The cycling group Critical Mass turns 15 years old this month! It all began in San Francisco in September 1992 as a bunch of bike commuters getting together for a ride, and quickly turned into a global cycling phenomenon with chapters in most major cities worldwide, as well as many smaller locales. The last Friday of every month, cyclists gather for a ride, often snarling traffic by accumulating in numbers that can’t be ignored, to reclaim the streets on behalf of human-powered transport and at least temporarily reverse the traditional cars-first hierarchy of the road. Critical Mass has been credited in some places with linking fellow alt-transportation enthusiasts who then together manage to advance pedestrian and bike-friendly initiatives in their city. In other locations, it’s a popular rolling party. Whatever it is, it’s a blast. Happy 15th!

Grist thanks its sponsors. Become one.