The last Friday of the month means one thing; the commute home is probably going to be delayed by Critical Mass.
Critical Mass is a monthly bicycling event.
Started in 1992; the event gathers hundreds of bicyclists together for a ride that takes over the streets of the City with no official route. Often times the route is determined by whomever is at the lead of the pack. The original purpose of the event was to draw attention to how unfriendly the City was to cyclists.
The riders usually have police escort and can tie up traffic for hours.
For the most part the event is peaceful and can be fun to watch if you happen to be stuck on one of the public transportation vehicles that is unable to move because of the cyclists.
See the above brief video from the beginning of last nights ride that stopped cars and Muni trains along the Embarcadero.
The Song Of The Day - My Body Says Yes by Titiyo