09/28/2012 02:15 pm ET

Friday’s Ride Marks 15th Anniversary Of Critical Mass

It's a big day for Critical Mass Chicago.

Today the bike ride that clogs up city traffic the last Friday of every month with several thousand bikers, celebrates it's 15th anniversary this September 28th.

"There are reports of Chicago Critical Mass rides happening in the early nineties, but no one has records of when the first rides started, so we mark our anniversary from September 1997, when cyclists first started massing up around the Chicago Picasso," states the Critical Mass Chicago website. "Fifteen years later, Chicago Critical Mass draws thousands of participants the last Friday of every month growing stronger each year."

In honor of the group's 15 year milestone, drivers should assume a higher volume of participants for this evening's ride. Perhaps one indication of the size of this month's ride is the fact the group is giving away 1000 free T-shirts to commemorate the event.