Wearing a black Jil Sander skirt matched with an elegant Velvet T-shirt, Lucy Wallace Eustice is pedaling her bike to work on a day as clear as Baccarat crystal. Her four-mile journey takes her along a bicycle path, one of the nation's busiest, that parallels the Hudson River on one side and the Manhattan skyline on the other side.

To get to her SoHo office in the middle of the city, she weaves through side streets, dodging UPS trucks, squawking taxis, and workmen perforating roadways with jackhammers. Ms. Eustice could easily take other methods of transportation – the teeming subway system or one of the New York's ubiquitous cabs.

But instead she chooses her silver Globe commuter seven-speed – even on cold days. "You are free when you're on the bike," says the fashion designer, whose bike, appropriately enough, has chic saddlebags. "You belong to yourself."

Yet a sense of emancipation in a city that can feel claustrophobic isn't the only reason Eustice bikes to work. She sees her ride as the equivalent of a trip to the gym and relishes the fresh air and "solitude." On weekends, she and her husband, John, and their two children often bike in the countryside as a family outing. "You gotta love your bike," she says.

Indeed, millions of Americans are doing just that – having a sudden romance with their bicycles. From the cliffs of Santa Monica, Calif., to the canyons of Wall Street, Americans of all generations and incomes are jumping on their Cannondales, Treks, and Specializeds in record numbers, turning to them both as a form of transportation and recreation.

The bike craze is being driven by a confluence of forces – perpetually high energy prices, the green movement, and an enduring fitness craze. At the same time, a new generation of mayors is pushing bike lanes, bike-share programs, bike garages, and other accouterments to wean people from their cars and couches onto the seats of Schwinns and Peugeots. In some cases, the municipal chief executives are engaged in friendly – but fierce – competition to get their city labeled the most "bike-friendly."

The result, on any given morning, is a growing legion of people in spandex, skirts, and Brooks Brothers suits coursing down urban streets from Minneapolis to Charlotte, Miami to Seattle in a way that is changing how the country travels to work. Enough of them are now filling roadways that they are creating new clashes with motorists and pedestrians for their share of asphalt in the nation's cities.

What, exactly, is America becoming here ... the Netherlands?

Well, not quite. The Dutch use their bikes for 26 percent of all their trips compared with 1 percent of Americans. Danes use bikes for 19 percent of their travel, while the Germans tap them for 10 percent.

The US is definitely gaining ground, though. Between 2000 and 2011, bicycle commuting in America was up 47 percent overall and 80 percent in communities that are bike-friendly, according to the US Census Bureau.

The largest increase has been in biking-pioneer Portland, Ore., where commuting on two wheels has jumped 250 percent over the same 12-year period. In Washington, D.C., where limos shuttle lobbyists around, biking to work is up 166 percent. Even the prospect of long dark winter days and icy roads hasn't stopped two-wheeled commuters: In Anchorage, Alaska, traveling to work on bikes (yes, much of it with studded tires) has increased 140 percent.

Nor is it just people riding their own 10-speeds. This year, 18 new cities will set up bike-share networks – a 50 percent increase over 2012 – allowing people to rent bikes for short jaunts across town.

"I think we're at a new standard for cycling in America," says Ray LaHood, the outgoing US Secretary of Transportation. "I think we've reached the tipping point, and we're going way beyond that."

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Barbara Bitondo commuted for years in Washington, D.C. – by car, by bus, but none of it worked for her. She found mass transit too crowded, the roadways too coagulated. So she started riding her bike across town to work at the World Bank, which has cut down significantly on her travel time, even if she doesn't do it wearing Tour de France attire.

"I have had people tell me it takes a lot of guts to wear those high heels," says Ms. Bitondo of her standard riding shoes. "It definitely garners a lot of attention."

Riding a bicycle in the cloying humidity of the nation's capital in August, of course, can be a challenge. But the World Bank, in its effort to encourage alternative forms of transportation, provides changing rooms, showers, and hair dryers for its two-wheeled commuters. "You just have to bring your own towel," says Bitondo.

The World Bank has a special parking area to lock up bikes. Need to inflate a tire or adjust a seat? No problem. It provides pumps and tools to make repairs.

Traffic-free commutes is only one reason Americans are turning to bikes for transportation. Debi Farber Bush of Eureka, Calif., had a more personal rationale. She recounts how five years ago she weighed 350 pounds. Then she started eating better and exercising with a personal trainer. She dropped 100 pounds.

Two years ago, she decided to add cycling to her regimen. The more she pedaled, the more pounds she shed. Soon friends were encouraging Ms. Bush to participate in a century ride, a 100-mile bike trek, and to join the AIDS/LifeCycle, a seven-day journey from San Francisco to Los Angeles to raise money for AIDS research.

Now Bush proudly shows photos of herself 200 pounds lighter and posing in a pair of voluminous jeans she wore just five years ago. "I hope to teach people that if I can do it, other people can do it," she says.

Many other Americans are turning to bikes for ecological reasons. They see a switch to two wheels as a way to combat climate change and fossil fuel use. Ken Reid of Santa Rosa, Calif., attended a showing of "An Inconvenient Truth," the documentary about Al Gore's campaign to alert the public to the dangers of global warming, several years ago. Shortly thereafter, he began riding his bicycle to work four miles each way.

"Heat, cold, rain – nothing has tempted me to drive," says Mr. Reid, who trains dogs to assist people who have physical disabilities. "It is one thing I could do on a regular basis."

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Like Reid, many also say biking simply makes life more interesting. David Waters, a pediatrician in Milwaukee, says commuting from his house to an inner-city section of the city gives him the chance to see people out jogging or walking their dogs. "I see humanity, and everyone looks at you and smiles and says 'hello,' " he says. "No one is honking at you."

Dr. Waters notes that Americans live in confined houses, drive around in confined cars, and work in confined office spaces. "Bicycling takes you out of the box, lets you smell, hear and experience the world around you," writes Waters in an e-mail. "Bicycling is part of the solution to every problem our society faces be it health, environment, economic, or social."

Certainly he won't get an argument from many urban planners and big-city mayors. Across the country, municipal leaders are promoting bicycle use to reduce traffic congestion, cut oil consumption, clean up skies, and slim down a flabby populace.

More practically, urban leaders view bikes as an inexpensive way to move people around in an era when it is getting harder and harder to convince taxpayers to fund anything – subways, roads, or even pothole repair – as well as pry money out of Washington. Since bike lanes take up only a fraction of the space needed for cars, it's appealing to promote commuting by bicycle rather than trying to garner funds for more blacktop.

"Our principal job is to make sure that people and goods can move through the city," says Roger Geller, bicycle coordinator for Portland, Ore. "If all the new residents coming into Portland drive at current rates, nobody is going to be moving."

Some of the burgeoning interest in pedal power is generational. Biking appeals to many of the nation's younger mayors, such as Anthony Foxx of Charlotte, N.C., the newly appointed head of the US Department of Transportation.

"There is a new class of mayors who understand urban systems better," says Shin-pei Tsay, who follows cities and transportation for the Carnegie Endowment for International Peace in New York. "They are asking what is the most efficient way to move people."

States are pushing alternative forms of transportation, too. In 2006, California passed a "sustainable communities" law that requires every metro area to come up with ways to reduce greenhouse gases related to travel. It is forcing cities to consider both more pedestrian and bicycle facilities.

"There will be way more investment in bikes than in the past," predicts Bill Fulton, vice president at Smart Growth America, a coalition of groups that advocate for less urban sprawl.

In some cases, biking is just part of the local ethos. Take Portland, probably America's premier cycling city. Bicycling magazine recently voted it – once again – the most "bike-friendly" city in America as part of a ranking it does every two years. Portland lost out last time around to Minneapolis.

According to the US Census Bureau, more than 6 percent of Portland's commuters (17,000 people) go to work on two wheels – 12 times the national average. Portland knows precisely how many of them ride each day over the Hawthorne Bridge, a main artery, since it installed a digital counter donated by a nonprofit group. Officials use the information to help plot bike routes and plan facilities.

It doesn't hurt, either, to know the numbers for bragging rights. Not long after Portland installed its counter, Seattle put in one of its own on the Fremont Bridge. Portland likes to note that it has more cyclists crossing the Hawthorne than Seattle does its span. (What's next in the great green rivalry of the Northwest – a bike path up the Space Needle?)

Part of the reason biking has boomed in Portland is simply geography. Compared with Seattle and San Francisco, the Oregon city is relatively flat. The weather, despite its reputation for being sodden, is bike-friendly, too. "It's pretty temperate year round," says Will Vanlue of the Bicycle Transportation Alliance, which promotes cycling statewide.

The city has also installed almost 400 miles of bikeways and added 5,000 bike racks. And then there is Portland's bohemian culture, which celebrates biking in its own wacky way.

Tall-bike jousting contests are held each summer in Col. Summers Park, where riders square off and try to lance one another using what look like giant cotton swabs. Groups organize dozens of rides annually celebrating everything from the worst weather day (in February) to biking in various stages of undress (mercifully, in June).

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Still, Portland may not want to get too complacent about its position as bike capital of America – plenty of other cities are trying to establish themselves as citadels of spokes, too. Start with Long Beach, in the heart of the southern California car culture.

On a sun-kissed day, Allan Crawford, a Long Beach "mobility coordinator," takes a visitor on a tour of some of the city's investments in two-wheeled transportation. As he pedals down tidy streets of manicured lawns with hedges that grace humble but well-appointed California bungalows, Mr. Crawford pauses at a traffic light until street sensors register the bike's metal frame. Cross traffic gets a red light. He pedals forward.

Around town, the city has hung huge canvas advertisements featuring the faces of local residents to promote the virtues of biking. Crawford points to another transformation about to ensue as well: a signed contract with Bike Nation, a firm that offers rental bikes in kiosks.

Like many cities across the country, Long Beach is setting up a network of rental stations that will allow riders to use bikes for short, one-way commutes. The first station will go in by the end of the year, followed by 250 others, a dense enough web that riders should be able to pedal anywhere they want and drop off a bike within 1-1/2 blocks of their destination.

One hope is that a more bike-oriented culture will boost downtown business. "For 20 years, we had one bike rack outside, and it was only used sporadically," says Kerstine Kansteiner, who owns two downtown coffee boutiques. "Now we have six, and they are jammed at all hours."

That's not surprising to April Economides of Green Octopus Consulting, who works with cities on bike-friendly business districts. She says one reason biking is good for small businesses is because cyclists travel at speeds that allow them to notice the world around them – meaning those window displays of brioche and boutique furniture. Studies show that people who take their bikes shopping usually stay within two miles of their house, a boon to neighborhood businesses, and shop more frequently.

"We see a business we would not have noticed if we were whizzing by in a car," says Ms. Economides. "We see people we know, and we connect with them and have a conversation."

Yet the biking invasion can lead to tensions with local businesses as well. San Francisco recently proposed establishing a bike lane on a busy road that runs through the Russian Hill neighborhood. When residents and merchants of Russian Hill heard that the lane could mean the removal of dozens of parking spaces, about 400 turned out at a meeting with representatives of the San Francisco Metropolitan Transportation Agency (SFMTA). The agency is now redrawing its plans to soothe area residents.

The pushback was relatively unusual for a city that views cycling as an alternative to the street car and pines to have more residents on bikes than Portland or Seattle. In the past five years, SFMTA reports that bike ridership in San Francisco has jumped by 71 percent. Currently 3.5 percent of all trips in the city occur on bikes. SFMTA hopes to more than double ridership by 2018, and the Board of Supervisors wants bikes to make up 20 percent of all trips by 2020.

To get there, the city is adding cycling lanes at a brisk pace – it now has 72 miles of them – and tilting traffic flow more toward riders: Some intersections have lights just for cyclists, while others feature lights synchronized for 13 miles per hour, closer to a bike's cruising speed. Yet the city admits that carving out enough space for riders is an uphill battle – literally.

"The hills are part of the problem, and the narrow streets," says Edward Reiskin, the SFMTA's director of transportation. "We are fighting down to the inch."

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On a typical end-of-day commute, cars along Boston's Massachusetts Avenue are as tightly packed as cobblestones. This is to say nothing of the mass transit buses and cyclists vying for their piece of roadway. Even though Boston has put in a bike lane along part of the busy thoroughfare, the coexistence among various travelers can be uncomfortably close – and contentious.

As one cyclist cruises down the roadway, a Jeep wanting to turn right on Marlborough Street creeps into the bike lane and cuts off the cyclist. The cyclist abruptly hits his brakes and signals his disgust, verbally and otherwise. The driver seems oblivious to the whole episode.

While no one was hurt, the incident is reminiscent of thousands of tense encounters that go on every day on the streets of American cities. As biking spreads as a form of transportation, the main issue is whether everyone can survive amicably on the same roadways. "No question, safety is the biggest issue facing biking," says LaHood, who has held two federal summits on bike safety.

Cyclists recite a litany of complaints: about encroaching cars, about pedestrians who jaywalk in front of them, about motorists who open their doors without looking to see if someone is riding by. Riders, though, have their lapses, too. Many run red lights or fail to signal when they are turning. Others treat roadways like their own private bike preserves.

The result of all this is a grim fact: Bicycle fatalities have not been dropping as quickly as automobile deaths. Between 2003 and 2011, cycling fatalities as a percentage of all traffic deaths were up by almost 24 percent, according to data compiled by the League of American Bicyclists, a Washington advocacy group. One reason for this may simply be that so many more cyclists are now on city streets.

Yet there is some good news in the statistics as well. The actual number of fatalities, 677 in 2011, was down from a peak of 786 in 2005. Darren Flusche, policy director at the league, speculates that this could be because vehicle drivers are getting more accustomed to having cyclists on the road with them.

Cities are doing what they can to improve bike safety. In Long Beach, road crews put in a roundabout at a dangerous and heavily used nexus. As a result, the city documented a 9 m.p.h. reduction in car speed, a doubling of children walking, and a tripling in the number of cyclists who now use the intersection.

In other cities, such as New York, some roads with bike lanes have special lights that allow cyclists to go through an intersection before vehicular traffic. Portland has installed what it terms bike boxes, which are geared to prevent collisions between motorists turning right and cyclists going straight. The boxes, which are areas of pavement painted green with a bike symbol inside them, put the cyclist in front of the motorist for better visibility.

Even Boston, famous for its rabbit-warren roadways and random one-way streets, is pushing to be more rider receptive. Between 1999 and 2006, Bicycling magazine rated Boston the worst biking city three times. So in 2007 Mayor Thomas Menino started instituting changes. In addition to the bike lanes on Mass Ave., the city installed a bike-share program and is working to prevent people from opening car doors on those whizzing by on two wheels. This summer, Boston will outfit some 1,825 taxis with a sticker on the inside of the car warning passengers to be aware of riders.

Yet cyclists need to do their part as well if the nation's streets are going to be safe for both SUVs and Schwinns. Some riders routinely cut in front of vehicles or weave through traffic. A few bike messengers are famous for grabbing onto taxis or drafting behind buses – activities that have been glorified in YouTube videos. "The rebel period has to end," says Peter Flax, editor in chief of Bicycling magazine.

If cyclists don't curb their behavior on their own, police might do it for them. In Santa Monica, Calif., a cyclist was recently sentenced to three years' probation and 30 days of community service after pleading guilty to assault with a deadly weapon. His offense: running a red light and striking and injuring a pedestrian in a crosswalk.

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Reckless or not, biking has long held a grip on the American imagination. In the 1880s and '90s, intrigued by the novelty of the new machines, Americans flocked to bikes in such numbers that it resulted in a push for paved roads and led women to wear practical clothing so they could ride as well.

In the 1960s and '70s, Americans donned spandex and jumped on lightweight European road-racing bikes as gasoline prices soared.

"We're really now at the beginning of what we think will be the third boom in biking," says Carolyn Szczepanski of the League of American Bicyclists.

The current wave is being driven by elements of the earlier ones: practicality, transportation, recreation, ecological awareness. But there is an added catalyst, too, especially for cities and institutions – raw economics.

Consider just these statistics: Chicago recently paid more than $600,000 for a single mass transit bus and has a fleet of 1,781 buses. New York is building a new subway line along Second Avenue. Just the first phase of the project is projected to cost $4.45 billion.

By contrast, Long Beach's entire system of bike lanes, bike racks, and special traffic lights that sense riders will cost about $25 million, most of it paid by a federal grant. New York is setting up a rental program, with 6,000 bikes, that will cost a fraction of other mass-transit expenditures.

"Citi Bike isn't just a bike network; it's New York City's first new public transit system in more than 75 years," said Janette Sadik-Khan, New York's commissioner of transportation, at a ribbon-cutting ceremony for the program.

Biking advocates also point to the benefits two-wheeled transportation brings to the planet. In a 2008 report, the Rails-to-Trails Conservancy, a nonprofit advocacy group in Washington, estimated that only a "modest" increase in walking and riding could save the US 3.8 billion gallons of gasoline per year. The annual savings in carbon emissions would be 33 million tons – the equivalent of 10 coal-fired power plants.

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The biggest change in all of this, however, may be how Americans view their own transportation. In the future, instead of hopping in the Land Rover to get a quart of milk, they may jump on a commuter bike. As Eustice, the handbag executive in New York, puts it: "Our children love to ride. And when I get too old to balance, I will just get a big trike, so I don't imagine a bike not being part of my life."

• Contributing to this report were staff writer Daniel B. Wood from Long Beach, Calif., and contributors Elizabeth Armstrong Moore from Portland, Ore.; Carolyn Abate from San Francisco; and Andrew Averill from Boston.