In one word: safety. And, as Sarah Laskow explains, the more folks take to riding, the safer the streets become, so it builds on itself. Critical to road safety is bike infrastructure, like protected bike lanes, bike boxes and bike traffic signals.
In order to stave off the worst climate change scenarios, experts have recommended that the world's countries must aggressively expand their efforts to limit greenhouse gas emissions. Some surprising countries are headed in the opposite direction.
Regulations and a natural gas boom have helped reduce America's CO2 emissions to their lowest level since 1994. But CO2, and plenty of other toxic particles, continue to pollute our urban air. Brentin Mock looks at 5 highly polluted urban areas.