The City of London has produced the first report on the wide area 20 mph scheme that was introduced in July 2014. It covers the whole City other than the A3211 (Lower/Upper Thames Street), but that has been slowed to a crawl anyway by the new Cycle Superhighway and associated road works.
Average speed has been reduced by 1.5mph which is higher than most such schemes (1 mph is more normal). But whether this was a result of the speed limit or more congestion generally is unclear. TfL have been complaining about more congestion from larger number of private hire vehicles in London and the Cycle Superhighway works and redevelopment of Aldgate have had wider impacts on congestion in the City also.
Provisional casualty data up to June 2015 suggest that there has been a continued increase in the number of slight injuries to people walking and cycling, according to the report. Increases in such accidents were one justification for the imposition of this scheme. The report argues that without the scheme, accidents might have increased even more, but that is a somewhat dubious statement is it not?
More data on accidents is not yet available (3 years before/after is the best measure of course although interpreting the data because of other changes in the City may not be easy).
The police have been quite vigorously enforcing the 20 mph limit and reported 370 traffic offences in the last 12 months which has resulted in 180 fixed penalty notices and 99 court summons.
The overall cost of this scheme was originally estimated at up to £150,000, and surely it has been a complete waste of money based on the evidence to date.