Amazing things happen to a city once people are encouraged to switch to bike commuting: the air quality improves throughout the city, which benefits everyone, not just cyclists. Quieter roads are more pleasant roads to be around, and they’re less congested for those who still insist on driving. And of course riding a bike every day brings all kinds of health benefits to the cyclists themselves.

A new study from researchers at Columbia University’s Mailman School of Public Health shows just how big those benefits can be. Per dollar spent, constructing bike lanes is a cheap way to improve public health. For instance back in 2005, New York City spent $10 million on curbing traffic as part of the federally-funded Safe Routes to School program. Sidewalks were widened, bike lanes constructed, and traffic lights re-phased to suit pedestrians. The “net societal benefit” of these changes? The study’s authors estimate it to be $230 million.

“Unlike helmet laws, bike lanes do not require behavioral change on the part of the cyclist, and they come with other benefits,” says the study. “For example, they ‘normalize’ exercise behaviors, reduce pollution and may help address the obesity epidemic in the USA.”

To evaluate the costs, the authors first determined that the 45.5 miles of bike lane constructed in New York in 2015, at a cost of just over $8 million, increased the probability of riding a bike by around 9%. They then modeled these figures to find out how cost effective more bike lanes would be in the future. Savings on health spending were calculated by estimating the effects of increased physical activity, and better air quality.

“We conclude that investments in bicycle lanes come with an exceptionally good value because they simultaneously address multiple public health problems,’ says the study. “Investments in bike lanes are more cost-effective than the majority of preventive approaches used today.” [emphasis added]

The study also points out some interesting facts. For instance, of the U.S.’s 67 million cyclists, making over 300 million trips per year just in big cities, there are 700 deaths and 48,000 serious injuries a year. That, say the authors, “makes the USA the most dangerous place among wealthy nations to bicycle.” Per mile traveled, U.S. cyclists are twice as likely to be killed as German cyclists, and three times more likely than Dutch cyclists. Bike lanes can cut all kinds of injury by 25%.

Of course, bike lanes don’t get really effective unless you build enough of them. Any number of good, protected lanes is better than none of course, but the benefits to the city as a whole don’t kick in until you reach a critical mass. Only when there are enough lanes that people can ditch their cars and start commuting by bike do things really change for the better.