Because Critical Mass takes place without an official route or sanction, participants in some cities have sometimes practiced a tactic known as "corking" in order to maintain the cohesion of the group. This tactic consists of a few riders blocking traffic from side roads so that the mass can freely proceed through red lights without interruption. Corking allows the mass to engage in a variety of activities, such as forming a cyclone, lifting their bikes in a tradition known as a "Bike Lift" (in Chicago this is referred to as a Chicago hold-up), or to perform a "die-in" where riders lie on the ground with their bikes to symbolise cyclist deaths and injuries caused by automobiles, very popular in Montreal. The 'Corks' sometimes take advantage of their time corking to distribute flyers.
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.