Why would a road, which is designed to carry vehicles, not be suitable for a car?!
Croydon does have some tight side roads, due to terraced houses with only on street parking, you'd drive down those at 20 just out of common sense.
I drove through Croydon recently, into Clapham via Streatham, 90% on main roads, pretty much 20mph all the way, the majority of drivers were going over 20, not by much, but enough to make the journey slightly less tedious. It still took bloody ages to travel just a few miles.
Perhaps I could have chosen better wording.
How about "not suitable for today's larger cars and heavier traffic densities"?
Especially when what should realistically be for the residents are used as rat runs and cut throughs.
