On the last Friday of every month, bicyclists gather for Critical Mass: to celebrate cycling and to assert cyclists' right to the road.
The June Critical Mass in Vancouver, BC, Canada is normally the largest ride, pulling thousands of riders and temporarily shutting down vehicular traffic in large swathes of the city. The June 2007 ride shutdown Lion's Gate bridge for 30 minutes and the Stanley Park Causeway that leads to it for 60 minutes.
Police helped to control traffic as Critical Mass relenquished control of Lion's Gate Bridge. Police aren't always so friendly to Critical Mass' around the World, and they weren't at one time in Vancouver, but it is definitely changing for the better