When you live in San Francisco, it’s not uncommon to see strange sights out your window. Once, I saw the mayor standing in the middle of the street, painting a bright green square on the asphalt.

This was a historic moment, I later learned—the inauguration of the city’s first “bike box,” a designated space for cyclists to wait at an intersection. That was in 2009. Cycling continued to grow in the following years. By 2015, San Franciscans took an estimated 82,000 bike trips per day, and the total length of the city’s bike lanes was 125 miles and growing.

The popularity of cycling in cities around the country, advocates say, is an encouraging trend that reduces traffic congestion and air pollution