Fairfax County leaders say they will ensure that a massive development of towers and townhouses planned next to the Vienna Metro station does not choke the area's verdant neighborhoods with cars.

County supervisors, who will vote on the project in the fall, say they could force Pulte Homes to scale back its vision of 2,250 homes, offices and stores if the developer cannot convince them it would coax enough people from their cars and toward trains, buses, car pools or sidewalks.

But the reality of MetroWest, the laboratory for how Washington's largest suburb is rethinking its future, is more complicated. Even as Fairfax embraces density next to Metro over sprawl, officials acknowledge that they cannot entirely predict or even influence the behavior of thousands of people who might live in the cluster of 13 towers off Interstate 66.

Fairfax, like other local governments, is used to pressing developers for new sewer lines, parks, road improvements or land for schools as payback for the traffic, children, police calls and other growth new homes bring.

But until now, the county has not required a developer to guarantee how condo owners or office workers will behave when they leave their houses in the morning. It's a test for a planning process with high hopes to steer new roofs from the county's fringes inward toward neighborhoods with transit to reduce driving.

It would be at least 10 years before MetroWest, with its narrow streets and coffee shops at the train's doorstep, is fully built. Only then will anyone know whether shoehorning up to 6,000 people onto 56 acres turns one of the most developed corners of Fairfax into a rush-hour parking lot or an enviable mini-city of walkers and transit riders.

"There's nothing like it out there," George E. Lovelace, a Vienna Town Council member, said last week after hearing a consultant's vision of how Pulte could reduce vehicle trips the project would generate. Lovelace is one in a highly galvanized community of neighbors watching the planning process.

With Pulte, the county is raising the ante as it struggles with where to put a population that's expected to reach 1.5 million in 20 years.

"It's a very important decision for us that will shape our future," said Board of Supervisors Chairman Gerald E. Connolly (D), who represented the Vienna area for eight years. "Most people feel the county squandered an opportunity to develop at the Metro. If we turn our back on transit-oriented development, it will have enormous implications."

A key piece of the puzzle will emerge this week, when transportation planners release their assessment of whether, as a report from consultant UrbanTrans says, Pulte can reduce enough of the forecasted car trips at MetroWest. The report lays out a menu of carrots and sticks, from expensive parking spaces to subsidies to ride Metro.

The county's goal is to shave by 47 percent the 1,356 rush-hour trips the project would generate if it were a traditional subdivision. For offices, the goal is 25 percent.

"We're confident we can get there," said Jon Lindgren, Pulte's manager for land acquisition. "If we try a set of ideas and some work and some don't, we'll try something new."

That will mean more aggressive enticements to pry people from their cars, even rewards to those who show they are riding Metro a lot. But critics worry that incentives might not do the trick. And they wonder: Can Pulte really be held accountable for controlling how much people drive?

The county could require Pulte to put money in escrow for possible fines against the developer, an idea under negotiation. Once the project is finished, a homeowners association would take over the job of reducing car trips.

Even if the county board insists on fewer homes or offices when it considers Pulte's rezoning application, traffic modeling can only be a guide. The county plans to count car trips when MetroWest is half built and again when it is completed. But there is no interim goal for reducing trips.

Because townhouses are quicker to build than condominium towers, they will be built first, Lindgren said, but at the property's outer edge instead of at the station's doorstep. That means people could be more likely to drive for several years.

The consultant's traffic calculations did not take into account existing congestion on roads around the station.

"There must be a reflection of the real world in which we live," said Will Elliott, a founder of Fairfax Citizens for Responsible Growth, a group pushing for fewer homes in MetroWest. He and others say they fear that it could be years before Pulte attracts the right mix of stores and commercial tenants.

The debate over whether the Vienna area can swallow a project on the scale of MetroWest has lingered over an often-contentious, two-year planning process that reached its first milestone in December, when supervisors changed the land-use plan at the station to allow dense, mixed-use development.

The change did not lay out specifics of how to minimize rush-hour car trips, although it required Pulte to come up with a plan. Pulte, facing skeptical neighbors, agreed earlier this year to pay $88,000 for a study the county would oversee.

Pulte's application will be considered by the Planning Commission, which recommends approval or denial. It then will go to the supervisors for a final decision. The public, as it did before the land-use vote, can weigh in at hearings. Supervisors say they will rely on the projections of transportation experts, who also will make certain assumptions.

"We're seeing people who want a change," said Supervisor Linda Q. Smyth (D-Providence), who represents Vienna. "They want to get out of their cars because they're tired of sitting on the road."

By all accounts, the idea makes sense. The Vienna Metro station, long bypassed for commercial development, is encircled in a moat of parking, fringed by single-family homes and townhouses, many a half-mile away. There is no Starbucks, no dry cleaner, no day-care center, no Dean & DeLuca market. Those services require a car trip. Many homeowners say they are thrilled at the prospect of grabbing a cup of coffee to drink before they board the train.

There are few MetroWests out there to serve as guides.

Arlington, which has encouraged dense building around its 11 Metro stations, is an urban county of 29 square miles. But Fairfax sprawls across 395 miles, with just five stations. Work and home are severed for most people, since not everyone can commute to an office on a train line.

"Just because you're living next to the Metro doesn't mean you can take the Metro," said Deborah Smith, who lives near the Vienna Metro station.

County supervisors acknowledge that they are seeking to fundamentally change the car-dependent mindset of every suburb outside the Capital Beltway. More traffic, however, is inevitable in Fairfax, which is expected to add 450,000 jobs by 2020.

"In the suburbs, you'll never totally pry people's fingers off their steering wheel," said Supervisor T. Dana Kauffman (D-Lee), chairman of the Metro board. "It becomes a question of minimizing the number of cars out there in the peak periods."

Even outside rush hour, MetroWest will generate thousands of car trips a day. That's a given. If it were built 30 years ago, the area could easy absorb more dwellings. Retrofitting a suburban landscape by dropping in a small city is a different story.

"In theory, it will work," said Lovelace, who lives a little more than a mile from the station. "But it relies on people giving up their automobile. . . . The concept that you can put all those people in a situation and they're going to act the way you want them to is not founded in experience and evidence."

Supporters of the project point to the self-selecting nature of would-be buyers and renters at MetroWest: federal government workers with jobs on the train line, retirees filling almost 400 age-restricted condos, single people who commute to the District or Arlington. And relatively few children.

Connolly acknowledges that his constituents are debating "whether we want transit-oriented development at all."

Philosophically, he said, he is in favor of it. But he said his board must make an educated guess about how much extra traffic the Vienna area can handle.

"It's a question of whether the concerns that have been raised can be addressed. We don't know that yet."