New study claims that more bicyclists on roads means fewer collisions

New research from Boulder, Colorado, backs up claims made in a “safety in numbers” study from eleven years ago. P. L. Jacobsen’s study from Injury Prevention in 2003 is still controversial, with many bicycle advocates disputing his results. The new study has been published in Accident Analysis & Prevention.

The new study was led by Bruce Janson, PhD, professor of civil engineering and Krista Nordback, PhD, PE of the University of Colorado Denver College of Engineering and Applied Science. Boulder, Colorado, has one of the highest rates of bicycling in America at about 12 percent of the population. That makes it one of the few US cities with enough bicycling to achieve the safety benefits already documented by researchers in Europe, said study co-author Wesley Marshall, PhD, PE.

And Boulder’s close proximity to CU Denver’s downtown campus offered an ideal opportunity for the researchers and students.

“I was glad to be able to do this practical, hands-on research on bicyclist safety while a student at CU Denver,” said Nordback.

The researchers wanted to create safety performance functions (SPFs) for bicycles in Boulder. SPFs model the mathematical relationship between the frequency of crashes and major factors related to them. Yet while there are SPFs for motor vehicles, there are none for bicycles.

The authors created their SPF for Boulder by studying crashes at intersections throughout the city where more than two-thirds of collisions occur. They compared the crash data to bicycle count data.

“Fortunately, Boulder was one of the first cities to establish a bicycle counting program back in the late 90s,” Marshall said.

The researchers found that the chance of collision decreased with more bicyclists. The risk of crashes was relatively high at intersections with less than 200 bicyclists per day.

“Anywhere above this threshold is where we are seeing the largest safety benefits,” Marshall said.

The reasons for this remain unknown. “Other studies have hypothesized that when drivers expect to see a significant number of bicyclists on the street, their behaviour changes,” Marshall said. “They are more likely to look over their shoulder for a bicyclist before taking a turn.”

Cyclists may also be attracted to safer areas.

“But we think there is even more to the story and we’ll be looking for that in our next study,” said Marshall.

“We are beginning to find that cities with a high level of bicycling are not just safer for cyclists but for all road users,” he said. “Improving the streets to better accommodate bicycles may enhance safety for everyone.”

Boulder has 300 miles of bike lanes, routes, designated shoulders and paths.