The idea of a speed limit is sound. But there are organizations that work to game the system. A great example is the multitude of sleepy towns right next to highways. They have a police force that is exclusively utilized on the five miles (or less) of highway that officially fall within the town limits, writing tickets to people who pass through. For example, Lindale, Ohio has a population of about 170 people and collects about $800,000 a year from speeding tickets.

My mother got snagged in one of these in Virginia a while back - they have a hill where the speed limit drops from 65 to 45 at the bottom of it, with a specially planted copse of trees where the police sit with their detectors. A quarter mile down the road, they have a specially built parking lot for people to get their tickets. They even provide envelopes and stamps for people to send their checks! Nice people.