Cyclists ride in the 15th Street NW bike lane in downtown Washington. (Keith Lane for The Washington Post)

The District’s 15th Street bike lane — the oldest and most popular protected bike lane in the city — has the “highest injury risk” among the protected lanes, according to a new study.

The lane’s southern section, between Massachusetts and Pennsylvania avenues Northwest, where the two-way bike lane travels alongside two-way vehicle traffic, stood out as particularly risky, according to a report by the Insurance Institute for Highway Safety, which looked at how well protected bike lanes are working in the United States.

“Surprisingly protected bike lanes vary in how well they shield cyclists from injury,” said the study’s author, Jessica Cicchino, IIHS’s vice president of research.

In other words, not all protected bike lanes are created equal. The lane on 15th Street NW, among the city’s most celebrated, has numerous risk factors that contribute to collisions and falls.

Its main problem, according to the report, lies in its configuration: a two-way lane on a roadway that has two-way traffic that crosses numerous major intersections and driveways. So many variables increase the opportunities for collisions.

“It is challenging for drivers to turn across because they have to monitor for oncoming traffic for cars and bikes,” Cicchino said. By comparison, the safer protected bike lanes in the study did not cross driveways or intersecting roads very often, she said.

Cicchino said the study provides yet more evidence that bike infrastructure works but that some areas could be made safer.

Some findings were no surprise. The risk of crashing or falling was much lower on protected bike lanes than on roads lacking the infrastructure; traveling on conventional bike lanes — those not protected — was safer than riding on the road; and traveling on local roads was safer than on major thoroughfares. The risk for injury went up when there were streetcar tracks present or obstructions such as cars and construction blocking cyclists’ way.

The researchers studied bike infrastructure in the District, New York and Portland, finding that bike lanes in D.C. crossed driveways and alleys more often than those in New York. Researchers interviewed more than 600 bicyclists who visited emergency rooms from falls or crashes in bike lanes. More than half were in the District. Of those injured using the protected lanes, about half were involved in crashes with cars and about half were in situations where they fell while trying to avoid a pedestrian or another cyclist, or while hitting bumps in the pavement.

The study found that nearly all incidents on street-level, two-way protected bike lanes occurred in the District. The 15th Street bike lane accounted for nearly half of the crashes or falls on street-level, two-way protected bike lanes in the study.

The 15th Street lane, which opened about a decade ago, is one of the most-used bike facilities in the nation’s capital. By some counts, it is the busiest, carrying as many as 2,500 people daily on nice days. It is the only north-south protected bike lane in the city, connecting neighborhoods in Northwest to downtown.

It has undergone multiple improvements in its life span. What began as a southbound lane was later converted into a two-way bike lane. To keep from taking any more road space from parking or driving, according to some accounts, the city opted to keep it a two-way lane on one side of the road rather than putting the southbound lane on the side of the southbound traffic and the northbound on the other, which would be safer. Through the years, the city also introduced bike signals at a couple of intersections to ease the passage of crossing cyclists. There has been some repaving and re-striping of pavement markings.

Bike advocates say the lane was the result of a compromise, squeezed in the road to take the least amount of space possible. And yet it become overwhelmingly the most popular bike lane in the city. It was the driver of bike commuting and the poster child of protected bike lanes, and it inspired the creation of a network of separate, protected infrastructure that research shows is critical to protecting cyclists and reducing serious injuries and deaths.

“People are far safer and far more likely to ride if we build separated, protected bike lanes,” said Greg Billing, executive director of the Washington Area Bicyclist Association. “We can’t let that message get lost in this discussion about, ‘Okay, we are building them. What is the best way to do it?’"

“Now let’s look at the early projects, like 15th Street, and learn from them and not repeat mistakes. I think that is the important message here,” he said.

Billing agrees that the 15th Street route can be improved. WABA has ideas for how to do that, from adding clearer signage to placing bumps at or near driveways and investing in more bike signals. Ideally, Billing said, the city would put the northbound and southbound lane on each side of the road.

The city has been taking some steps to make riding safer. It has announced plans to crack down on drivers who park in or block bike lanes by creating a team of bike-lane enforcement officers with the power to issue tickets on the scene and by mail. And it issued a set of new rules clarifying that stopping, standing and parking in bike lanes, as well as loading and unloading passengers and other obstructions of bike lanes, are prohibited. City transportation officials say they are embracing new technologies and planning processes to build safer infrastructure.

“Where there is existing infrastructure, where we have heard that there are issues with specific intersections, we have looked at ways to make modifications and improvements,” said Jeff Marootian, director of the D.C. Department of Transportation. Any plans for immediate improvements to the 15th Street bike lane, however, are unclear.

The study’s author praised some of the city’s recent policies to lower the risks of injury to cyclists.

“This study supports D.C.’s efforts to try to keep parked cars out of bike lanes, to keep obstructions out of the way,” Cicchino said. “We also need to talk more about what features of these protected bike lanes make them safer and less safe so that we can continue to build infrastructure for cyclists that protect them. It seems that putting this kind of infrastructure in roads that have fewer intersections and fewer driveways is ideal. It is not always possible. But if it can be done, that will protect the cyclists the most.”