Far it be from us to bring up the subject of the old Three Sisters Bridge, but as you may have noted the other day, a study on over-the-Potomac traffic predicts much more of it in the next decade. Predictions can go awry, of course. Officials at the Metropolitan Washington Council of Governments, which put out this study, know this all too well from their past predictions of growth in this region. But never mind; the conventional wisdom now is that the Metrorail system will ease the traffic pressures.

And maybe it will. But it will depend on some obvious things - not the least being the convenience and service that Metro is able to offer. For example, routes fron Vienna-Fairfax City and Springfield-Franconia - once planned, now being restudied - are important in this pattern. Other incentives aimed at pooling automobile rides, such as higher parking rates, gasoline price increases and an end to subsidized parking at government buildings, should also have some dampening effect.

Also, many people no longer commute into the city but instead live and work in Northern Virginia. Nevertheless, we hope the river jams don't grow to the point where they encourage a new wave of isolationism in the region or, just as bad, cries for more bridges. The emphasis should be on generating stronger public support for an efficient rail and bus network.