A new British Medical Journal research article reports the effect of introducing 20 mph zones on casualties within those zones and in adjacent areas. It was based on police road casualty data between 1986 and 2006 and adjusted for the underlying downward trend in traffic casualties. The new 20 mph zones were associated with 40% reduction in death and serious injury. There was no increase of casualties in adjacent areas to the 20 mph zones. There casualties also fell slightly (8.0%).
This research is available here: http://www.bmj.com/cgi/content/full/339/dec10_3/b4469 .

UK Government to allow wider use of 20 mph zones
New proposals to allow more 20 mph schemes across the UK were announced today by Road Safety Minister Paul Clark. He cited the study reported above (BMJ 2009;339:b4469). The Government is encouraging local councils to introduce 20 mph schemes into residential streets and other roads where cycle and pedestrian traffic is high, such as around schools, shops and parks.
Further information is available here:
http://nds.coi.gov.uk/content/detail.aspx?ReleaseID=409648&NewsAreaID=2&HUserID=895,777,895,857,782,868,710,705,765,674,677,767,684,762,718,674,708,683,706,718,674&ClientID=-1