IN A REPORT early this week to the Virginia Department of Highways and Transportation, a consulting firm said a toll road from just inside the Beltway to Dulles Airport is "financially feasible." No doubt it is - if you accept all the assumptions of the consultants. A more basic question is whether it makes sense. We don't think so.

The idea is to build half of this toll road on each side of the Dulles access road. Ten interchanges would be provided, begining at Route 123 where the access road now begins, and ending at Route 28, just east of the airport. Local traffic would be charged a fee, depending upon the distance traveled, while through traffic to the airport (or Wolf Trap Farm Park) would continue to travel free on the existing highway.

The proposal raises two quite different questions. One is the rate at which the part of Fairfax County around Dulles Airport is to grow. The numbers used by the consultants in their study project a population increase of 50 percent and a job increase of 60 percent in the next dozen years if the toll road - or some other road like it - is built. The strain that kind of growth would put on public facilities in that part of the county would be enormous - almost 250,000 people already live there.

The other question deals with the wisdom of building a toll road for local traffic parallel to an existing super highway and a potential Metro route. Since the new road would tie into I-66, it would provide still another funnel for pouring automobiles into Arlington and the District of Columbia. While the rationale for the road is to provide internal access in the Dulles corridor, its effect on the way people will choose to travel around the entire area would be substantial. It is clear that major improvements will have to be made in local roads in the Dulles area, but those improvements should not be designed to aim still more traffic at the center of this metropolitan area.

There is another reason for wondering whether this is really a serious proposal. That is the casual way in which the consultants assumed the federal government would donate most of the right-of-way the road would require. Without that donation, financing the project with such low tolls would be simply impossible. Yet we have seen no indication that the federal government, which has refused to open the Dulles access road to local traffic, has any intention of giving away the land it bought several years ago.

What is most distressing about the project, however, is what it reveals about the thinking of Virginia's state government. It is permissible, it appears, to have Northern Virginians pay for a highway through tolls, but it is wrong to let those same citizens have the taxing power they need to pay for Metro. The state could have put its money to better use by buying a study on how Northern Virginia is to fund Metro - the transportation project it needs most - instead of a study on how four more lanes of concrete can be built in a corridor where four already exist.