SIRENS WAILED. RED LIGHTS FLASHED. Police chased some alleged bad guys, and traffic on the Pasadena Freeway came to a dead stop. Typical Los Angeles. What happened next wasn't.

With all those cars going nowhere, drivers turned off their engines and got out to stretch. The members of a mariachi band started strumming and singing. Ice-cream vendors pushed their jingling carts through a hole in a chain-link fence. Then passing bicyclists rolled their vehicles of choice onto the freeway turned parking lot to join the spontaneous celebration, reclaiming a route their kind had once ruled.

As it happens, this 2004 event was the second time in as many years that bikes had taken over this stretch of freeway. I had helped orchestrate the first.

IN 1900, SOUTHERN CALIFORNIANS CREATED a futuristic traffic structure catering to the mechanical marvel of the day--the bicycle. It opened along a corridor known as the Arroyo Seco, named for the seasonal stream that flows from the San Gabriel Mountains and enters the Los Angeles River just north of downtown Los Angeles.

It was part of a grand plan to connect Los Angeles to Pasadena through an eight-mile "great transit artery." A Pasadena mayor, Horace Dobbins, provided the start-up funds to create an elevated, multilane, wooden "cycleway," complete with streetlights and gazebo turnouts.

When the first leg opened, swarms of bicyclists handed over the 15-cent toll. A Los Angeles Times commentator gushed that the countryside it passed through "is the loveliest in Southern California, the route having been chosen with an eye to scenic beauty as well as to practical needs."

The Los Angeles region, with its mild Mediterranean climate and relatively flat terrain, was in fact considered an ideal home for the bicycle, with more than 20 percent of the population biking for pleasure or to work when the cycleway was proposed.

"There is no part of the world where cycling is in greater favor than in Southern California, and nowhere on the American continent are conditions so favorable the year round for wheeling," one 1897 newspaper article commented. The bicycle use complemented the city's streetcars.

Soon the automobile gained popularity, however, and the elegant bicycling structure was eventually dismantled. Early discussion of car routes, meanwhile, highlighted the concept of a "parkway" as part of a mixed-transit system, built along scenic corridors with adjacent parkland.

Designers incorporated some of these features into the Arroyo Seco Parkway--the first freeway of the West, as it came to be called. It roughly followed the route of the old bikeway.

By the 1940s, Los Angeles, like other regions, had begun to reorient its transportation planning to exclusively favor the car, and the parkway officially became the Pasadena Freeway in 1954. With the passage of the Federal-Aid Highway Act of 1956 and its dedicated Highway Trust Fund, the car and the utilitarian freeway triumphed.

"America lives on wheels," Treasury Secretary George Humphrey proclaimed in 1955, "and we have to provide the highways to keep America living on wheels and keep the kind and form of life we want."

For the next several decades, transportation policy in Los Angeles and nationwide focused almost exclusively on where and how to build and expand the freeway system. Highway construction molded and shaped the land-use patterns, commercial and industrial activities, and spatial identities of cities and the countryside.

By the 1980s and '90s, however, the economic, political, legal, and environmental costs of such massive construction projects were causing officials to doubt their continued viability. Planners shifted their focus from system expansion to system management, as hours-long commutes began to stir public outrage.

Nowhere was this more painfully conspicuous than on the once scenic Arroyo Seco Parkway. This hybrid--part parkway, part modern freeway--had become the symbol of dysfunctional motoring. Cars routinely overshot hairpin exits and entrances designed to be approached at five miles per hour. Its curves, pleasant at 40 mph, often sent vehicles traveling at freeway speeds careening into the cement-lined Arroyo Seco, and a light rain invariably caused an unsightly ballet of pirouetting SUVs.

Community and environmental groups had for years mobilized around the freeway's problems. As a professor at nearby Occidental College and the director of its Urban and Environmental Policy Institute, I began strategizing with these organizations and other academic institutions. Particularly appealing was a subversive idea: Why not reclaim the freeway from automobiles, if only for a morning?

It took years of discussion to articulate the full-blown plan for ArroyoFest, an event we hoped would, among other goals, help Angelenos imagine bikes once again playing an important role in moving people around the city.

The project unfolded like a community-organizing thriller. The first question at any meeting: "Do you really think Caltrans is going to allow this to happen?" It seemed unlikely. Yet one by one, an array of organizations overcame the obstacles: securing liability insurance, finding ways to divert freeway traffic, and obtaining permits from various jurisdictions through which the route passed.

At a meeting just days before the event, Caltrans staffers announced that they had issued a permit to close the freeway. This astonished other agencies including the California Highway Patrol, which had assumed the state's transportation authority would simply say no. Momentum now shifted improbably but inexorably to yes.

A HEAVY FOG SETTLED OVER the Arroyo corridor in the early hours of June 15, 2003, muffling the voices of more than 3,000 cyclists who came pedaling in on mountain bikes, racing bikes, tandems, trikes, unicycles, and recumbents to line up at the beginning of the freeway.

It was Father's Day, and a familial mood settled over the multigenerational, multiethnic crowd. Local schools had produced almost 100 murals and draped them from fences and overpasses. Community groups set up dozens of booths and passed out literature under the sycamores in a park along the route. It was a festival to celebrate a freeway taken back from the car, and with the sounding of a horn at 7:30 A.M., whooping bikers and pedestrians streamed onto forbidden turf.

"I could feel the cool air coming out of the tree-covered parks," one participant said. "I always knew the parkway was built to be beautiful, but seeing it at the appropriate speed clarified my vision."

Today, just a few years after the takeover of the Pasadena Freeway, a diverse bicycle movement is flourishing in L.A. It includes neighborhood and ethnic-based cycling clubs, policy advocates, ride-to-work and bike-along-the-river events, and several gatherings at which hundreds of riders take to the streets each month. Many of the groups are less than a year or two old.

The Bike Oven, for example, started as a free repair and do-it-yourself bicycle maintenance shop, but the garage where it operates has now become a social space and meeting center where neighborhood rides are launched, monthly art shows are held, and "bike-in" movies are screened. And while policymakers still largely ignore the bike's potential as one alternative to the car, L.A. cyclists have begun to coalesce into a force that promises to become more formidable in the months and years to come, as the congestion, pollution, and cost of driving become the movement's most effective recruiting tool.

On the morning of ArroyoFest, however, reclaiming a major route from automobiles seemed like an impossibility overcome. Sure, Los Angeles had shut off streets to cars for marathons and bikeathons. But this was a freeway, the internal combustion engine's sacrosanct realm. Now riders chatted and flirted. Others pedaled hard, reporting that bicycling the 8.5-mile stretch of open freeway took far less time than when they commuted along the same route by car.

Gone with all those engines was the freeway's roar. Riders and spectators said they relished the relative silence. One nearby resident noted how disorienting and exhilarating it was to "open my window in the morning and hear birds and the wind and breathe the air in a way I had never experienced before."

Some say the event was like turning back time. I prefer to think of it as a glimpse of the future, an opportunity to be seized.