More bikes means fewer collisions, study finds

University of Colorado study explores critical mass threshold

The more cyclists on the road, the less likely accidents are to happen, a recent study by CU Denver concluded.


The study focused on Boulder, Colorado, which has one of the highest rates of cycling in the United States at about 12 percent of city’s population. That makes it one of the few U.S. cities with enough riders on the road to achieve the safety benefits already documented by researchers in Europe, said study co-author Wesley Marshall, PhD, PE, assistant professor of civil engineering at CU Denver’s College of Engineering and Applied Science.

The researchers set out to model the mathematical relationship between the frequency of crashes and major factors related to them, known in the auto safety world as safety performance functions, or SPFs. Yet while there are SPFs for vehicles, there are none for bikes.

The authors created their SPF for Boulder by studying crashes at intersections throughout the city where more than two-thirds of collisions occur. They then 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 cu study took place in boulder, colorado, which has a high percentage of cyclists compared to the rest of the us:
Ben Delaney / Future Publishing

People on bikes are a common sight around the roads of Boulder, Colorado

The researchers found that the chance of collision decreased with more bicyclists. Conversely, the risk of accident was relatively high at intersections with fewer than 200 cyclists per day. “Anywhere above this threshold is where we are seeing the largest safety benefits,” Marshall said.

The reasons for this remain unknown, the researchers said.

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

The researches postulated that cyclists may also be attracted to safer area. 

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

About 12 percent of people in boulder, colorado get around on bikes, the study said:
Ben Delaney / Future Publishing

The more riders on the road, the safer they are, the study concluded

The results could have national implications, not only for cyclists but drivers, too.

“In fact, 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.”


The study was co-authored by CU Denver’s Bruce Janson, PhD, professor of civil engineering and Krista Nordback, PhD, PE. It was published last month in the journal Accident Analysis & Prevention.