In a StreetsWiki entry on Safety in Numbers, Andy Hamilton points to the theory stating that the more cyclists and pedestrians use the streets, the less likely they are to be injured. It’s an observation advanced by public health consultant Peter Jacobsen, but others have weighed in as well.

Traffic engineer and amatuer bicycling expert John Forester believes
the relationship is a spurious correlation, with no proven cause.
But safety experts appear to disagree, and believe that motorists drive
more cautiously when there are a larger number of walkers and bicylists
in their environment. For example, Dr. Chris Rissel of Sydney
University in Australia stated in a 2008 interview, "It appears that
motorists adjust their behavior in the presence of increasing numbers
of people bicycling because they expect or experience more people
cycling. Also, rising cycling rates mean motorists are more likely to
be cyclists, and therefore be more conscious of, and sympathetic
towards, cyclists."

Jacobsen cites evidence from three studies that show
drivers slow down when they see bicyclists and pedestrians, and speed
up when they don’t.

An unscientific look at New York stats appears to support the Safety in Numbers theory. While the number of city cyclists has increased dramatically in the past 10 years, the number of fatalities and injuries have dropped. Though most agree that one death is too many.

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