What is "Reasonable and Prudent?"
There are lots of idiots on the roads who floor the pedal and go shooting by everyone. Not everyone is like this, however. There are plenty of people who drive at a "reasonable and prudent" rate, regardless of the posted limit. It's impossible for a set speed limit to be able to apply to every time of the day and the situation. For example, you're going down a busy city street with the speed limit posted at 30. There are a lot of pedestrians and bicycles, and traffic is busy. It's a situation where going at the posted limit or less is the best option.
How about when you're on a country road with miles of straight stretches and no vehicles as far as eye can see, yet the speed limit is only 60? You will find out that your driving performance and the number of tickets you get in your lifetime depends not on your following of the posted limits, but your ability to judge the situation and drive at a speed fitting for it. Police themselves regularly use common sense for what speed is suitable, which is why they frequently let people get away with highway speeds 10 mph over the limit, as long as the driving situation doesn't make that speed dangerous.