In Colorado, that new vision was catalyzed by climate change. In 2019, Gov. Jared Polis signed a law that required the state to reduce greenhouse gas emissions by 90 percent within 30 years. As the state tried to figure out how it would get there, it zeroed in on drivers. Transportation is the largest single contributor to greenhouse gas emissions in the United States, accounting for about 30 percent of the total; 60 percent of that comes from cars and trucks. To reduce emissions, Coloradans would have to drive less.