The same sort of arse that sits right behind you riding in the bus lane, then overtakes and pulls in immediately at a bus stop? I know them SO well.
I don't have any issues with making a whole bunch of roads 20mph, ALL residential roads for a start. But Brockley Rise, Stondon Park, Brockley Road..... seriously? And that is just a single example. I did wonder if the off peak bus timetables would be rewritten to allow for the decrease in permitted speed. Have yet to look into this further.
The average speed in London is a very awkward thing to calculate, as it is based on hot spots, and not entire journeys. Also an average speed is very different to the speeds you are permitted to travel at on a clearer road. "making up time" is the phrase most used.
The ability to drive at a predetermined speed along a long road in London and hit all greens in pretty much a thing of the past. With junctions and roads lights being controlled by various bodies, and each having a specific aim. Not to mention the junctions controlled by TfL which are frequently adjusted depending on traffic volumes and flow.
At the end of the day, the roads are a nightmare. They will never be "safe". The sooner people take responsibility for their own actions, and take more care, the safer each person will travel. Cycling, I am torn between my #1 enemy. Pedestrians can be ignorant and bloody dangerous, crossing where they please and with no care of the dangers around them. Distracted drivers are another, but can usually be spotted with ease. But malicious drivers who don't wish you to pass them while stuck in traffic, they are appalling, and drive all manner of vehicles.
Anyway, I digress from the 20mph limit. IMO, it is too wide spread, especially for roads which move large volumes of traffic. There are always going to be people pushing the limits, so I fear that as we get used to slower moving traffic, there is less awareness of the dangers that roads pose.