CU study finds more bicycles on the road lead to fewer accidents

CU finding mirrors European results

DENVER - Surprising results from a CU study that compared the number of bicyclists to the number of accidents -- and the results match those already found in Europe.

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

The study focused on Boulder, which has one of the highest rates of bicycling in the country. About 12 percent of the population rides bikes. That makes it one of the few U.S. cities with enough bicycling 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.

"I was glad to be able to do this practical, hands-on research on bicyclist safety while a student at CU Denver," said study co-author Krista Nordback, PhD, PE.

The researchers wanted to create safety performance functions 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 vehicles, there were 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 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 risk of accidents was relatively high at intersections that saw fewer than 200 bicyclists per day.

"Anywhere above this threshold is where we are seeing the largest safety benefits," Marshall said.  That's when the risk of accidents begins decreasing.

"That makes total sense to me because the more bicyclists there are, the more drivers become trained to be aware of bikers," said Jason Bartley, a bicyclist in Fort Collins.  "Fort Collins, it seems like people that are biking, it's more like transportation and so, seems like many more people have a bike that they use for transportation when they can; that means when they're driving a car, they'll know what it's like to be a biker."

Researchers say the reasons for the stunning study finding remain unknown.

"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."

Cyclists may also be attracted to safer areas.

"In the same way that maybe a traffic light would make you look up and be aware I think probably cyclist are the same way, the more of them the more likely you are to be paying attention for them," said Elan Jimenez-Waters, a driver in Fort Collins. The home to CSU also has a large percentage of bicyclists.

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

As bicycling increases in cities across the U.S. each year, the results could have national implications.

"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 published last month in the journal Accident Analysis & Prevention. 

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