The case for 20 MPH roads

The safety effects of the 20 mph zones have been enormous for pedestrians, cyclists, and drivers alike. In London, serious traffic injuries and fatalities have fallen by 46 percent within the zones, according to the prestigious British Medical Journal. Deaths and serious injuries sustained by children have dropped 50 percent. There's even a small spillover effect, with areas immediately adjacent to 20 mph zones seeing an eight percent reduction in total injuries and deaths. The science is so clear that in 2004 the World Health Organization endorsed 20 mph speeds as an essential strategy to save lives.
From: http://www.streetsblog.org/2010/03/22/how-london-is-saving-lives-with-20-mph-zones/

Related: http://urbanplacesandspaces.blogspot.com/2010/07/20-mph-speed-limit-sign-on-stanford-ave.html