Critical Mass is a controversial bicycle ride that occurs on the last Friday of every month. It originated in San Francisco in the 1990s as an effort to let drivers know that cyclists have equal rights to the roadway. The ride has spread to major cities throughout the world. The controversy stems from the fact that some riders can be confrontational and the ride typically does not stop at red lights. Those in favor of the ride point to the fact that its very existence gets its message across. Those against point to the fact that the ride impedes pedestrians and public transit as well as cars.
Regardless of your position on the issue, Critical Mass typically has an impact on the Friday evening commute for drivers, cyclists, pedestrians and public transit riders. Being forewarned, planning for extra time and being patient will make the evening easier.
More information about the history and politics of Critical Mass can be found at: http://en.wikipedia.org/wiki/Critical_Mass