Critical Mass

An informal, leaderless bike ride that meets on the last Friday of every month, in cities around the world. There is no route planned; the ride simply follows those who happen to be at the head of the group. The ride in London meets on the South Bank. 

The term is also used to describe a volume of cycle traffic on a particular street or road that might guarantee a certain degree of safety. If there are many cyclists using a particular street, it is held that drivers will be paying more attention, and are less likely to hit an individual cyclist. It is closely allied to (but should not be confused with) the Safety in Numbers theory.