So, the benefits of cycling greatly outweigh the risks. When asked this question, Peter Croft, Manager of the Road Environmental Safety, Roads and Traffic Authority of New South Wales identified lowering motor vehicle speeds as the answer. It would give more time in which to avoid the accident. It would reduce the speed of impact. Impact speeds in a collision with a motor vehicle largely determine if a cyclist will live or die.
Speed also affects the ability to stop.
Between 25 and 50 mph, the distance required to stop triples from 60 feet to 180 feet. Thus accidents are more likely to happen at higher speeds. I strongly recommend reading Pedalling Health or at least looking at the excellent charts. The original website in Australia.