The rerouting of traffic on three of Washington's busiest downtown streets this week is providing motorists and pedestrians with the first glimpse of what the city's major new park and terraced plaza on Pennsylvania Avenue will look like when they are completed at the end of 1980.

The $20 million construction of Pershing Park and Western Plaza will divide the nation's historic presidential parade route into two one-way streets, around the park and plaza, much as commuters found them Monday morning after District highway workers spent a whirlwind weekend reshuffling traffic lanes, signs and signals.

Sections of 14th and 15th streets around Pennsylvania Avenue also were made one-way, changes which will last for about three months while the major street construction work is underway. Highway officials said there were few rush-hour traffic jams caused by the temporary one-way street system, despite the diversion of southbound 14th Street traffic onto already busy 15th Street.

Work on the plaza, park and adjacent streets began in April, but only a small section of E Street was closed. The plaza and park construction are part of $70 million in public improvements being made all along Pennsylvania Avenue from the Capitol to the White House. The improvements are to be completed by the fall of 1980 - in time for the 1981 inaugural parade down Pennsylvania Avenue.

There also will be an additional $500-to- $700 million in private construction generated by the revitalization of Pennsylvania Avenue, with seven major hotel, commercial and office complexes planned or already under construction, under the direction of the Pennsylvania Avenue Development Corporation (PADC).

PADC is spending about $130 million in public funds, much of it for relocation of businesses and for historic preservation of the Willard Hotel and six blocks of old buildings, according to PADC spokeswoman Rita Abraham.

Besides Western Plaza and Pershing Park, two additional parks are being created along Pennsylvania Avenue. One is next to the U.S. District Court building at Fourth Street, and the other is the former Kann's department store site on Indiana Avenue, where trees and shrubs were planted last week. That par, as yet unnamed, is to open early next month.

Western Plaza will be an open, paved area, to preserve the vista down Pennsylvania Avenue, except for some greenery around the statue of Gen. Casimir Pulaski. The statue will mark the eastern end of the plaza.

Pershing Park is planned as an extension of the White House grounds and will be ringed with trees and grassy areas. Beside a memorial to Gen. John J. Pershing - a bust and an 11-foot wall to commemorate his World War I campaigns - the park will include a waterfall and reflecting pool that will serve as an ice rink in winter.

Other improvements along Pennsylvania Avenue will include a double row of trees on the avenue's south side and a triple row on the north side. Plans call for wider sidewalks and a new and brighter lighting system for streets and sidewalks.

The summer traffic changes around 14th Street and Pennsylvania Avenue include making Pennsylvania one-way westbound between 13th and 15th streets NW. All eastbound Pennsylvania Avenue traffic has been diverted to E Street for the two blocks, much as it will be when Western Plaza is completed.

Fourteenth Street is now one-way for about 200 feet, between E Street and Pennsylvania Avenue, where only northbound traffic is permitted. Fifteenth Street is one-way between Constitution and Pennsylvania avenues, where only southbound traffic is allowed.

Some minor traffic changes will be made as construction continues. Thirteenth Street will be closed this weekend to all traffic in the short block between E Street and Pennsylvania Avenue, according to the District Department of Transportation. CAPTION: Map 1, map shows the temporary traffic pattern established this week to allow construction work on Pershing Park and Western Plaza. Maps by Beth Ann Thornburgh for The Washington Post; Map 2, map shows traffic flow after park and plaza are completed.; Picture, The view down Pennsylvania Avenue.By Criag Herndon - The Washington Post; Illustration, Drawing shows projected plans for new Pershing Park and Western Plaza.