Mostly just freeways. I don't think it's heavily enforced. The idea is that cars traveling at drastically different speeds on the same road are more likely to cause an accident. It's best to drive "the speed of traffic" because that's what is predictable. Roads should be designed in such a way to make the target speed limit the fastest speed at which most people feel comfortable anyways, rather than just obeying a sign. So a 20mph road should be skinny and not straight. A 70mph highway should be wide and straight. Back to the point, though, in a traffic jam, all the cars are slow and therefore the speed differential is small already and therefore no reason to ticket anyone.