You could also decrease speed limit to something unreasonable. For example, 15mph and issue tickets at 21mph.
You can also hide a max speed sign behind something, like a bush, and install it in otherwise higher speed zone.
You can also install speed trap on the down-slope road, where drivers would naturally speed up without any conscious input.
You can also offer "early payment" discounts on tickets, where if you pay and plead guilty your fine reduced to the point of not worth the time fighting it.
You can establish a ticket challenge procedure that would conflict with working hours, making it logistically difficult for people to challenge.
You can intentionally mail tickets to old addresses, then rake up late fees and interest.
Oh, all of these happened in one or another municipality at some point in time.
The truth is that very few speed limit signs are hidden in the manner you describe. Lets be honest, the vast majority of the time us drivers exceed the speed limit we do so knowing we are doing it, we just do not think we will get caught. We know what the speed limit is on most roads we drive down, we just sometimes push them as we are in a hurry. We are probably driving in a generally safe manner, we are just doing it too quickly.
The thing is though, speed limits do exist for a reason. They are there to force us to account for the unknown: children running into the road, morons pulling out of side streets and not looking, us getting a blowout due to something to small to see in the road. The chances of these things happening are all pretty minuscule but since we all drive everywhere so much these small odds multiple so it always happens to someone in the end and the fallout can be catastrophic and expensive for the city to clean up afterwards.
Speed limits are also actually a way of us being more flexible with other driving rules, like paying attention to where you are going. Sometimes people do some really stupid crap on the roads. Like trying to find a CD to put on and veering on to the wrong side of the road, answering the phone in our pocket that is awkward to get to, looking at maps, turning round and shouting at the kids in the back, this list could go on for ever. As it is we can do this with a certain degree of impunity as the worst that would happen is we wrote off our car and someone else's the vast majority of the time. If everyone could drive everywhere as fast as they liked the police would have to be far more ruthless at enforcing other aspects of the traffic laws, maybe even down to banning persistant offenders until they got the message.
Sometimes I actually think this might be a better idea, then I catch myself doing some of the stupid crap I describe :)