Back in drivers ed, we were taught that it's more important for safety to match everyone elses speed, not to go the speed limit. Police cars parked along the highway cause everyone to slow down, and everyone knows that it's nothing but a source of revenue.
I drive an hour each way to school, five days a week. From my experience, and I know from yours as well, you slow down as soon as you see a cop (if you're speeding or not), and then when you're a quarter mile past them, you resume doing 10 over. The net effect is that people obey the speed limit for a quarter to a half mile, the state gets more money, drivers are pissed off, and everyone's rapid change in speed makes the roads less safe.