... when driven continuously without stopping, like a on cross country limited access highway.
When driving in the places people live, with cross walks and stop signs and children playing outside requiring frequent slowing & stopping, there's no efficiency benefit from racing 0 to 50mph every block then slamming on the brakes, only to repeat for each block after.
I frequently drive through 20 mph areas with little stop start traffic. I rarely drive at busy times.
There is nowhere in the UK I can think of that has had a 50 limit in my lifetime that requires frequent breaking. 20 mph limits are invariably reduced from 30.
