San Francisco Police Chief Greg Suhr said he was told to expect 10,000 cyclists or maybe even more. Those that take part in Critical Mass say they are fighting a car-centered society -- one calls it a culture war.
"We were really second-class citizens on the road," said Chris Carlsson.
So 20 years ago this month Carlsson and about 50 other bicyclists set out to make their presence known. The Critical Mass movement was born.
"Some people have jokingly called it 'We bully cars.' It's like, 'Give me a break. All day long, every day, motorists are bullying bicyclists.' So once a month we turn the tables, and not to be bullies, but to say, 'We're here, respect our space, respect us for once a month for just a few hours while we take the roads,'" said Carlsson.
There's no predicting who will show up on the last Friday of every month or which streets they will take over. Sometimes there are hundreds of cyclists, sometimes thousands. Most rides are peaceful, but frustrating for motorists. Yet others have turned into ugly confrontations over congestion. One ride in 1997 ended with 250 cyclists arrested for everything from traffic violations to assault. Another in 2007 caused then-Mayor Gavin Newsom to say the movement was losing the hearts and minds of the community.
"The kind of inconvenience that critical mass is providing, I think is counter-productive now," said Newsom.
Still, Critical Mass has prevailed and now has international influence. Fabio Corrennte is here from Italy to celebrate the 20th anniversary.
"We have Critical Mass in five to six towns," said Corrennte.
And where city leaders once talked tough, now the President of the Board of Supervisors, David Chiu, is an avid cyclist.
"I certainly think in recent years, the biking community has gone mainstream and really diversified," said Chiu.
And San Francisco has added miles of new bike lanes, but Rob Anderson is trying to stop any more from coming in. He's taking the city's bike plan to court.
"One of the problems with the bike plan is that it takes away traffic lanes and street parking on busy city streets. That creates a problem, that's an environmental impact," said Anderson.
But on Friday night, bikes will rule the road in parts of San Francisco.
"We will, we'll have extra people with them. We'll use our motorcycles, both dirt bikes and big bikes and actually officers on bicycles themselves," said Suhr.
This is not a static rally or protest. This is a moving movement, which means if you want to avoid traffic delays, you might want to get out of Downtown San Francisco around 6 p.m. Friday or take public transportation.
If you don't already have it, now might be a good time to download the Waze traffic app -- exclusive to ABC7 News. It's available on iTunes and Google play and you can use it to guide you around the congestion Friday night.