"Of all interventions to increase cycle safety, the greatest benefits come from reducing motor vehicle speeds. Interventions that achieve this are also likely to result in casualty reductions for all classes of road user. This may be achieved by a variety of methods, including physical traffic calming; urban design that changes the appearance and pedestrian use of a street; and, possibly, the wider use of 20 mph speed limits."
Seems even the government now agree that lower speeds will make cycling (and walking) less risky
http://www.bikebiz.com//news/read/to-increase-cyclist-safety-reduce-motor-vehicle-speeds
http://assets.dft.gov.uk/publications/infrastructure-and-cyclist-safety/infrastructure-and-cyclist-safety.pdf
"Of all interventions to increase cycle safety, the greatest benefits come from reducing motor vehicle speeds. Interventions that achieve this are also likely to result in casualty reductions for all classes of road user. This may be achieved by a variety of methods, including physical traffic calming; urban design that changes the appearance and pedestrian use of a street; and, possibly, the wider use of 20 mph speed limits."