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Dressed as a zombie nurse, Katrina Loera, 50, readies for San Jose Bike Party, a mass nighttime bicycle ride, at a starting location in Santa Clara, Calif. on Friday, October 15, 2010.

Almost stealthily, they begin their trek -- bike riders, thousands of them, wheeling through quiet neighborhoods, busy intersections and major thoroughfares.

Their route is so secretive that even on the day of the big ride, not everyone knows the way.

But as daylight turns to darkness, volunteers appear at the starting location -- different spots every third Friday of the month -- and produce a detailed map for what's become a San Jose phenomenon: a huge bike party that has no official leaders or sponsors.

In many ways, the San Jose Bike Party is the antithesis of San Francisco's Critical Mass -- an often disruptive bike ride through rush hour in downtown San Francisco, with hordes of bicyclists intentionally blocking traffic in their attempt to promote alternative transportation.

"We are less of a protest and more of a celebration of everything that makes this city great on bikes "... as long as folks are willing to follow the rules of the road and how we ride, we welcome everyone," said Ian Emmons, one of several hundred volunteers.

On this particular Friday, one of the starts for the San Jose Bike Party is at Lafayette and Benton streets in Santa Clara -- a special ride that marked the third anniversary of the monthly event, a cruiser dubbed "Ride of the Living Dead," in homage to Halloween.

"I've been riding here since it started," said Rob King, 19, a San Jose State student, preparing to ride on his aluminum-framed, black and white bike that he built himself. "It's a good time, not as intense as Critical Mass in San Francisco. I feel it conveys the same message but without the anger."

For the most part, the San Jose Bike Party appears void of any hard-core political bike message, even starting at 8 p.m., well after the evening commute.

"It beats the bars and the club scene," said Sarah Holquin, dressed as a blood-soaked zombie for the pre-Halloween ride. "I look forward to my bike party instead of abusing my body with alcohol."

In an extraordinary feat one recent Friday, an estimated 4,000 bicyclists managed to circle through the city in a rather orderly fashion, with volunteers shouting directions, reminding riders to steer clear of traffic, stop at red lights and let vehicles pass.

Only a few times did things get dicey -- at the Plant Shopping Center in South San Jose, a huge crowd filled the lot, with bikers doing tricks on a makeshift ramp that quickly drew police and the ire of a store manager and his customers.

"They don't care about anyone but themselves," said Terry Fernandes, manager of Babies R Us. "They are extremely inconsiderate."

Quickly shut down by police, the riders continued their journey as some hauled boom boxes blaring an eclectic mix of music, from reggae and gangster rap to golden oldies. Even a bagpiper played as he rode along toward San Jose State -- the party's final destination.

At times, the unmistakable smell of marijuana wafted through the air as riders stopped at various destinations during the 17-mile ride. No one seemed to mind. And, at least once, a rider rolled through a red light.

The San Jose Bike Party has its origins in 2004, when about 15 to 30 bikers held a Halloween "get out the vote" ride. A year later, many of the same bikers reunited, sporadically holding rides. But it wasn't until 2007 that it blossomed into a more organized event, with volunteers promoting the event through e-mail and social networking sites and touting monthly themes and planned routes.

There have, however, been a few road bumps along the way, with reports of serious bike crashes and riders drinking alcohol, urinating in public and littering the streets. Many of them choose not to wear helmets.

Even so, San Jose police have not assigned any special patrols to monitor. "It hasn't been that big of a problem," San Jose police Lt. Mike King said.

With the number of bicyclists in the thousands, San Jose State police Sgt. Manuel Aguayo would like to see some changes.

"It's grown way too large and it's getting too big for our law enforcement resources," Aguayo said of the ride, which has met several times at the campus.

Typically, large groups who gather on campus must get clearance, but with no one claiming responsibility for organizing the bike party and the routes so secretive, it's unlikely that will ever happen. One of the reasons for the secrecy, volunteers say, is not to avoid police but to deter the unruly factions from showing up and marring the ride -- something that happened last year, when the number of riders nearly doubled.

Santa Clara police Lt. Phil Cooke said his department has an informal agreement with other law enforcement agencies to assist in case the ride becomes too unwieldy and dangerous.

"We're there for safety reasons," said Cooke, whose department had a contingent of officers monitoring -- at a distance -- the bikers on the most recent ride.

It isn't always safe.

Felicia Aldana, 25, was riding her bike on a recent ride on Race Street when a sedan struck her and sped away. Though the driver was caught and Aldana escaped relatively unharmed, the episode left her shaken.

"It could have been a lot worse,'' Aldana said. "I was really lucky."

On this night, however, even drivers caught in the whirl of bikers didn't seem to mind -- and many honked in good cheer.

"It's kind of cool," said Shannon York, who curiously smiled at the bike riders from her car as she waited for them to pass.

That isn't a sentiment shared by Catherine Christofferson, who said she was caught in a maze of bikers in August as she tried to leave Santana Row.

Christofferson said she watched helplessly as bikers weaved through traffic, blocked lanes and rolled through red lights, with some screaming into the window of her car.

"As a driver, I didn't feel safe," she said. "It was unsettling. I don't have a problem sharing the road, but I do have a problem when they take over the road."

City Councilman Pierluigi Oliverio, a frequent rider, said he believes unruly bikers are the exception. "You're always going to get someone who is a bad apple," he said.

Along the quiet street of Saddle Brook Drive last Friday, spectators came out of their homes to cheer. Others sat on the curb, watching bikes of every kind glide by, including those riding with children in tow.

Other spectators cheerfully slapped hands and fist-bumped the riders who rode through the streets, occasionally yelling out "Bike Party!"

"I came out to look. It's like a parade," Jim Ramos said while standing outside his house. "I think it's cool. I might join them next time."

With the ride nearly complete last Friday, Dixie Baus stopped and leaned against her bike, exhausted.

"This is my fourth time and I fade out every time," said the 41-year-old woman from San Jose.

"But I can't stop doing it, it's so much fun to be with this mass of people," she said as she slowly resumed her ride.