What Jacobsen found was surprising. "The number of crashes involving bikes correlated with the number of riders in a community. As ridership fluctuated, so did the crash rate. More riders, fewer crashes; fewer riders, more crashes."
Through his research, Jacobsen has theorized that the more bicyclists there are on the road, the more they are noticed by motorist thus causing motorists to become more attentive, drive a bit more slowly, take extra precautions when passing, and pay more attention to their blind spot.
To read more about what Elly Blue has to say about safety in numbers, go here: http://www.grist.org/article/2010-10-11-theres-safety-in-numbers-for-cyclists
To read Peter Jacobsen's report go here: http://www.cycle-helmets.com/safety_in_numbers2.pdf