Barbara Ross with Time's Up Volunteers Wendy Brauer, Ariane Burgess and Steve Klein

As private cars, taxis, trucks and other motor vehicles continue to clog New York City streets, a growing number of Big Apple residents are hopping on their bicycles to get around town. Doing so, they say, is good for their health, good for the city's environment, and good for the planet, too. One busy grassroots environmentalist group called Time's Up promotes biking as a way to "go green" all year long.

Indeed, at nine o' clock in the evening, when most New Yorkers have left the downtown district and gone home, Time's Up's ramshackle headquarters in Lower Manhattan is abuzz with bicycle enthusiasts and urban environmentalists. For Wendy Brauer, a longtime member of the organization's all-volunteer staff, those two roles go together like the two wheels on her slightly battered but beloved bike.

"We are really at a turning point for all cities as climate change awareness is growing, and so much of it has to do with our transportation choices," Brauer says. "We all need bicycles to make greener cities."

Time's Up volunteer Bike mechanic and teacher Mark simpson

One commonly cited reason why bicycle use makes environmental sense is that they use no fuel. "They give off no emissions of any kind," says Brauer, who points out several additional advantages to bicycles. "They are cheap. They are reliable. They are fun. They are healthy. And it's very easy to keep a bike for a long time, and you can keep it in repair yourself, and that's a great thing."

Time's Up offers free bicycle repair workshops taught by volunteer mechanics like Mark Simpson. He says that self-sufficiency is a "green" value. "Because if I can transport myself to and from my job, I am not relying on diesel fuel for the bus or the car, and I am not relying on electrical power for the subway."

Bicycles are already a popular form of personal transport in many world cities, from Copenhagen to Lagos to Beijing. And activists say that getting New Yorkers on bikes should be a relatively easy sell. They note that the city is mostly flat and very compact, and New Yorkers already use mass transit more intensively than residents of most other U.S. cities.

But riding a bike in the Big Apple can also save time. Another environmental group called Transportation Alternatives holds an annual competition where three people, one on a bicycle, one in a car, and one on a subway depart at the same time during rush hour from the borough of Brooklyn to a designated spot in Manhattan. The bicyclist has arrived first every year for the past four years.

Time's-Up group rides often take place in city parks

But in spite of the advantages, most New Yorkers do not ride bikes regularly, other than for recreation. Concern over safety bicycling in New York's heavy traffic is one major reason. That is why, according to Times's Up director Bill DiPaola, the organization sponsors frequent "group rides" in city parks and greenways.

"We have rides to Coney Island, and moonlight rides though the park. You can go on a tour of Lower Manhattan, and there are lots of other offerings. They are all slow and fun and easy," he says. "Group rides are a great new way that a lot of people get introduced to riding."

Group rides are also a way to build community. For Scotland-born Ariane Burgess, "full moon" rides have also been a way to see her adopted city in a magical new way. "It's quite a thrilling experience, and quite delightful too," she says. "Everyone's got their little red lights flashing, and when they go off in the dark they look like fairies flying around."

'Critical Mass' group ride on Halloween night

Many Time's Up! group rides are designed to support the group's progressive environmentalist agenda. In "Critical Mass," rides, for example, hundreds of bicycle activists converge on busy streets and thoroughfares such as Times Square, then ride the streets en masse, slowing or stopping traffic as they go. Group members have often been arrested or issued summonses for parading without permits.

In one upcoming event, Time's Up members wearing swimming trunks and underwater gear will bicycle to a Manhattan site that some climatologists predict will one day be under the ocean as a result of global warming. Although Earth Day is past, for these environmental activists, it is, as one of them put it "Earth Day every day."