Small increase in cyclists means large decrease in accidents.
 

I have read this as a truism, but was there ever a study to show this, and if so is it available online?

The only research paper on the Internet that I am aware of is "Safety in numbers in Australia: more walkers and bicyclists, safer walking and bicycling" in Health Promotion Journal of Australia 2005;16:47-51 which is available in full text as a PDF document HERE. This is one of the studies cited regarding mandatory helmet laws. If mandatory helmet use reduces the numbers of cyclists, the net effect on safety for people who continue to bicycle can be no improvement or even less safety.

Studies of this nature tend to be observational because a controlled experimental trial is difficult to do. I would think that if there were good numbers on number of bicyclists and motor vehicles on the road by day of week or month or season, that might be another way of looking for an effect.

Edit: Another link for the same paper LINK. Graphs in the paper in higher resolution LINK.

Giro, thanks for posting that. I had forgotten to put it on the research page, probably because it has a near-identical title to this paper by Jacobsen -- Safety in Numbers: More Walkers and Bicyclists, Safer Walking and Bicycling (abstract) -- from 2003, which I believe is a different animal from the Robinson paper, although I've only looked at the abstract.

Robert