With the Fourth of July looming, Vikki McGraw and Terry Barbot spent much of yesterday engaged in one of the most critical elements of planning for the big event: the placement of portable toilets.

You can't stick a portable toilet just anywhere, explained McGraw, who works for the National Park Service. It must be done in an "environmentally sensitive" manner, so that it does not disturb tender shrubs or seeded grass. Most important, "You don't want it in the blazing sun. They bake."

These things matter when you're expecting 450,000 guests. As the United States prepares to celebrate its 211th birthday, a date that coincides with the bicentennial of the Constitution, nowhere are preparations more frenzied -- or more meticulous -- than in the place the youthful nation made its capital.

After previous celebrations marred by nightmarish traffic jams and an overwhelmed subway system, city and Park Service officials are crossing their fingers and vowing that, although there will undoubtedly be delays, this year's event will be different.

"I would like to predict that this will be a safe and happy Fourth for the people of our region," ventured Beverly Silverberg, a spokeswoman for Metro.

In tangible terms, that means additional bus service and trains to complement the previously established practice of extending hours and simplifying fares at Metro stations.

It means 280 portable toilets on the grounds around the Washington Monument and other memorials, 60 more than last year.

In the realm of entertainment, the Park Service promises a dramatic, 35-minute fireworks display, with twice the number of shells as last year, according to a spokeswoman.

Other events on the Mall include three concerts, among them a performance by the National Symphony Orchestra on the west lawn of the Capitol.

A "Welcome Back" concert for Vietnam veterans is scheduled Saturday afternoon at the Capital Centre and will feature such performers as Linda Ronstadt, John Fogerty and Crosby, Stills & Nash.

Then there are the numerous sideshows, official and otherwise, that make up the pageant of Independence Day in Washington. Religious groups have already laid claim to several areas in the Mall, and the Youth International Party, otherwise known as the Yippies, will make their annual appearance near the Lincoln Memorial.

Government offices, banks and most businesses will be closed Friday.

The American Automobile Association advises those heading out of town for a long weekend at the beach to get an early start. "Ocean City, Rehoboth, every motel at the beach is booked solid," said Mary Anne Reynolds, a spokeswoman for the Potomac chapter of AAA.

While the holiday is typically a slow time for Washington hotels, an industry spokesman said yesterday that many of the city's hotel rooms are filled because of a convention of the International Kiwanis Club.

Thus far, the National Weather Service is offering a generally rosy forecast. While predicting a chance of thunderstorms or showers Friday and Saturday, the service is calling for hot and sunny weather with temperatures Saturday in the 80s.

The Park Service began planning this year's event soon after the debris was swept up from last year's celebration, said McGraw. For days now, park rangers and maintenance employes have been erecting snow fences for crowd control and trash receptacles.

McGraw, 36, spent much of yesterday morning supervising the installation of two construction site trailers that will serve as command posts during the celebration, then left with ranger Barbot, 33, to scout the best sites for portable toilets.

First stop was the Lincoln Memorial. "We've said we wanted to put more at the Lincoln, so let's do it," said McGraw. "Let's try 10 right here." She gestured toward a patch of pavement adjacent to Lincoln Circle.

Six of the toilets are for handicapped people, and the women tried to distribute them evenly up and down the Mall. Barbot marked the placement on a map that she planned to show the rental agency when the toilets are delivered.

After more than an hour of driving back and forth, they found places for all 280, which were rented at a cost of $19,000. "We were kind of feeling our way around last year," said McGraw. "This year, I think we'll hit it right on the nose."

Metro has been similarly occupied with the Fourth as officials seek to avoid past mistakes. Silverberg, the Metro spokeswoman, described the holiday as "our report card," the day the system functions at capacity and many out-of-town visitors ride it for the first time.

Metrorail service will begin at 8 a.m. Saturday and will continue to 3 a.m. Sunday, with plans for additional service if necessary. A flat fare of 75 cents will be collected in barrels to speed passage through gates. In addition to rail service, buses will run from several downtown staging areas near the Mall.

Silverberg cautioned against raised expectations, noting that the rail system cannot possibly absorb everyone at once in the period immediately after the fireworks.

"People need to have a little patience," she said. "If the platforms are full, we will not let people into the stations because there's no place to go." She advised people to leave large coolers at home.

In planning the fireworks display, the Park Service narrowly averted a snag that could have proved disastrous, a shortage of adequate liability insurance. Finally, however, the service accepted an arrangement where the government takes some of the risk, thus resolving the problem.

With that out of the way, Park Service officials are focusing on more mundane details, such as toilets and trash receptacles. But McGraw was not about to let her job dampen her enthusiasm for the holiday. "I don't know, I like the Fourth," she said. "I guess I'm sort of corny at heart."