BY CLOSING DOWN the improvished parking lot at the Pentagon City Metro stop, the Arlington County Board has turned a useful arrangement into a mess. Several hundred commuters had been parking happily, for nothing, in a dusty field beside the station. Now that the lot has been ruled out of bounds, some of them have apparently stopped using the station - and perhaps the Metro system - entirely. Those who still come are strewing their cars all over nearby fields and roadsides, causing unnecessary disruption and headaches for the police.

It's true that the parking lot was irregular, it had never been planned. But that reflects a basic miscalculation by county and Metro officials, who saw the Pentagon City stop as mainly meant to serve existing and future development nearby. Somehow they failed to anticipate that anyone might want to drive in from elsewhere, park, and pick up the Blue Line there.

Since the traffic did develop, why is the county board being so inhospitable? Two notions seemed to be behind the 3-to-2 rejection of the parking lot. One was that commuters should be encouraged to ride buses to the subway and leave their cars at home. That's fine in theory - but nobody knows where the Pentagon City crowds are coming from and whether adequate bus service is available to them. By discouraging them from using Metrorail for part of their trips to work, the board may just be driving them back to their cars.

The second notion was that parking lots cause neighborhood problems. Even where that is true, the needs and convenience of Metro users should not automatically take second place. Excessive deference to neighborhood reaction caused the RFK Stadium Metro stop to built in the wrong place, much too far from the stadium's parking lots. At Pentagon City, however, the choice ought to be easy because the station isn't really in a neighborhood at all. The closest apartment houses are about two blocks away, across some weedy fields. The only commuter parking problem there is the one that the county created by closing the most logical lot. Instead of putting up more and more "no parking" signs, the county should take the first ones down and invite the Metro patrons to come back.