THE CAPITAL BELTWAY is clearly too tight for the expanding girth of Greater Washington. The answer is not overwhelmingly attractive, especially to those who will find themselves in its path: another 150 miles of superhighway around the Maryland and Virginia outskirts. It wouldn't quite qualify as "Beltway II: The Sequel," because the experts are not looking at an outer ring but at two arcs that could become an oval. One, the favorite of Maryland officials, would be an eastern route; the other, preferred by their Virginia counterparts, would go west around the capital. Ultimately, both will be necessary -- and there lies the significance of a new agreement between the two states.

A long standstill has already pushed any visible progress well into the 21st century, no matter how swiftly the wheels of bureaucracy may yet roll. But Maryland and Virginia transportation officials have agreed to joint financing of an engineering study aimed at forging "consensus on need and priority" for the two bypass options. That may not seem the greatest leap forward, but if the contemplated legs are ever to move, coordination is critical: either state could effectively block the other's proposal.

Precise routes are nowhere near being mapped. But, roughly, the western arc would cut off I-95 near Quantico in Prince William County, steer toward western Loudoun County, cross the Potomac near White's Ferry, connect with I-270 and I-70 in Maryland and join I-95 again north of Baltimore. The eastern bypass would veer off I-95 near Dumfries in Prince William, cross the Potomac and travel along Rte. 301 in Charles and Prince George's counties, then continue on Rte. 3 into the Baltimore area.

Credit for the new cooperation goes to Maryland Highway Administrator Hal Kassoff and Virginia Transportation Commissioner Ray D. Pethtel, who spent months in negotiations. This month, Govs. Schaefer and Baliles discussed the bypass informally. Some residents of both states have formed a coalition under former Maryland congressman Michael D. Barnes to push for progress. Either they all prevail in the name of sensible growth, or the region will be left to cope with uncoordinated expansion and bumper-to-bumper traffic.