An R&W Welsh poll finds backing for the 20mph speed limit
My guess is that a similar question in a national poll would find clear support. It is striking that in this poll that Tory voters while backing the plan are not as supportive as the rest.
If this does go through you can see it getting similar support outside Wales. There are of course many more road users than drivers.
As a driver, a cyclist and a pedestrian I would be supportive because it could transform urban environments. In any case it is quite difficult to get to 30mph when driving in many towns and cities.