David Alpert is editor of the blogs Greater Greater Washington and Greater Greater Education.

If a shrinking number of people want to fly in and out of an airport, is the solution to spend a billion dollars to build a road there? Or is the better approach to build infrastructure where people do want to go?

The former doesn’t seem to make a lot of sense, but that’s exactly what we’re hearing from the Metropolitan Washington Airports Authority (MWAA) and Virginia officials about the proposed Bi-County Parkway.

The Post explored the question in the July 14 Metro article “Could a Pr. William-Loudoun road revive Dulles?” The basic issue is that people seem more eager to fly in and out of Reagan National Airport than Dulles. Congress recently added exemptions to Reagan’s perimeter rule that has allowed for airlines to add more long-distance flights, helping to spur a 5 percent increase in passenger traffic last year. Meanwhile, Dulles saw 2 percent growth in international traffic, but domestic traffic dropped 8 percent.

Let’s say you run a business and have two products. A lot of people want to buy one product, but your manufacturing capacity is limited, so you can’t make all of the goods that you know would sell. The other product has flat or sluggish sales despite adequate inventory. You might do some marketing or try to tweak the less-popular product, but the best way to grow your business is clearly to find some more suppliers and make more of the popular product.

MWAA, which operates both Reagan and Dulles, instead seems to want to find ways to get people to use Dulles more.

Since more passengers aren’t willing to fly there, MWAA officials talk about attracting more air cargo and building offices and hotels on airport property. Local leaders propose building big roads so that trucks can drive to warehouses.

They are looking at the issue completely backward.

Virginia and airport officials seem to behave as though their mission is to make more stuff happen at Dulles, whether that stuff wants to happen there or not. We need to look at it the other way: If people want to use Dulles, great. But if Dulles lacks demand, our region needs to invest its limited transportation and infrastructure funds where that demand exists.

Northern Virginia’s strength isn’t warehouses. Its strength, and that of the District and Maryland, is knowledge jobs in high tech, biotech and defense. Rather than a $1.5 billion parkway at the edge of the region where demand isn’t strong, let’s put transportation dollars toward growth that will attract knowledge workers and jobs that are the engine of Virginia’s success. That would help workers from all across the region get to those jobs, not congest our roads with trucks driving refrigerated materials to Dulles to fly them somewhere else.

The debate about Dulles and the Bi-County Parkway is actually a debate about whether Northern Virginia should spread out or grow more in the center. There are groups of people with strong interests in each. Property owners of large rural tracts want to be able to fill them with houses. Is that in the best interest of the region?

The market has spoken clearly. People want to live in walkable communities, whether a smaller suburban town with a concentrated center near a Metro line, or a booming neighborhood in the center city. They want grocery stores and restaurants and hardware stores and bars and gyms and playgrounds and schools and churches in close proximity. People haven’t stopped buying traditional suburban houses, but the unmet market demand for walkable communities is far higher. We simply don’t have enough of these.

When we spend money on transportation, that sends a signal to developers about where to build new housing and to residents about where to live. We have two choices: We can grow communities in existing or emerging walkable places, such as Tysons Corner, Old Town Alexandria, Silver Spring and Columbia Heights, or we can grow outward with another wave of houses in places that create even longer commutes than residents currently face.

Efforts to force more economic activity at Dulles will push growth outward to the detriment of nearly everyone. Opportunities exist in Fairfax County, Arlington, Alexandria, Prince William and the rest of the region. Dulles is a great asset to Washington. Let’s help it stay strong and fill the demand that exists, not contort ourselves to push for extra activity where there is not demand.

David Alpert is editor of the blogs Greater Greater Washington and Greater Greater Education.