The World Naked Bike Ride (WNBR) is an international clothing-optional bike ride in which participants plan, meet and ride together en masse on human-powered transport (the vast majority on bicycles, but some on skateboards and inline skates), to “deliver a vision of a cleaner, safer, body-positive world.” The largest iteration of the WNBR in the world takes place in Portland, Oregon averaging approximately 10,000 participants and thousands of spectators.

https://en.wikipedia.org/wiki/World_Naked_Bike_Ride