Tonight, as happens every last Friday of the month, a minor revolution will break out on Atlanta's streets.

Starting about 6:30 p.m., more than 300 bicyclists plan to take over several lanes of traffic, shoving King Car aside.

They call it Critical Mass. It's a rolling message board that says "Bicycles have a right to the road too!"

During last month's ride, a few bicyclists "corked" the intersections at every traffic light, blocking cars for the mass of bikers as they pedaled through a couple of light changes.

Surprisingly few drivers honked. Most just stared with quizzical looks. One elderly woman behind the wheel of a large sedan appeared startled, possibly frightened by the bicycle version of Freaknik gliding down 10th Street near Georgia Tech.

The event is both political statement and rollicking street festival, with some civil disobedience thrown in. (Bicyclists are supposed to follow the same rules as drivers, including stopping for red lights). There's no organized leadership. Routes are not planned, and anyone who wants to lead the pack is welcome to try.

Critical Mass started in San Francisco in 1992 and is now replicated in hundreds of cities worldwide. An Emory University graduate student brought the ride to Atlanta in 1999.

Darian Somers, 34, a Virginia-Highland resident, has seen the mass ride through her neighborhood.

"I don't have a problem with them trying to get the point across," she said. "Feel free to share the road, but follow the same rules that apply to motor vehicles. Don't go flying through a red light while we're all waiting."

To help defuse hostility and spread the joy of their preferred mode of transportation, riders during last month's ride shouted "Happy Friday!" and "Thank you!" to waiting motorists.

The message is not necessarily judgment-free. "Sucks to be in a car," said one young rider, in a tone meant more for his pedalling companions than for drivers.

When the group began to fragment as it rolled through Georgia Tech, calls of "Mass up!" began. A few fast riders had broken away from the pack as the rest clumped up behind them, trying to hang on. The group re-massed at the next red light.

The ride lasted about an hour and a half, winding through downtown, Midtown, Inman Park, Little Five Points and Virginia-Highland.

Even in the bicycling community, the unauthorized rides are controversial. Some believe the tactic gives bicycling a bad name. To the critics, Critical Mass creates a bike vs. car mentality that doesn't help when bicyclists are lobbying for more and better biking facilities, including bike racks and designated bike lanes and paths.

But Rachael Spiewak, 27, a regular Critical Mass'er and executive director of SoPo Bicycle Co-op, a nonprofit bike shop in East Atlanta, said the rides are an important tool for empowering bicyclists and making drivers more aware of them.

"The collective message is, 'We're here, look at us!' Whether people mean for it to be or not, it winds up sending a political message. ... It's kind of a way to take the power back in a peaceful way."

Spiewak said bicyclists are accustomed to second-class status in Atlanta. Bike lanes, intermittent at best, often are made hazardous from road debris, and cars don't always want to share the road. But biking, Spiewak said, "is affordable, clean, it's good for you, and it puts you in touch with other people using the street."

The rides have been relatively calm in Atlanta compared with other cities. In San Francisco, where thousands of bicyclists participate in the monthly last-Friday rides, motorists and bicyclists clash occasionally, both verbally and physically. In July 1997, 250 bicyclists were arrested when they snarled traffic for hours.

In Chicago last August, police took control of a ride and forbade bicyclists to complete the route.

The Atlanta Police Department takes no position on Critical Mass. Last month, officers directing traffic for the Bruce Springsteen concert at Philips Arena stopped traffic on Techwood Drive to let the bikers through.

Officer J. Childers with the Special Operations Unit said he has ticketed one rider, after he saw him run several red lights and get into arguments with drivers waiting at the intersections. His main concern is altercations between motorists and bicyclists.

"Obviously it's a problem, but it's one of those things you pick your battles on," Childers said. "It's irritating to be driving and have them on the street. One of the problems is the sheer numbers of them. ... Bicyclists are supposed to maintain the same laws as vehicles."

Happy Friday!

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IF YOU GO

Critical Mass is the last Friday of every month. Bicyclists gather starting at 6 p.m. in downtown Atlanta's Woodruff Park and begin riding about 6:30 p.m. For more information, go to www.criticalmassatlanta.org.

WHY IS IT CALLED CRITICAL MASS?

The name "Critical Mass" comes from Ted White's bike-umentary Return of the Scorcher. This video shows intersection crossing etiquette in China's big cities. Cross bike traffic waits until it has enough riders, i.e., a critical mass, to push its way through the intersection.