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.