The preeminent protest ride, Critical Mass is typically held on the last Friday of the month in cities around the world. Critical Mass started in San Francisco in September 1992 to reclaim the streets when cyclists saw themselves being marginalized by automobiles.

Critical mass rides are not really organized or led; they just happen. Usually only the meeting place and time is fixed, and then the ride is on without official blessing or authorization.

Sometimes this mass ride disrupts the flow of automobiles as the group streams through an intersection. The common response? A phrase which forms the heart of the Critical Mass philosophy: "We aren't blocking traffic; we are traffic."

Critical Mass has drawn the attention of the police on a number of occasions, in particular with high-profile clashes with the law in New York City during the Republican National Convention in 2004 and in London in 2005 when authorities demanded that a group of people with no formal organizational structure somehow apply a week in advance for parade permits for specified route. In both cases, an extensive legal discussion ensued about the right of people to freely assemble, and the case was ultimately argued before the U. S. Supreme Court.

Cyclists in a Critical Mass ride literally take over the streets of the city, swarming through traffic, bringing automobiles to a halt. Lead riders approaching an intersection will "cork" the cross streets, physically blocking vehicles that want to cross with their bikes while their fellow riders stream through without no thought to the actual traffic signals.

Equal parts political statement and a demonstration of cyclists’ unity, taking part in a Critical Mass ride is something that you will not likely soon forget.