Per capita traffic fatalities tend to decline as the portion of nonmotorized urban travel increases.
Wardlaw (2001) and Jacobsen (2003) find that the per capita collisions between motor
vehicles and nonmotorized travelers declines with increased nonmotorized travel. Jacobsen
calculates that the number of motorists colliding with pedestrians and cyclists increases at
roughly 0.4 power of the number of people walking or cycling (e.g., doubling NMT travel
in a community will increase pedestrian/cycling injuries by 32%), and the risk of being hit
as a pedestrian declines 34% if walking and cycling double in an area. Robinson (2005)
found similar results using Australian data: doubling bicycle travel reduces cyclist risk per
kilometer by about 34%; and conversely, halving bicycle travel increases risk per kilometer
about 52%.
Several studies indicate that motor vehicle external accident costs average 2¢ to 12¢ per
automobile mile, depending on vehicle type and driving conditions. Collision rates per
vehicle mile tend to increase with traffic density, although fatality rates tend to decline as
congestion reduces traffic speeds.
Estimated Benefits: Net benefits of a shift from driving to walking or cycling are estimated
to average 5¢ per urban peak mile, 4¢ per urban off-peak mile, and 3¢ per rural mile.
Although people who shift from motorized to nonmotorized modes may experience some
increased accident risk, this can be minimized if mobility management programs include
appropriate safety education and facility improvements, and can be offset overall by
reductions in risk to others, and increased caution by drivers.