Average car journey in the uk is about 8 miles. @30mph = 16mins @20 mph = 24 mins In the context of a whole day, 8 minutes is statistically insignificant. Sadly folks don’t think in those terms. Same with speeding. You ain’t gaining significant time doing 80mph but risk is exponential.

And of course the average speed over the 8 miles will be well under 20/30 resp, so the difference will in practice be even less.

Is there is a whole 8 mile stretch limited to 20mph in Wales? A long drive through Wales recently met with only a couple of speed restrictions, and the sooner the English govt copies Wales the better. People should set out a few minutes earlier.