Cycle campaigning

Typically within Hull, 20 mph zones have achieved reductions[106] in injury accidents of:
— Total accidents -56 per cent
— Killed & seriously injured accidents -90 per cent
— Accidents involving child casualties -64 per cent
— All pedestrian accidents -54 per cent
— Child pedestrian accidents -74 per cent.
It is estimated that at the end of 1999, 390 injury accidents had been prevented within the 20 mph zones which had been previously installed. 122 of these would have involved injuries to children.

http://www.publications.parliament.uk/pa/cm200102/cmselect/cmtlgr/557/557ap80.htm

The new 20mph zones will be policed by ANPR average speed cameras so will also detect uninsured cars, tailgating, erratic driving etc.

London isn't Hull. I've never been there's but my experience of provincial towns is that motorists obey the speed limits and wait till lits go green before setting off. That never happens in London.

There are already ANPR cameras all over London. There have been for years. They are only effective against people who diligently register cars in their own name, and the type of person who drives without insurance doesn't do that. And no camera can detect erratic driving or tailgating. Average speed cameras cannot work in London because the average speeds are about half of the speed limits anyway. To actually catch anyone going at an average of over 20mph between two points there would have to be more cameras than there are lamp posts, and not only would that be an eyesore, there will never be anything like enough money to pay for them. Which means that the 20mph zones will be unenforced or enforced only by anti cyclist measures such as speed humps or chicanes.