My legs start to burn as I hit a hill, but the soft yellows of dawn are filtering through the city streets, and a cool breeze is brushing my cheeks. My mind is clear. At the end of my five-mile bike commute, my blood is flowing, I'm focused, and I'm excited for work. This is how I start my day: fully alive. How many Americans can say the same about their commute? I have a hunch: all the cyclists.

Until recently, bike commuting in the United States was something of a cult, mysterious to outsiders who assume it's only for the young, the superfit, and the spandex-clad. But that's rapidly changing, as urban planners are designing streets where bikes are as welcome as cars, and employers are offering bike facilities as a way to attract creative professionals. "Quality of life today is the most important tool of economic development," says Gil Penalosa, the former parks commissioner of bike-friendly Bogota, Colombia. "People save all year to go on vacation to places that are walkable and bikable. Why not live in a place where it's walkable and bikable?"

If you already bike to work, consider yourself part of the vanguard that's pushing the country toward a postcarbon future of high-density, vibrant communities. If you haven't biked since childhood, it's not too late to rekindle your passion for that most efficient of human inventions.

In Copenhagen, home to Danish urban planner Jan Gehl, 37 percent of all commuting is done by bike--partly thanks to Gehl's interest in "human scale" urban design. "You have to start with people," he says. "You can't add the people after you have made the cars happy. A city with a lot of bicyclists is a city with a lot of life. A city with a lot of cars is a city with a lot of metal and speed."

In 2008, according to the U.S. Census, 720,000 Americans commuted to work by bike--43 percent more than in 2000. It would be nice to say that the growth was driven by a concern for the climate, but the main reason is economics. "People bike because it's fast, cheap, and easy to get around," says Penalosa, "not because it's good for the environment." Christopher Leinberger, a land-use strategist at the Brookings Institution, notes that people who are auto-dependent spend 25 percent of their income on transportation, compared with 9 percent for those who walk, bike, or take public transit.

Incentives like that change the way our cities look and work. As Leinberger says, "transportation infrastructure drives development." Builders and real estate developers take their cue from the roads, highways, and public transportation options available in an area. Since the 1950s, U.S. transportation policy has focused almost exclusively on the automobile. Sprawling suburbs were sold as freedom but ended up trapping their residents in traffic jams. "We should build cities that we want to live in," insists Penalosa, now executive director of the Canadian nonprofit 8-80 Cities. "Do we want to be stuck in cars?"

Building a new bicycle culture requires extensive infrastructure, he says. "And I don't just mean painted lines. Bicycle lanes are not enough--you need physically separated bikeways." In this country, we call them "cycle tracks"--lanes physically divided from vehicular traffic by islands. Although more expensive to install, they provide an extremely high level of safety for cyclists, both real and perceived. In October 2007, New York City unveiled the country's first cycle track on Ninth Avenue in Manhattan, something the advocacy group Transportation Alternatives had been pushing since 1993.

The city has also added 200 miles of standard bike lanes--even more than in bicycle-friendly Copenhagen. Cyclists have responded. "There has been a huge surge in the number of cyclists on our streets," says Wiley Norvell, communication director of Transportation Alternatives. "You find yourself waiting at a light with four or five cyclists behind you. And that makes for safety in numbers, because everyone else on the street is starting to anticipate cyclists being on the road."

Cities like Seattle take a cheaper approach, separating cyclists and cars with a wide painted lane, as opposed to a mere line in the road. "Safety is important, but so is perceived safety," says John Pucher, a professor of urban planning and public policy at Rutgers University.

Many other cities--including Syracuse, New York; St. Paul, Minnesota; and Salt Lake City--are taking another tack, rechristening streets through residential neighborhoods as "bike boulevards." Car traffic is still allowed but is "calmed" with reduced speed limits, engineering fixes like planters and sidewalks that bulge at intersections, and signage that makes it clear to motorists to expect lots of bikes. ("Traffic calming may be just as important as bike lanes," Pucher says.) Bike boulevards adjacent to major traffic arteries provide crucial direct links between destinations. While bike paths sometimes meander, bicycle commuters need routes that connect them to workplaces, shops, and schools.

Another easy fix for cyclists seeking safety is adding "bike boxes." These large, green-painted zones extend the bike lane into the vehicle lane at intersections, alerting drivers to the presence of bikes and allowing cyclists to make safer left-hand turns. Simple changes like these can have an enormous effect, given that the main factor suppressing the number of regular cyclists in the United States is fear. This is especially true of women, who are generally more risk averse than men. Pucher has found, for instance, that three times as many men as women use bikes for transportation in the United States, while in Europe, where infrastructure is better, the numbers are equal. Penalosa evaluates infrastructure on the "8-80 Rule": "If you would not send an 8-year-old along with an 80-year-old on a walk or a bike ride on that infrastructure, then it is not safe enough."

My Initiation into the world of bike commuting came after college. Five days a week, I enthusiastically pedaled 17 miles each way between my home in Boulder, Colorado, and my office in Longmont, on a highway that cut through golden prairie to the foot of the Rocky Mountains. Given the uncertainties of Front Range weather, a sunny morning could give way to snowflakes by noon. But I had a secret weapon: the bus. Since the Boulder buses had bike racks, I could always bring my bike home with me. Bike racks are crucial to a green transportation system because they enable people who live or work far from public transportation to build cycling into a multi-vehicle commute. They're good for bus systems too, upping the number of potential passengers.

Trains and light rail are also inviting bikes on board, although often, as on Chicago's Metra and the San Francisco Bay Area's BART, only folding bikes are allowed during rush hour. Pucher's research shows that U.S. cyclists prefer to bring their bike on the train with them, but the availability of secure bike parking could encourage them to ride to the train station and lock up their steed there. Enter Bikestation, a nonprofit that has created cycling hubs in seven cities. In Washington, D.C., for example, Bikestation provides 130 parking spaces and changing rooms for cyclists at Union Station in a modern glass-and-steel structure with secure, automated access--all for an annual membership fee of $20.

Of course, bicycle commuting isn't for everyone. And most people--cyclists included--need buses and trains and, yes, cars to get around at least some of the time. But cyclists can be a catalyst in the green-transportation revolution; they fight passionately for safer infrastructure because their lives depend on it. ("And by the way," Gehl adds, "if you bicycle to work every day you can live seven years longer.")

Here is a future we can choose to make: Towns and cities where the streets are full of cyclists pedaling to work and the sidewalks vibrant with pedestrians walking to cafes, movie theaters, and farmers' markets. Healthier communities connected by rail lines and bus routes. A low-carbon transportation system that helps us avert a climate catastrophe.

And there you are, in the middle of it all, pedaling, your legs burning a little, sure, but a smile creeping across your face.