Evidence is mounting that switching from cars to taking trains or buses, bicycling, or walking is the fastest way to cut greenhouse gases. Reducing the number of cars on the road and, more importantly, the number of total trips, can cut the 27 percent of greenhouse gases that originate from transportation vehicles in the U.S. Meanwhile, pollution in cities can be reduced by some 40 percent by 2050, putting about $100 trillion back into the economy. Here’s an in-depth look, with only one conclusion: think twice before you drive.