A two-year study of Constitution and Independence avenues is expected to recommend major improvements and several traffic changes for the wide ceremonial streets that flank Washington's Mall.

Extending Constitution to the Potomac River near the Lincoln Memorial -- the way it was 20 years ago -- and eliminating "the spaghetti of ramps" built there for the now dead Inner Loop Freeway, will be the major proposals, according to a spokesman for the landscape architectural firm of EDAW.

The $84,000 study which is being prepared for the National Capital Planning Commission, also will recommend eliminating at least one of the eight lanes on Independence Avenue and enlarging the avenue's sidewalks. The study also will propose closing sections of both Pennsylvania and Maryland avenues at the foot of the Capitol which are clogged wit tour buses and street vendors. EDAW proposes that buses be banned from the area as ugly intrusions on the Mall, while recommending that vendors remain if their trucks are replaced by attractive stands.

Preliminary plans for both avenues, to be presented to NCPC this spring, call for planting large numbers of sidewalk trees, similar to what is now being done on Pennsylvania Avenue by the Pennsylvania Avenue Development Corp. Although Constitution and Independence mark the edge of the histroic Mall, the avenues are now largely barren because about 50 percent of the trees have died, EDAW planner Joseph Brown told the federal planning agency earlier this month.

The plan to beautify the avenues and to reopen Constitution to the river, long favored by NCPC, would end three decades of highway planning turmoil near the Lincoln Memorial and restore the area as much as possible to the design of the turn-of-the-century MacMillan Plan for downtown Washington. Constitution Avenue, formerly known as Canal or B Street, was constructed on top of the canal that ran past the foot of the Capitol to Southwest Washington until the Civil War. The lockkeeper's house still sits at 17th and Constitution.

Although any changes will have to be approved by federal and city agencies, EDAW is proposing to eliminate at least three ramps and one bridge, built as part of the proposed Inner Loop Freeway, E Street Expressway and I-66 (Roosevelt Bridge) highway projects. Motorists would be able to follow the same traffice routes they drive now but the sweeping freeway-style ramps off Roosevelt Bridge would be replaced by small parkway-style ramps and the dangerous S shaped curve on Rock Creek Parkway -- just north of the Lincoln Memorial -- would be eliminated.

While two new traffic lights might be needed on the extented section of Constitution, EDAW's Brown says traffic would not be greatly affected by the changes. "Look waht you have there now," he said, a dangerous S-curve for the heavy morning and evening rush-hour traffic along Rock Creek Parkway.

EDAW also proposes the straightening of the intersection of Constitution and Pennsylvania avenues, near the foot of the Capitol, as well as closing the short sections of Pennsylvania and Maryland avenues by the Capitol Reflecting Pool. The one-block sections are now used primarily for tour bus parking and vendor trucks.