Minimum amount of a given fissionable material necessary to achieve a self-sustaining nuclear chain reaction under specified conditions. Critical mass depends on several factors, including the kind of fissionable material used, its concentration and purity, and the composition and geometry of the surrounding reaction system.
Learn more about critical mass with a free trial on Britannica.com.
Critical Mass is a bicycling event typically held on the last Friday of every month in over 300 cities around the world. While the ride was originally founded with the idea of drawing attention to how unfriendly the city was to bicyclists, the leaderless structure of Critical Mass makes it impossible to assign it any one specific goal. In fact, the purpose of Critical Mass is not formalized beyond the direct action of meeting at a set location and time and traveling as a group through city or town streets.
Critical Mass rides have been perceived as protest activities. A 2006 New Yorker magazine article described Critical Mass' activity in New York City as "monthly political-protest rides", and characterized Critical Mass as a part of a social movement; and the UK e-zine Urban75, which advertises as well as publishes photographs of the Critical Mass event in London, describes this as "the monthly protest by cyclists reclaiming the streets of London. However, Critical Mass participants have insisted that these events should be viewed as "celebrations" and spontaneous gatherings, and not as protests or organized demonstrations. This stance allows Critical Mass to argue a legal position that its events can occur without advance notification of local police.
Critical Mass rides vary greatly in many respects, including frequency and number of participants that directly result in the endangerment of legitimate cyclists. For example, many small cities have monthly Critical Mass rides with fewer than twenty riders which offer safety in numbersto cyclists in those locales, while on the opposite extreme, in what have been the largest events using the name Critical Mass, cyclists in Budapest, Hungary hold only two rides each year on 22 September (International Car Free Day) and 22 April (Earth Day). The 'Budapest style' attracts tens of thousands of riders. The April 20, 2008 Budapest ride participation was generally estimated at 80,000 riders.
The first ride took place on Friday, September 25, 1992 at 6 pm in San Francisco. At that time, the event was known as Commute Clot and was composed of a couple of dozen cyclists who had received flyers on Market Street.
Shortly after this, some participants in that ride went to a local bicycle shop for a screening of Ted White's documentary Return of the Scorcher, about bike culture overseas. In that film, American human powered vehicle and pedicab designer George Bliss noted that, in China, both motorists and bicyclists had an understood method of negotiating intersections without signals. Traffic would "bunch up" at these intersections until the backlog reached a "critical mass", at which point that mass would move through the intersection. That term from the movie was applied to the ride, and the name caught on, replacing "Commute Clot" by the time of the second event.
By the time of the fourth ride, the number of cyclists had increased to around 100 and participation continued to grow dramatically, reaching about 1,000 riders, on average.
The name was soon adopted as a generic label by participants in similar but independent mass rides that were either initiated in various locations around the world at around the same time, or had already existed before 1992 under other names. It is estimated that there are Critical Mass-type rides in more than 325 cities to date. The term "masser" is sometimes applied to frequent participants.
Due to the unorganized nature of Critical Mass, no standard vocabulary exists. This section outlines words and phrases that have become popular in some cities that hold Critical Mass events.Bike-lifting (Also known in Chicago as the Chicago hold-up) : Bike-lifting is when a participant raises his or her bicycle in the air. This occurs when an intersection is corked, when a cyclone is occurring, or at any point a participant desires to hold a bicycle in the air.Corking : Corking (described in detail below) is a tactic used to prevent traffic from entering the path of the cyclists.Cyclone : Cyclones form when the mass begins to circle an intersection. Popular in Chicago's large six-point intersections, cyclones can be used to "mass up" the critical mass, so that it can maintain the density of cyclists necessary to prevent the flow of automotive traffic. The tactic also allows splinter masses time to rejoin the group.Die-in : Die-ins are when participants lay down on the ground with their bikes to symbolize cyclist deaths and injuries caused by automobiles. "Mass up" or "Mass it up" : In the middle or at the end of the group of cyclists, the number of cyclists traveling on the road can grow thin, resulting in dangerous conditions for riders if automotive traffic attempts to cut through the middle of the mass while cyclists are still passing. Participants in some cities will yell "mass up" or "mass it up" in order to tell the front of the group to slow down. Different tactics are used to mass up such as simply slowing down or stopping as well as cycloning.Splinter mass : Splinter masses are a common occurrence due to the unorganized nature of the event. A splinter mass occurs when a smaller group of the larger critical mass separates from the main mass inadvertently. They often no longer possess a mass that is "critical" enough to stop traffic. "Massing up" allows splinter masses to rejoin the group.
Critics argue that the practice of corking roads in order to pass through red lights as a group is contrary to Critical Mass' claim that "we are traffic", since ordinary traffic (including bicycle traffic) does not usually have the right to go through intersections once the traffic signal has changed to red. Corking has sometimes led to hostility between motorists and riders, even erupting into violence and arrests of motorists and cyclists alike during Critical Mass rides.
In California's San Francisco Bay Area, there have been several incidents of conflict during Critical Mass events. Similar conflicts have arisen during critical mass rides in Winnipeg, Manitoba and Seattle, Washington.
Near the end of the ride that evening, near the Japan Center and Western Addition neighborhoods, a resident of Redwood City, California tried to drive through the mass of bicycle riders. An eyewitness claimed to have observed the driver strike a cyclist and flee the scene of the accident before cyclists chased after and surrounded her vehicle. The driver denied striking a cyclist and alleged that hundreds of bicyclists surrounded her minivan while she and her 11- and 13-year-old daughters were inside, banged on the sides of her car, "keyed" the paint, and threw a bicycle through the rear window of the vehicle, causing $5,300 in damage.
San Francisco Mayor Gavin Newsom, in April 2007, requested that Critical Mass riders "police themselves." "It does the bicycle-advocacy community no good to have people that are aggressive and dispirit the entire movement,” Newsom said. “I would encourage the bicycle coalition to say, ‘Look, we don’t put up with this, enough is enough.’”
A group of mass riders caught up to the vehicle, broke its rear windshield, and assaulted the motorist when he got out of his car. At least one cyclist slashed the motorist's tires. Damage to the car was estimated at $1500. The motorist was struck in the back of his head by a bike lock after being pulled from his vehicle. The motorist was taken to a local hospital where doctors stapled the wound shut. Two cyclists were arrested for suspicion of vandalism to the car. Seattle Police are not considering the motorist to have committed a crime.
Police in New York have claimed that Critical Mass bicyclists corking intersections to allow the mass of bikes to pass may delay emergency vehicles unable to move in the gridlock. Motorists driving cars on cross streets cannot practicably move over to the side of the streets in the way a bicyclist can due to the length and square footage of a car, and current traffic lane configurations on Manhattan streets which, at present, allow parking and driving of private automobiles in some areas.
On July 25, 2008, a NYC Critical Mass rider was arrested and charged with attempted assault, resisting arrest and disorderly conduct. A video surfaced that showed NYPD officer Patrick Pogan shoving the bicyclist to the ground, raising doubts about the objectivity of the officer's arrest report where he claims the rider "drove his bike into the officer's body, knocking him to the ground and causing cuts. although the video clearly shows that the officer never falls to the ground. The officer, who had only left the academy three weeks prior to time of the incident, was stripped of his badge and weapon and placed on desk duty pending an investigation.
During the ride approximately 250 riders were arrested for unlawful assembly and jailed, allegedly after being cordoned off by lines of riot-helmeted San Francisco police officers and without being given a lawful order or chance to disperse, and had their bicycles confiscated by the police. San Francisco Deputy Police Chief Rich Holder was quoted as saying that elsewhere during the ride some of the cyclists "stormed" the San Francisco – Oakland Bay Bridge, which lacks a pedestrian and bicycle right-of-way such as the one on the Brooklyn Bridge or the Golden Gate Bridge. Brown was humorously portrayed by one news report as hypocritical and lacking credibility on the issue of enforcing traffic laws against Critical Mass bicyclists since Brown gained a noted reputation for flying through town with a motorcycle escort just to make his meetings on time. At a July 31st, 1997 press conference, Brown referred to Critical Mass cyclists as "little weenies," and implied that bicyclists do not vote or comprise a significant portion of his constituency. Brown later explained the remark as occurring "in a moment of evidence of (his) annoyance." By the middle of his second term in office, Brown was seen by some as having evolved on bicycling issues. On the 10th anniversary of Critical Mass on September 27, 2002, the city officially closed down four blocks to automobile traffic for the Car-Free Day street fair. Brown remarked: "I'm delighted. A new tradition has been born in our city."
Bennett Hall, a professional photographer, witnessed a San Francisco police officer writing a citation for a bicyclist he claimed had committed no offense. Hall alleged that while he was photographing the event a police officer arrested him without cause and seized his camera. Another pedestrian attempted to take the camera to bring it to the San Francisco Chronicle, but was also arrested.
In September 2005, Metropolitan Police in London gave out notices announcing a requirement that the organisers of the mass provide a route six days before the event. In addition, they stated that the Mass may be restricted in the future, and arrests would result if their orders were not followed. The threat was retracted when politicians and cyclist groups voiced objections. The following ride, that of October 2005, had close to 1200 participants. There was a long stop in Parliament Square, part of the Government's exclusion area in the Serious Organised Crime and Police Act 2005. However, this event also led to a particularly slow and cumbersome ride which brought some debate from London cycling groups.
One ride participant sought a declaration from the High Court of England and Wales that police need not be notified about the Critical Mass rides, in a "friendly action" in which neither side sought damages. The Court ruling agreed, exempting Critical Mass from notification under Section 11 of the Public Order Act 1986. The ruling was reversed on appeal. In early 2008 Friends of the Earth, who supported the legal action, said the case would be appealed to England's highest legal authority, the House of Lords.
Police in Chicago are often seen riding with the Critical Mass to monitor the event and squad cars are seen blocking intersections to provide safe passage of the mass.
However, on August 31, 2007, seven riders in the Chicago Critical Mass were arrested on charges of obstructing traffic and disobeying police. The seven were held overnight. According to some of those arrested, they were released at various points in the late night/early morning.
Some bicycling advocacy groups have expressed concern that the "subversive" nature of Critical Mass and altercations with motorists could weaken public support for bicyclists. Though it does not condone incidents of violence and rudeness, the San Francisco Bicycle Coalition credits Critical Mass with spotlighting bicycle issues and aiding their efforts in advocating for cyclists.
Still other cyclists and motorists believe that altercations with motorists during the rides have led to a perception among motorists that cyclists are willing to defend themselves by physical force. News reports and videotapes of altercations with motorists have turned off some cyclists, and aroused the sympathy and solidarity of other cyclists. Often, such news reports bring many new, first-time riders out to the rides the following month, and in the future.
In San Francisco, an event known as "Critical Manners" was created as a response to Critical Mass. Critical Manners rides through the city on the second Friday of the month, with riders encouraged to obey all traffic laws such as stopping at red lights and signaling. Tucson, Arizona holds the Tuesday Night Community Bike Ride as their alternative to Critical Mass. The weekly ride encourages bicycle commuting and motor vehicle awareness in a peaceful and friendly way.
In 2007 there were conversations about starting Critical Manners in Portland, Oregon. According to the Critical Mass book, edited by Chris Carlsson, rider Michael Bluejay of Austin, TX has worked on one of his projects, Courteous Mass described as "an alternative to Critical Mass."
An alternative ride named RideCivil formed in Seattle, Washington in late 2007 Rides are on the 2nd Friday of every month, and focus on encouraging civility between motorists, pedestrians and cyclists. Information may be found at
Critical Mass rides have generated considerable controversy and public opposition. However, there is also public support, with many spectators cheering and enjoying the colorful festival-like events.