A unique decision by the Arlington County Board proves that enlightened public policy can be effective in shaping development of commercial areas in general and the composition of the retail marketplace in particular.

Concerned over the rapid decline in the number of supermarkets in Arlington's Metro corridor, the board last week approved plans for a high-rise office building in Rosslyn, with the proviso that the structure contain a supermarket on a lower level. The board has agreed to increase the density limit for the project in return for assurances from the developer that a supermarket will be housed in the building.

It is a decision that merits the attention of elected officials in other urban communities where redevelopment, among other factors, has had an adverse impact on many traditional and essential services.

"I think what you're seeing is Arlington County's early-warning signal to what we see as a potential long-term problem," an official suggested in discussing the need for more supermarkets in the county.

Compounding the problem are increases in residences and employment in Metro and development corridors that are already "essentially underserved" by supermarkets, he noted.

Although the Arlington County board's initial approach to the problem may be unique, the problem itself isn't. The number of supermarkets operated by major food chains in the District has declined from almost 100 in the late '60s to only 37. The problem, in fact, is almost universal in urban areas where land is either too expensive or inadequate for development of supermarkets.

The food chain industry concluded some time ago that operating larger supermarkets is more cost-effective in a business where the profit margin is just over 1 percent. But the industry maintains that replacing smaller stores in urban areas presents a problem because of difficulty in assembling land parcels large enough to accommodate the typical suburban supermarket of 40,000 to 55,000 square feet.

Typically, the loss of supermarkets is seen as a big-city problem. But as one Arlington County official points out, suburban jurisdictions that are either fully developed as office centers or are undergoing extensive commercial redevelopment also lack the kinds of sites required by supermarket chains.

D.C. and Arlington officials have made retaining existing supermarkets or persuading major chains to build new stores in their respective jurisdictions a priority. The District commissioned a study by the University of the District of Columbia more than two years ago to help it deal with the problem. Ironically, Arlington County has come closer to following one of the more innovative recommendations in that study by intervening in the development process.

The study concluded that public policy should be "geared to public-sector intervention in the market mechanism to create adequate land for food service stores."

That is precisely what the Arlington County Board did in insisting, as a condition of its approval, that the developer include provisions for a supermarket and parking in his building.

The concept has unlimited possibilities in the District. It may not be practical to install a supermarket in some office buildings in downtown Washington, but the concept certainly has merit if applied properly. After all, Safeway Stores Inc. proved several years ago that it can be done when it opened a Safeway International store in an office building in the heart of downtown.

There are other choices, however. The UDC study suggests, for example, that supermarkets can be included in large apartment complexes where it is possible to have ground-level visibility and ample parking.

What is most instructive in the Arlington County example, however, is the use of public policy to intervene in the marketplace to assure retention of essential services. It is that kind of policy that the District has been reluctant to adopt in guiding the redevelopment of downtown Washington, where small businesses are "hanging by their fingernails," as the co-chairman of the Downtown Retail Merchants Association described the situation to the D.C. City Council last week.

Small businesses that wanted to remain in the downtown area have been removed by city land policies, Joseph E. Threatt of the merchants association charged in the letter to council members. Implicit in his criticism is the failure of the D.C. government to use the leverage of public-sector intervention to retain small businesses as part of the mix in downtown Washington.

Arlington County's action is more than a signal. It is a beacon to those who haven't found a way to deal with one of the critical problems of urban development.