Exciting new ideas are marching down Pennslyania Avenue.This time they may even get somewhere.

With sculpture coming to the rescue where urban design has failed, we may now have a dignified, even inspired western terminus after all. The White House would be represented by an ethereal entrance gate, so that the downtown stretch of Pennsylvania Avenue will indeed become "the grand axis" between the Capitol and "the President's House" L'Enfant intended it

This symbolic solution to a problem that has haunted Pennsylvania Avenue designers for 15 years doesn't require the removal of a single building, let alone the bulldozing of entire city blocks to create a vacuous "national square."

It is based on a simple and direct straightening of the chaotic traffic pattern, where cars now are entangled between a confusion of little traffic islands dotting the avenue between 13th and 15th Streets.

The proposed new alignment was quietly worked out some time ago between the staffs of the Pennsylvania Avenue Corp. and the city's Municipal Planning Office. It routes east-west traffic on two separate arteries. The main artery continues Pennsylvania Avenue traffic alongside the District Building and south of the White House lawn toward the Potomac. A second artery, continuing E Stret, runs along the National Theater and Willard Hotel and turns north into 15th Stret.

The north-south streets divide the area between these arteries into three neat squares: Sherman Park, Pershing Park and what is, for the time being, called "Western Plaza."

Sherman Park, the wooded mound south of the old Treasury Building where Gen. William Tecumseh Sherman on horseback is guarded by four Union Army soldiers on foot, will remain unchanged. But there is talk of building stairs up to the lovely Old Treasury garden terrace along 15th Street, so that we, too, can share the grand view of the grand avenue with the general.

Pershing Park, between 15th and 14th Streets, is being designed by landscape architects M. Paul Friedberg and Jerome Lindsey. It will be a green oasis in the center of the city, with a new monument to Gen. John Joseph Pershing, commander-in-chief of the Allied Expeditionary Force in World War I, shaded by a grove of trees. The monument was designed by Wallace K. Harrison, one of the architects of Rockefeller Center in New York City.

The present design of Pershing Park also calls for a pond, but objections by the Municipal Planning Office may dry it up.

The "Western Plaza" is the dramatic and therefore perhaps most controversial piece de resistance of the scheme. It is being designed by architect Robert Venturi in association with landscape architect George Patton.

The plaza is to raise three marble steps (3 1/2 feet above street level. It would be edged by low-cut shrubbery, but otherwise entirely paved.

On this platform are to be two huge 70 or 80-foot-high tablets or slabs, no more than three feet thick, which will just stand there in a staggered position, one behind the other. On the plaza you see them simply as two not very conspicuous slabs, an abstract sculpture.

From the distance, however, they frame the view up or down Pennsylvania Avenue. They are so placed that, looking east, from the Treasury, they focus the eye on the Capitol.Looking south, they focus the eye on the south portico of the Treasury as it peeks out on 15th Street and hints at the White House behind it.

The simple slabs are thus a kind of trompe Voeil , giving the illusion that the avenue has a clear direction, a beginning and an end. It creates the effect which arctitect Nathaniel A. Owings hoped to create with his enormous National Square, for which the Willard Hotel was to be sacrificed. Venturi's slabs evoke visions of the Arc de Triomphe at the end of Champs Elysees. They can be imagined, as an entrance gate to the White House precinct.

On the pavement of his plaza, Venturi would place a mosiac showing part of the L'Enfant Plan - the portion from the Capitol to the White House. At the eastern end of the plaza, Brigadier General Count Casimir Pulaski, who helped us win the Revolutionary War, would remain undisturbed, surrounded by trees.

Venturi's idea of using sculpture not just as an ornamental adjunct to architecture - the way we plunk a Henry Moore or Calder in front of a building - but to put it to work as an instrument of urban design, is very much in the baroque spirit of the L'Enfant plan. The Rome of Pope Sixtus V, which inspired baroque city planning, uses obelisks and colonnades to exaggerate perspectives and to dramatize the sense of procession and arrival - to guide the choregarphy of the city as it were.

But enough is enough.

And Venturi, who is famous for extolling the virtues of spontaneous, popular architecture, weakens his brilliant concept by overdoing it.

His marble pylons, which should be elegant understatements, are to be dressed up with relief stars and a black marble stripe, reminiscent of a "kandy-colored" hot-rod car.

If the pylons need decoration - and I am not sure that they do - the Great Seal of the United States and the Seal of the President might be carved into them to reinforce the idea that they represent the president's end of the avenue.

In addition to the pylons, Venturi proposes big flagpoles and big flags, which I consider an inappropriate redundancy. It is like having chocolate pudding in addition to butter pecan ice cream for dessert. Venturi says he will hide floodlights in the flagpoles to illuminate the pylons at night.

There is also too much of a good thing on the pizza. On his L'Enfant plan pavement, Venturi proposes to place small marble models of the White House and the Capitol, turning the nation's most cherished buildings into silly toys. There is a place for pop art and kitsch, but this is not it.

I don't think, on the other hand, that this Western Plaza - which should be called L'Enfant Square even at the risk of confusion with L'Enfant Plaza - needs to be solemn and bare, as some city officials seem to want it.

These city officials see the plaza as the front yard of the District Building, a place for the people of this city to rally and make themselves heard. They refer to Venturi's scheme as "a miniature golf course," too busy and undignified.

Flower beds, fountains and benches in Lafayette Park in front of the White House, don't stop people from rallying there to shake their fists at the president, so why should Venturi's marbles stop them from shaking their fists at the mayor? All Venturi's scheme needs is craping off some of the amusement park excesses.

Besides, there is talk of moving the mayor's office and the city council down the avenue to the Municipal Center, so that the glorious Beaux Art District Building could be converted into a Washington Opera House. With the Kennedy Center Opera filled to capacity, there is beginning to be an acute need. The idea was first proposed by Patrick Hayes, the city's foremost impresario. An opera in addition to the National Theater, would give Pennsylvania Avenue the cosmospolitan importance it deserves.

A serious problem with Venturi's elevated plaza is the inaugural parade every four years. Tanks and drum majorettes cannot easily run across this obstacle. But if the plan is otherwise attractive to people, why worry about the parade? Why not, in fact, build the president's reviewing stand on this handy elevation and end the parade at 15th Street where the ceremonial avenue ends?

While all this is studied and debated (the project will be presented to the National Capital Planning Commission next week) the Pennsylvania Avenue Corp. has begun to build an impressive new sidewalk along the National Gallery East Building. Brick pavement with granite curbs, two rows of Willow Oak trees with iron castings around them and attractive new light fixtures will give the avenue a smart new dress.

The zoning board, meanwhile, has approved a new interpretation of the height limitation Congress imposed on Washington buildings in 1910. Buidlings located between the FBI Building and 15th Street may now go as high as 160 feet, with a uniform 50-foot setback. The height limit in the rest of the city will be strictly maintained unless Congress, heaven prevent, changes the 1910 law.

Most of the new sidewalk as well as the new squares on the western end of the avenue are to be completed for the next inauguration.