullshit. Especially on the wide-open highway, which is the most common place to see speedtrap-warning light-flashing, anyway. Bullshit laws should be violated, and it is in everyone's interest for everyone else to ignore them, disrespect them, resist them, mock them, undermine them, and shit upon those who enact them and enforce them. "Fuck you pigs" and so on. I want people to be able to drive 90 MPH in arbitrarily-marked 75 MPH zones, and I want them to get away with breaking the law, just as I hope to get away with breaking the law, too.
Usually. There's a catch, and this is where it gets really fun.
I've been caught speeding several times over the last few decades, and yet despite what I wrote above, about it being good for people to get away with it, I also think that every time I got caught, I deserved it and it was also probably in society's interest that I didn't get away with it. WTF?! Contradiction?
No. I deserved it, not because I was speeding, but because I was inattentive enough to get caught. I really was driving unsafely, but speed wasn't the main safety problem.
Speedtraps are an "are you paying attention?" TEST! And while I want people to generally get away with speeding and for speeding laws to not be successfully enforced, and for the cops who try to enforce them to have horrible days of frustration and failure with nothing to show for it, I don't want people to get away with failing "are you paying attention?" pop quizzes. I'd like everyone to pass those pop quizzes, but of course, we all know not everyone will. And if people fail their pop quizzes too often, I want 'em to flunk out of driving on public roads. So maybe not-so-fast on the "fuck you pigs" that I said above. I usually mean it, but this time, there's a lot less conviction behind the angry words. Sorry, pigs! I suppose I owe one of you a beer for all the "fuck you"s.
Flashers: you're helping daydreamers, space cases, vision-impaired, mirror-makeup queens, drunks, dumbfucks, etc cheat on their pop quizzes. Do you sincerely think that is a good idea? I am totally with you in disrespecting the law and thinking the government is, on average (with some admitted exceptions) is generally evil and that usually good for everyone, that it be opposed, restricted, resisted, mocked, shat-upon, disregarded, etc. But "are you paying attention?" tests really are a good idea and in the interests of public safety, no?
You're not worried about people speeding, but I don't think I have ever sat next to any other driver for very long, who didn't eventually rant about one of those people out there, who is driving like a dumbfuck. The cellphone jabberer is almost cliche, where just driving behind them watching how their car moves, you can tell what they're doing without even needing to see them holding the phone. Testing is how you're going to weed those fuckers out.
All we need, is just for it to be a fair test. It can be sudden and demand some quick reactions, but if it's further away than what a car should reasonably be expected to ever have to deal with, then whoa there. The cop shouldn't have concealment or look like something other than a cop (an "innocent" van, for example). But if you want to implement the test as an "innocent" van that suddenly looks like a cop and then starts measuring your speed so people have one second after that to get under the limit, ok. That sounds like a good test.