critical mass

I was reading a very funny article about why SF sucks by a blogger who's lived here but has since moved on. One writer commented - too many dorks/hipster/hippies. 

Its true, I see them on my way to work. They clog the streets every last Friday of the month to make a point, what that point is I have no idea, it's called Critical Mass where a HUGE group of cyclist ride en route from downtown market through the mission, they set out this trek to make a point of how unfriendly cities are to bikers, at the expense of causing major traffic jams, over the last few months there had been incidence of violence between bikers and motorists, and from reading some of these articles its the bikers who usually provokes these fights.