Faced with bad drivers, government sets speed limits low so that any crashes are less destructive. Driving is a conditional right. It should be conditional on being a competent and considerate driver. The way the law works now is that driving is an arbitrary privilege granted by government. If someone has an infraction or minor violation unrelated to driving, and he does not pay it, the state can refuse to renew one's driving license.
Government uses the driving privilege to enforce unrelated laws.
Since driving is a privilege, government can set arbitrary rules of the road, and this is a good source of revenue, since most folks violate unreasonable road rules. I heard that some small Southern towns make a good living with hidden speed limit sights. Reckless driving or driving without a license should be stiffly punished. In the city, speed limits should be set for reasonable safety. They now seem to be about 10 miles per hour too slow everywhere.