HIGHWAY EXPERTS all around Greater Washington are looking down the road to the 21st century -- where they see, in one form or another, a second Capital Beltway perhaps 10 miles outside the one that's so congested today. Their vision is significant, because without another bypass this region will have serious trouble coping with the considerable development taking place along the outer edges of the metropolitan area. Even if a road plan can win the necessary support and be converted swiftly into a real highway, the experts note that it would take at least another 15 years to complete.

A bypass of this kind would not be merely for the convenience of the region's outer parts and/or through-traffic north and south. Unless there is some alternate route for all this traffic, areas inside and around the existing beltway will be a mess. Already there are regular heavy-traffic tie-ups on the beltway as well as other roads in Northern Virginia and suburban Maryland. About one-third of the 120,000 vehicles now using the Cabin John Bridge each day are going to places beyond the local area; T. Eugene Smith of McLean, a member of the Virginia Highway Commission, says one purpose of a bypass would be to separate this traffic from the strictly local flow.

In Virginia, state highway commissioner Harold C. King is urging Congress to consider a bypass that would dissuade motorists traveling on I-95, I-70 and I-270 from getting on the existing Capital Beltway. While various routes are being studied, the bypass generally would extend northwest from I-95 near Dale City to an area near Dulles International Airport, then cross the Potomac near the Loudoun-Fairfax county line and connect to I-70 near Mount Airy, Md.

Maryland state highway administrator Hal Kassoff says his agency is skeptical of this plan but is looking at ways to build some kind of 15-mile bypass and bridge over the Potomac from Dumfries in Prince William County to Indian Head in Charles County.

The question, then, isn't whether Greater Washington needs new bypass routes, but whether Maryland, Virginia and the local governments can agree on a coordinated, sensible approach that will serve their competing interests in economic development. And the sooner they begin trying, the better.