Every once in a while, someone tries to cross Route 7 in Tysons Corner on foot.

It isn't easy. At 170 feet curb to curb, the suburban strip is far wider than the Champs-Elysees in Paris, Las Ramblas in Barcelona or Fifth Avenue in New York. Worse, crosswalk and "Walk/Don't Walk" signals, which engineers say would impede traffic, are deliberately scarce.

The few pedestrians willing to cross typically will scan the horizon for a break in the flow of cars, sometimes tentatively dangling a toe over the curb, and finally, engulfed by the rumble and noxious breath of rushing traffic, bolt.

"It's crazy," said John Hampton, 35, a shipping and receiving supervisor, after having half-sprinted across diagonally between the Toys R Us and the McDonald's one morning last week. "You could get killed out there."

Fairfax County business leaders and planners now want to transform Tysons Corner, the vast mall and office hub where people drive to get around, into a traditional downtown where people feel comfortable walking about. Their efforts are considered critical to the success of the $1.5 billion Metrorail extension through Tysons Corner, because most potential passengers must be willing to walk at least as far as the train station.

But creating a traditional city from a place laid out almost exclusively for automobiles has never been done on this scale, planners say, and the challenges of Route 7 alone illustrate the difficulties.

Its scale is more intimidating for pedestrians than roads in traditional cities, but giving pedestrians time to cross -- when each second for walkers takes away precious "green time" for cars -- is bound to frustrate drivers.

When some neighbors requested more pedestrian signals on Route 7, engineers for the Virginia Department of Transportation studied their request but built only one in the one-mile stretch between International Drive and the Dulles Access Road. Many more are needed, pedestrian advocates say.

"They were afraid that pedestrians crossing the road would slow down traffic," said Wade Smith, a board member of the McLean Citizens Association, who has doggedly catalogued missing sidewalk segments in the area. "The biggest reason people say they can't walk around Tysons Corner is that they can't cross the main roads. It's very intimidating out there."

Route 7 is one of the central traffic arteries of Tysons Corner, a place that has most of the raw ingredients of a traditional city -- it's the Washington region's second-largest jobs center -- but lacks a city's physical setup. Buildings are separated by berms, side yards, parking lots and wide roadways that sacrifice pedestrian ease for vehicle convenience.

The vast drive-through operation at the McDonald's on Route 7, for example, is a marvel of auto-oriented convenience: To keep the cars moving at lunchtime, five headset-wearing clerks roam the pavement outside, taking orders and delivering food. For pedestrians, however, the Tysons environment can be stressful and sometimes deadly. A Sterling man was killed crossing Route 7 in April 2004, and a District pedestrian was fatally injured trying to cross Chain Bridge Road near the Tysons Corner malls in January 2004. Fairfax County police did not have statistics on hand for pedestrian accidents in the area.

Since 1994, county plans have envisioned Route 7 becoming an "urban boulevard." The proposed Metrorail line, which would have two stops on Route 7, has given the plan more prominence.

It calls for new buildings to be set closer to one another and to the sidewalks, as in a traditional city.

The county is also studying a raft of developer proposals to permit denser, more urban building. Opponents say this will choke the area with traffic; advocates say Tysons is the most logical place in Northern Virginia's economy to accommodate growth.

"Workers and residents will be able to do everyday errands, or meet a friend for dinner and a movie, without getting into an auto," the county plan for Tysons says. "Out-of-town visitors will be able to take rapid rail from Dulles International or Reagan National Airport to Tysons Corner, stay in a hotel, and attend a convention in a trade center: they should be able to take clients to dinner or relax at the local health club, all without renting a car."

But in the view of many planners -- and anyone who has ever navigated Route 7 on foot -- that evolution won't be easy.

"It's good to make big plans, but the challenge is formidable," said Gerrit Knaap, director of the National Center for Smart Growth at the University of Maryland. "I don't know of any other place in the country that has achieved that kind of metamorphosis in a place of that size."

When it comes to building cities, he said, "it is better to get things right the first time. It's much more difficult to retrofit."

Including its frontage roads, Route 7 is wider than some of the best-known streets in the world's biggest cities, and that alone spells trouble.

"The problem with having it so wide is that it becomes an inhuman fast-paced thing that no one would ever want to walk along," said Allan B. Jacobs, an urban planning professor at the University of California at Berkeley, who has spent much of his career studying streets and is the author of Great Streets.

Narrowing Route 7 might make pedestrians more comfortable, and VDOT planners are studying new configurations to accomplish this, though they are loath to make changes that would create more tie-ups on already jammed roads.

The issue of walkers vs. drivers on Route 7 has come up before.

In response to neighborhood advocates who asked for more pedestrian signals, which cost roughly $25,000 to install, VDOT planners performed a study and they concluded that adding other signals would create unacceptable traffic delays.

Assuming that the average adult walks four feet per second, a pedestrian crossing the central portion of Route 7 -- not including the frontage roads -- blocks the thoroughfare's traffic for about 30 seconds. Tysons Corner traffic backups are already notorious without taking a half-minute of green time away whenever a pedestrian punches a button.

"We studied the effects of putting pedestrian lights at other locations," said Claudia Llana, a VDOT engineer. "The delays would have been higher."

Some of the few who dared to cross the roadway last week had little but scorn for what they interpret as a careless attitude toward pedestrians.

"Yeah, 'People: Who cares?' " said Geoffrey Lindstrom, 54, a garden designer from Arlington who was walking around after dropping off his car for service at a nearby dealer. "I wouldn't be out here on foot unless I had to be."

Fear of impeding traffic has thwarted petitions for more pedestrian crosswalks along Route 7 at Tysons Corner shopping and business district. Run, don't walk: Pedestrians brave the traffic to cross Route 7, which at 170 feet wide makes for a risky dash.