From Alderman Reilly Reports:

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Don't forget, the monthly Critical Mass Bicycling Event occurs tonight, starting at Daley Plaza. Critical Mass is a recreational bike ride that typically attracts several hundred bicyclists. As tonight is the final Friday before Halloween, the turnout is expected to be much more significant.

The ride begins at approximately 7:30 p.m. and typically lasts about three hours, during which time we anticipate longer drive times for those driving in downtown Chicago.

Please note that during the times of 7:30 pm and 8:30 pm, the Loop and Near North neighborhoods may be more congested than normal. I urge you to allow additional travel time and exercise caution during this time period.

Police will be on-site to enforce the restrictions to the route, namely that cyclists will not be allowed east of Michigan in the Central Business District, nor will they be allowed to travel on Michigan Avenue, north of the Chicago River.
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More information about Critical Mass on Wikipedia -

"Critical Mass is an event typically held on the last Friday of every month in cities around the world where bicyclists and, less frequently, unicyclists, skateboarders, inline skaters, roller skaters and other self-propelled commuters take to the streets en masse. While the ride was originally founded with the idea of drawing attention to how unfriendly the city was to bicyclists, the leaderless structure of Critical Mass makes it impossible to assign it any one specific goal. In fact, the purpose of Critical Mass is not formalized beyond the direct action of meeting at a set location and time and traveling as a group through city streets."

Critical Mass - Wikipedia, the free encyclopedia