For me entering my 8th decade, driving at 30 at night now seems much faster than it used to especially with all the parked cars where I live in Inner London affecting my judgement. It is very different in the day time when 30 sometimes appears pedestrian to me.

20-25 at night seems more comfortable for me in built-up areas than 30. The 20 mph zones we have locally are probably sensible after dark, though not in the daytime. Maybe, there should be variable speed limits according to the time of day and lighting conditions?