Jacobsen 2003 - “Safety in Numbers: More Walkers and Bicyclists, Safer Walking and Bicycling”

Jacobsen, Peter Lyndon
“Safety in Numbers: More Walkers and Bicyclists, Safer Walking and Bicycling”
Injury Prevention
September 2003; v.9,n.1; pp.205-209.
One the Web
Relevance: high

Jacobsen analyzed various datasets from Europe and California to compare accident rates per capita across differing rates of walking/biking.  He found that as the rates of walking/biking increase, the total number of accidents increases as expected, but the likelihood that an individual pedestrian or cyclist will be hit by a car decreases.

The Math
Jacobsen used the relationship I=aE^(b), where I is the injury measure, E is the measure of walking or bicycling, and a and b are parameters to be measured.  For an individual, the risk can be estimated as; I/E=aE^(b-1).  The author found that the total number of pedestrians or bicyclists struck by motorists varies with the 0.4 power (b=0.4) of the amount of walking or bicycling.

So a community doubling its walking can expect only a 32% increase [2^(0.4) = 1.32] in total injuries and a reduction in injuries per person to 66% [2^(0.4)/2 = 2^(-0.6) - 0.66].  So each walker/biker is actually safer.

"This result is unexpected.  Since it is unlikely that the people walking and bicycling become more cautious if their numbers are larger, it indicates that the behavior of motorists controls the likelihood of collisions.  It appears that motorists adjust their behavior in the presence of people walking and bicycling.  Policies that increase the numbers of people walking and bicycling appear to be an effective route to improving the safety of people walking and bicycling."


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Eric de Place

My inveterate skeptic wonders if this is not motorists adjusting their behavior so much as something like a chicken-and-egg phenomena with pedestrians and pedestrian infrastructure. I'd assume that in places where people walk more there are better facilities--wider sidewalks, traffic calming, barriers, etc. etc--so that accidents decrease where people walk a lot. But not because people are walking a lot. People are walking a lot because facilities are already safer. It's not at all clear to me that we can infer from this data that motorists change their behavior.

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