WE HAVE 15 days to end the auto age, and I'm a little nervous. "World Car-Free Days," an event launched in 2000 by Prague-based activist group Carbusters, started on Sept. 13 and ends on Sept. 27 in San Francisco's Financial District, where, for a few hours, Montgomery Street will be closed off to cars between Market and California for a transportation fair. And a few hours later, at 6 p.m. or thereabouts, Critical Mass will ride off in a jubilant parade of metal beauty, honking horns, and hopefully a few good costumes. And after that ... a portal will open in the ground and all the Chevron stations and abandoned hubcaps and two-car garages will fall through it in a blaze of tire fires and exhaust fumes.

Or maybe we'll go back to work on Monday, using whatever mode of transportation we used on Friday, passing through a landscape of too many parking lots, too many worker bees trapped in gridlock, and too many bicyclists taking their lives in their hands every time they hit the streets.

San Francisco's Critical Mass turns 10 on Sept. 27, and what I'm hoping for is something in between the hell-mouth scenario and business as usual. The people who show up at Justin Herman Plaza that day on their beaters, rock hoppers, two-wheeled art projects, and world-class racing bikes will have something very serious to celebrate. The first Critical Mass took place here in 1992 and involved 48 bicyclists; flyers for the event invited people to ditch their cars for the day and "bike home from work together." Over the past 10 years, Critical Mass riders, in groups of tens, then hundreds, and eventually thousands, have taken the long way home on the last Friday of every month. The ride has traveled up the city's steepest hills, through the Stockton and Broadway Tunnels, to the ballpark, the Wave Organ, the bay, and the ocean, across the Golden Gate Bridge, and into as many of the city's neighborhoods as its participants collectively felt like visiting.

In spirit C.M. has traveled a whole lot farther: Jym Dyer's Critical Mass Web site (www.critical-mass.org) lists 316 rides (some dormant, some dead, many flourishing) in cities and towns throughout Europe, Asia, Africa, Australia, and North and South America. There's even one in my hometown of Rochester, N.Y., where, in my youth, riding on the sidewalk was often forcefully encouraged by passing motorists – not a problem, since there weren't any pedestrians to speak of. Ten people clustered together on their bikes in Rochester would radically change the way the streets look, if only for an afternoon. I say that while thinking about how San Francisco has changed. Though this is not yet the city of my dreams, the streets here look different because of Critical Mass's influence, and I wonder what things look like to passing motorists, pedestrians, and bicyclists in Melbourne, Tokyo, Boston, or any other city where a Critical Mass has taken place or taken hold.

Many of those cities make their way into Critical Mass: Bicycling's Defiant Celebration (AK Press), an anthology edited by Chris Carlsson, one of the original S.F. Critical Mass riders. A cofounder of the revered zine Processed World and coeditor of the multimedia project Shaping San Francisco, Carlsson put out a call over e-mail for contributions and got back a deluge of essays, anecdotes, photos, and flyers. Critical Mass participants in cities like Montreal, Bombay, Tel Aviv, and Lubbock, Texas, sent in their report-backs.

There are photos of a costumed Halloween ride in Manhattan, a "bike lift" in London. A flyer from Madison, Wisc., asks, "Worried about terrorism? Want to know what you can do? Help fight back, ride a bike!" From San Francisco comes an essay by Dildo Man, a local C.M. celebrity, whose interactions with the cops were like a performance art piece. Another flyer comments on the murder of Nigerian activist Ken Saro-Wiwa. A reproduction of a 1993 "corking" sign with the text "Thanks for Waiting!" exemplifies one approach C.M. riders use to encourage motorists to stay out of the intersection while the bikes go by. A photo captures the February 1993 art installation The Door Is Always Open; the work of a mysterious "Dept. of Public Art," the sculpture depicts the painful embrace of bicyclist and car door. Bike activist Jason Meggs contributes "A Well-Camera'd Society Is a Polite Society," a helpful piece on documenting police abuses and knowing your rights. Many locals comment on the wild ride of July 1997, when some 5,000 participants showed up, the mayor got pissed off, and a police dragnet sent a hundred cyclists to jail.

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The rides have been almost dull lately, Carlsson says, though he doesn't sound that fazed by it. No motorist assaults, not much trouble from the cops, no media attention. It's hard for me to imagine. My first Critical Mass took place in August 1997, and the TV helicopters were like a cloud of gnats up in the sky.

Neither jail time nor the possibility of being captured on local TV against my will scared me then as much as the fear of punking out. But as it turns out, we weren't going anywhere fast. The next day I was sore from the odd experience of having biked up several of the city's fabled 14 hills in slow motion, wheel to wheel with my biking compatriots. I hated car commercials, tire junkyards, shoddy mass transit systems, and society's dependency on oil, but I have to admit: at times we were kind of a traffic jam all on our own. Road rage also spoiled a few moments – macho motorists getting into culture clashes with macho cyclists, people facing off in intersections, veins popping out of their necks. Still, filling all that space in the road where cars had been – it was exhilarating and addictive. I wish the whole city could know how that feels.

I fell really hard for my bike after that. I named it, put stickers all over it, started riding everywhere. I became slightly sanctimonious and developed a really bad case of bike-messenger envy, with delusions of badassedness that usually involved running lights at top speed and riding in between Muni buses. A growing fear of death put a damper on that behavior, but the obsessive bike love has outlasted many other relationships.

A few of us rode up Twin Peaks on Wednesday evenings. On the weekends after parties, we would cruise down to Third Street where the travelers lived in campers and the Lefty O'Doul Bridge sat in a steel sulk. And every month there was the sea of bikes in Justin Herman Plaza, kids riding around on their hind wheels, people in costumes, flyers circulating like rumors. For a while you could figure out where the ride was by watching the skies. My friend Michael T., who carried a 12-pack of beer in his backpack on every ride, would send out reminders to all his friends, with ASCI art of a bicycle and the words "Ride On" at the bottom. I admired his Critical Mass spirit.

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In "Why They're Wrong about Critical Mass," Adam Kessel from Boston talks about social progress needing the presence of a group that "occasionally acts 'as if' what they wanted to be true were true." Whenever I rode in Critical Mass, I had the sense that our presence was essential, that we were playacting something deadly serious, using hilarity to show the motorists something we urgently felt needed to be true. That's probably why I was always so hyperemotional afterward, not knowing quite what to do with myself. We'd spend the evening laughing and hollering, passing out fake traffic-violation tickets, making a comedy routine out of the commute home. And then it was over for another month, and the space we had made into a compelling shape sort of dissipated while Friday night took over and the city came out to entertain itself. We could still feel it, but it was hard to tell if it mattered what we felt.

This is what I love and find frustrating about Critical Mass. It creates a bike-friendly zone and makes a picture of what many of us feel the city should always look like. It dodges in and out, eluding regulation. But it's only a two-hour holiday from the carbon monoxide smackdown. People tie their mountain bikes onto their SUVs; hipsters tool around town in their vintage cars, dodging smog tests. On Monday the motorists have gained back the feet of roadway that were stolen from them, and the intersections downtown are corked with cars again, instead of people holding signs that read, "Honk if you love bicycles."

"Even if you've only ridden in Critical Mass one time, it's going to alter the boundaries of your imagination and what's possible," Carlsson says. "There's no end to that process. What's to prevent you from coming up with brilliant ideas about how city life could be better?"

I ask him to name some. "The notion of a dedicated grid of bikeways just for that purpose, that don't have to share the road with motorists," he responds. "General eco-friendliness. Opening up the creeks and creating running streams and restoring wetlands. Growing more food in the city and tearing up streets. Wild corridors through the city." I have images of maps with rivulets of water running down them. The bumper cars fly past on the 280 outside my window at work.

I think about the San Francisco Bicycle Coalition, working its ass off on a network of lanes for the past 10 years, on dedicated paths, an urban greenway in the earliest of planning stages. More cyclists are out on the streets these days, something Carlsson links to Critical Mass and Leah Shahum of the SFBC links to the new bike lanes on Polk, Arguello, and Valencia, where the city reported an increase in bicycle travel of 41, 67, and 144 percent respectively. They're probably both right, and I think they'd agree. The coalition's numbers rose in the wake of Critical Mass's emergence, and as the ride grew bigger, the coalition's clout with the city grew too, meaning that necessary work is being done to create alternatives to driving, meaning that we may someday have that greenway. Still, the paint on the road is wearing thin, and the rush-hour ride home on Valencia feels like a video game. What we might want is a blood oath from every driver to look before opening his or her door.

"Fifteen Days to End the Auto Age," the Carbusters Web site tells me. But 10 years of Critical Mass and thousands of hours logged by bicycle advocates and activists has not been quite enough to change a car-devoted city's mind, not to mention the rest of the country or the rest of the world. I want to end the auto age by riding my bike around the streets of San Francisco trying to look like traffic to people who forget to check their blind spots. I want to end it by making all this look easy and fun, making people feel like finding a way not to drive. Carlsson, a man who dreams of a San Francisco with all its creeks resurfaced, half its streets gone, talks to me about radical patience. "It doesn't mean waiting," he says. "It means things take a long time."

The SFBC, Walk San Francisco, Rescue Muni, City CarShare, and Transportation for a Livable City throw a 'Car Free Party' featuring Rube Waddell and DJ Dave Byron Sept. 20, 6:30-10 p.m., 26 Mix, 3024 Mission, S.F. $5, free for organization members. Valet bike parking provided. www.sfbike.org. 'Car-Free Day' is Sept. 27, 10 a.m.-2 p.m., Montgomery between Market and California, S.F. www.carbusters.ecn.cz/carfreeday. Critical Mass meets Sept. 27 (last Friday of every month), 6 p.m., Justin Herman Plaza, Market and Embarcadero, S.F. Some unofficial sites: www.critical-mass.org, www.criticalmasshub.com, www.talkfastrideslow.org. E-mail Lynn Rapoport at lynn@sfbg.com.