This is a great idea and, personally, I think is the optimal solution to the issue.
It'd be cool to script in a feature where high ranks in law enforcement factions can put these speed cameras up, and only have a finite amount available; five, maybe ten? (Maybe allow PD/SD to buy up to X additional ones — the faction itself, i.e. on a budgetary level, rather than individuals buying them — and make them expensive).
This enables traffic enforcement oriented divisions (in both PD and SD) to adjust these as necessary (i.e. on streets that often feature 100mph+ drivers), while also preventing people from learning where exactly these speed cameras are (on LS:RP, speed cameras were in the same position for years and years until their eventual removal, resulting in people knowing where they can speed, and where they cannot). Since they are dynamic in nature, this provides an omnipresent traffic enforcement feature — one that does not stagnate. I think it's more than possible, script-wise, since law enforcement factions already have a speed radar, which allows you to set up a small-ish red circle that, when a vehicle drives through it, displays its speed. Could simply make the circle bigger to cover a whole lane/intersection/whatever.
I also think it's a good idea to make them activate only for vehicles travelling at a truly inordinate speed. 20 mph or over seems OK, but I wouldn't mind it being even more (30 or 40mph+). Drivers that go 10-15mph over the limit aren't really part of the problem at hand, so they can just be handled by actual players role playing law enforcement (e.g. Traffic Division).
I do agree, though, this is a big problem. As a PD faction member I see it all the time, and if we were to pull over every single person speeding, we wouldn't have much time for other law enforcement activities. There's simply too many people who do this. Another approach would be to massively increase the fines for speeding and other traffic violations. I feel that right now, people just shrug off most fines issued for speeding. A $2,500 fine really isn't that bad, nor is $5,000. In an ideal world we'd look at tripling (or even more) these numbers.
This is also a great point — the fact the issue stems from the impatience of players rather than characters. Unfortunately I don't see many solutions to enforcing this on an out-of-character level. Admins can spectate, but that takes away their time to handle reports (which, arguably, are often more important). Perhaps admins can be given a command to toggle specific notifications of players speeding throughout residential areas at inordinate speeds for a prolonged period of time. That way, they can just spectate them for less than 5 seconds and make a judgment.