Safety in Numbers, The theory

Overview

Safety in Numbers is the observation that the risk of an individual pedestrian or bicyclist being hit by a motor vehicle decreases as the number of pedestrians or bicyclists increases, respectively. This idea runs counter to what one might expect -- that the more pedestrians and bicyclists there are, the more collisions with motor vehicles will occur. Data show there is not a proportional relationship between these two variables. In fact, the safety in numbers relationship has been observed across a wide range of geographic study areas, from individual intersections to continents, in the U.S., Europe, and Australia.

The Data

A widely cited 2003 paper by public health consultant Peter Jacobsen examined injury rates, pedestrian and/or bicycle volume, and population over time in several different settings, with the following results:

• The likelihood of injury to a pedestrian or bicyclist in 68 California cities decreased as the percent of commuters walking or bicycling increased.
• A study of walking, bicycling, and moped use in 47 Danish towns found that walking was safer where there were more walkers and bicycling/moped use was safer where these modes were higher.
• The number of bicyclist fatalities per distance bicycled in 14 European countries decreased as the distance of bicycling per capita increased.
• In 8 European countries where data were available, the number of bicycling and pedestrian fatalities each decreased as per capita biking and walking trips increased, respectively.
• In Britain, bicycling varied up and down with different factors, such as the Arab Oil Embargo and new traffic speed laws, from 1950 to 1999. Whenever bicycling increased, per capita bicycling fatalities decreased, and the inverse was also true.
• In the Netherlands, where bicycling facilities and traffic law changes from 1980 to 1998 have greatly increased the amount of bicyclists and bicycle mileage, per capita bicycle fatalities have fallen equally dramatically.

Similar to these results, a 2006 study of 247 Oakland, California, intersections found that pedestrian collisions decreased with increasing pedestrian flows, and increased with increasing traffic volume.



Jacobsen, P. 2003. Safety in numbers: more walkers and bicyclists, safer walking and bicycling. Injury Prevention, 9:205-209.

Gaffney, D. September 3, 2008. A virtuous cycle: safety in numbers for riders says research. Science Daily (University of New South Wales).