Powering cars with clean electricity is one way to make transportation more sustainable. Going back to basics with human powered transportation may be a bigger step. 

But getting more bicycles on the road has been an uphill battle for many communities. As waving fists and occasional hanging boots attest, car drivers and bicyclists often don't get along.

Now a new study conducted jointly by the University of Colorado, Denver, and University of New Mexico, proves some assumptions false—showing that cities with active bike lanes are safer for both bicyclists and cars.

The study looked at 12 U.S. cities with populations over 400,000 including Denver; Austin, Texas; Chicago; Dallas; Houston; Memphis, Minneapolis, Portland, Oregon; Oklahoma City; San Francisco; and Seattle,  over 13 years—2000 to 2012—and found that cities that are safest for bicycles are safest for everybody.

The 12 cities all saw a dramatic increase in bicycling over that timeframe as more people took to pedal power both for exercise and for financial and environmental reasons. The cities took different approaches to integrating cyclists, and had different outcomes in terms of safety.

The most dramatic was Portland, Oregon, which saw a five-fold increase in bikes on its streets and a 75 percent reduction in road deaths.

The study follows on a 2014 study by the University of Colorado, that showed cities with more bicycles were safer for cyclists—the so-called "critical mass" theory. It showed that intersections with at least 200 bicycle crossings a day were safer for cyclists than those with fewer crossings.  But that study couldn't conclusively say what about the number of bicycles made the city safer.

They latest study, by College of Engineering and Applied Sciences at the CU, considered the different approaches the cities took toward integrating bicycles into traffic and found that cities that increased the number of dedicated bike lanes saw the largest safety benefits.

The study's co-author, Nicholas Ferenchak, an assistant professor of Civil, Construction, and Environmental Engineering at University of New Mexico laid out the findings: "When we believed it was the old safety-in-numbers concept, that meant we had to figure out how to get more people on bicycles to make a city safer,"  he said. "That's not easy. But this research has boiled it down for city planners: create cycling facilities, and you'll see the impact."

Those cycling facilities benefited most when they had physical barriers to divide bike lanes from cars, not just stripes and road paint.

So why were the cities safer for cars as well? Cars have to drive slower, says the study's primary author, Wes Marshall, an associate professor of civil engineering at CU Denver. He told Automotive News (subscription required), that the correlation didn't just have to do with the number of bicycles crossing intersections, as his earlier study did, but with the number of intersections per mile of road, things which "typically correspond to more compact and lower-speed" environments, he says.