Sydenham hill possibly comes under Southwark which, Red routes aside, is a 20mph borough.

Until we are all in google cars, automatically able to adjust to different speed limits on a road by road basis and perhaps also depending on the time of day and weather conditions, then blanket speed limits are the easiest to implement.

Having said that, with the main entrance to the Horniman play triangle at the bottom, I think a lower limit is justified towards this end. Sorry.