Commuter Behavior Key to MetroWest

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.