It's not the speed that is the problem - it's the relative difference in speed between two nearby vehicles that you should be concerned. Someone doing 55 in a 5

It's not the speed that is the problem - it's the relative difference in speed between two nearby vehicles that you should be concerned. Someone doing 55 in a 55 when everyone else is doing 70 is being a hazard, in their case following the speed limit is outright dangerous. This is of course in context of freeways - residential streets etc do require a hard speed limit.