Critical Mass is an event held regularly in cities around the world, where bicyclists and other non-motorised vehicle-users take to the streets in a large enough numbers that they displace cars from the road, creating an unusual degree of safety and freedom in a car-free environment. While some view this as a deliberate attempt to obstruct traffic, and cause a disruption of normal city functions, one of Critical Mass's standard, philosophical replies to this is: "We aren't blocking traffic, we are traffic."
Despite this contention, however, Critical Mass differs from ordinary traffic in that it takes liberties with respect to traffic laws which ordinary traffic, both motorized and unmotorized, are expected to obey. In particular, Critical Mass riders may carry out an action known as "corking", which involves blocking cross traffic so that the riders can freely run red lights. Although many bicyclists and bicycle activists promote the principle that the road should be shared between motorists and bicyclists, Critical Mass, in attempting to take over the road in the formation of a "mass", does not seek to share the road with motorists. Critics would also argue that the Critical Mass experience also does not reflect the actual day-to-day conditions of bicyclists who do not have the luxury of "corking" to grant them the privilege of ignoring traffic lights and other traffic restrictions. Thus some bicycle activists consider Critical Mass to be couterproductive towards achieving the goals of greater respect for bicycles as bona fide traffic. They would thus argue that the "we are traffic" slogan is misleading.
Critical mass differs from many other social movements in its rhizomal, (rather than arboreal) structure. Critical Mass claims to be an "organised coincidence", with no leader, no organizers, and no membership. For example, the term xerocracy was coined to describe the process for how the route for a Critical Mass is decided: Anyone who has an opinion makes their own map, and distributes it to the cyclists participating in the Mass.
The route is decided on the day of the Mass, by consensus. This frees up the movements from the overhead costs involved in a hierarchical organisation: no meetings, no structure, no internal politics, and so on. In order for it to exist, all that has to happen is that enough people know about it and turn up on the day to create a 'critical mass' of riders large enough to safely occupy a piece of road to the exclusion of motorised road users.
Critical Mass started in San Francisco in 1992 and has since spread to more than 325 cities around the world. In San Francisco, the traditional beginning of the Mass is at Justin Herman Plaza, at the end of Market Street, on the Embarcadero.
The term "critical mass" in this sense, was adopted from an observation made by American Human Powered Vehicle and pedicab designer George Bliss while visiting China. He noted that traffic, both motorists and bicyclists, in China had an undeterstood method of negotiating unsignalled intersections. Traffic would "bunch up" at these intersections until the back log reached a "critical mass" at which point that mass would move through the intersection. This description was related in the Ted White documentary Return of the Scorcher (1992) and subsequently adopted by the Critical Mass movement.