The subways and buses may not be running, but the bike business is booming.

As New Yorkers take their daily commutes to the streets and sidewalks, cycling advocates are hailing the transit strike as a boon for two-wheelers, citing a spike in sales and estimates that city bike riding is up by an average of 500%.

“It’s making the best of a bad situation,” the project director for Transportation Alternatives, Noah Budnick, said. “We’d prefer transit workers not strike, but we’re encouraging people to make the best of it.”

Transportation Alternatives, a 5,500-member nonprofit that advocates reduced car usage, is estimating that as many as eight times as many people crossed the Brooklyn Bridge on bike yesterday during the morning rush than on a regular December weekday. The group counted riders on the East River bridges, Sixth Avenue, and the Hudson River Greenway during the evening peak period Tuesday, and again yesterday morning. Bike riding spiked between 250% and 850% from a typical December morning, the group reported. That translated to 800 riders an hour on the Brooklyn Bridge; normally, about 100 an hour cross the span during the morning rush.

The group’s survey also indicates that bike riding will only increase. Counters reported twice as many riders yesterday as on Tuesday, the first day of the strike.

“I know that other people are suffering, but it was a great day for bike riders,” a bicycle advocate, Judy Ross, said. Ms. Ross, 41, is an outreach volunteer for Time’s Up, an environmental group that pushes for cycling and pedestrian rights.

Using the slogan, “Bike the Strike,” Time’s Up and other groups have mobilized for the transit strike, coordinating events like bike-pools, maintenance workshops, and used bike sales.

About 20 people joined the group as it rode yesterday morning from the Manhattan Bridge to Columbus Circle, dropping workers off at their offices on the way. For Ms. Ross, the mass of bikers on the road was a revelation. “I stopped at 23rd Street for the light, and there were eight bicyclists behind me,” she said. “It was like a little mini-traffic jam for bikes.”

Bike shops across the city also benefited from the walkout. The owner of C&M Cycles on East 17th Street, Carlos Dall’Orso, said he sold and rented three times as many bikes yesterday as on a typical winter day. Three-quarters of his rental stock sold out, he said. “It’s almost unheard of that I would rent a bicycle during a weekday,” Mr. Dall’Orso said, adding that he offered a deal in which customers could rent a bike – usually $25 a day – for the duration of the strike for $100.

As thousands of commuters descended on the Brooklyn Bridge for the evening trip home, bicyclists bared the cold, and bristled at the strike. “I don’t think this will wake anybody up to the pleasures of biking in the cold,” a Park Slope resident, Ryan Cunningham, 25, said as he shivered on the Manhattan side of the bridge.

A Carroll Gardens resident, Michael Tosov, 52, was more defiant. “I would ride to work through the whole winter rather than see them settle with these people,” he said.