In the US, speed limits on main roads vary from 55 - 80 mph. The US police enforce limits more rigorously than the U.K. I think in the U.K. we should be less rigid with the uniformity of limits, for instance the M6 through Birmingham could arguably be 55 mph, but north of Manchester it could be 80.
Salisbury was one of the first IN THE UK to implement a 20 limit, complete waste of money as it’s almost impossible to get up to 20 mph.
Just need a bit more thought