The expression "Rush Hour" suggests an hour of particularly heavy and congested traffic almost certainly due to commuter and school run traffic.
I commute from my village to Liverpool city centre every day, a distance of 19 miles on mainly A roads. Even when I start at 7.30am I am usually in traffic jams. There also now seems to be a lunchtime rush hour where the roads are jammed up between 11.30am and 1.30pm and the rush hour home starts at 3.30pm and it is still chocker until almost 7pm.
It's probably rush hour because everyone is in a rush to get somewhere but they can't.
I think it may be an hour in some places at some time. Commutes seem much longer these days. I think it was assumed folk'd leave work and everyone would take a whole hour to get home and clear the roads.