I dont like 'blanket' 20 mph. There have always been many roads where 30 mph is too fast , and some where even 20 mph is still too fast for the circumstances. But plenty of roads where the current 30 mph is about right. I like to think I am a responsible driver. I often drive well under the 30 mph limit on certain roads,and situations . But it can be quite tempting to exceed the limit if you regard it as too slow. We all think we know better. I always resist the temptation of course ,

I live on a main road into town . It is 30 mph. Its 40 mph just up the road, where the houses are identical. Indeed my house is on a short stretch that is dual carriageway. As a resident I am NOT calling for MY road to be 20 mph. It would be ridiculous. Its a road where its tempting to drive faster, and thats why it has a speed camera. I just wish they could stop those on illegal e scooters hurtling along the pavement at 30 mph, so I can emerge safely from my driveway, by car or on foot. Its a bit of a hill too. Would cyclists respect the 20 mph limit?
Its normal in the uk to assume if there are lamp posts its 30 mph unless there are speed signs to the contrary.
Saves on the cost of putting up so many signs. But i think drivers are more likely to respect speed limits if they are always well signed, and accepted as appropriate for the immediate locality. Even if its 10 mph in places.
This doesnt mean I am anti there being a lower limit on many roads. But over use because thats the default speed can breed contempt.