20mph zones and congestion in London

Cyclists in the City report that the City of London is considering introducing a 20mph zone. This could be transformative if it goes ahead, and not just because of the positive impact on road safety. A 20mph zone would probably also lead to less congestion, because road collisions are responsible for around 28% of congestion in London, and 20mph zones reduce collisions by an average of 37% (p. 50 here). There is little point ever going above 20mph in the City anyway, yet at the moment many drivers do accelerate sharply whenever there is a bit of open road ahead of them. Cutting out that kind of behaviour would be a small but significant step to a saner city.