Here in Quebec we have signs at every freeway entrance, 100 MAX 60 MIN. (~62 max, ~37 min mph)
Most people do 120 (~75mph) and the cops don't bat an eye but you can bet they'll stop you doing 59 when traffic is moving freely.
No state has a law on the books to my knowledge that has 5 under the speed limit as "impeding traffic." Utah specifically (and other states, now) has been ticketing people for doing the speed limit in the left lane. The left lane is not the "fast lane," but is for passing only. Camp in the left lane in Utah, you'll likely get a ticket.