But putting up 20mph signs isn't really the same as having a 20mph speed limit.
We already have notionally 20mph roads everywhere but they are largely ignored as there is little to no enforcement
It's probably one of those generational things that will take a long time to settle into cultural norms. We've got, what, 70+ years of motorist supremacy to undo? Can't expect behavioural change this quickly, but it will come.
All laws are mostly enacted by consent. It's impossible to enforce something universally without if unless you go dictator, so best to push hard for change to norms with light touch. I reckon.