I saw one on I-405 recently hiding behind a broken down pickup truck. He has his car parked in front of it in such a way to where the truck was hiding it. Saw the same truck broken down again, with another officer hiding behind it, a couple of weeks later. Must be a pretty unreliable truck...
Too bad this is really the only exposure Joe Average Citizen ever gets to our police. The various municipalities make the decision to deploy their police in this manner; most cops I know hate traffic duty and hate writing tickets. I think they know what this does to destroy the good will of the people with them. Jim has the right idea. Maybe even follow up with a letter to city hall, or the county seat, explaining why you go out of your way to avoid their area. Of course if they do it where you live, that's not an option, but you can write your city or county and explain how this needs to change. Good luck with that, by the way...
As far as the temporary lowering of limits, simply return after the trap moves on and photograph the area. Get the speed limit sign with other things to verify its location. Have a newspaper or magazine with the date visible in the foreground of all photos. Use film, not digital. Bring your photos to court. Make the cop explain why it was lower than its norm on that particular day. Make them bring the ticket count from that location on that day, and another day outside of their speed trap dates for comparrison. The judge, if he/she is honest at all, will have his ass. I watched a guy do this one day in court; it was priceless.