The first workday after the closing of Pennsylvania Avenue in front of the White House began more easily than commuters expected yesterday, but the evening rush hour pointed up the difficulty of rerouting downtown traffic across the city.

After a relatively smooth morning rush hour, during which thousands of people arrived late and some apparently took the day off, the afternoon was a different story. H Street, the new east-west route across town, was jammed between 14th and 17th streets NW even as police officers tried to move traffic at key intersections around the White House.

An illegally parked delivery truck and a broken-down Airport Connection bus without emergency flashers robbed westbound drivers of one lane on H Street about 6 p.m. Metrobuses fell behind schedule in the afternoon after a morning with few complaints. Metro spokeswoman Cheryl Johnson said traffic light patterns need to be adjusted.

According to D.C. Public Works Director Larry King, some lights had been adjusted to give drivers more green time. City officials will continue to monitor traffic in the next few days to determine whether more steps are needed, he said.

One plan certain to get more attention after last night's rush hour is a proposal to make H Street one way eastbound while changing I Street to one way westbound. That would give drivers in both directions five lanes.

Robert L. Morris, a Bethesda traffic consultant who studied the closing of Pennsylvania Avenue for the Secret Service, said yesterday that it was a mistake for the city not to make H and I streets one way immediately. It will be harder to make the change after traffic has adjusted to the new pattern, he said.

"I must say I was surprised they didn't do it right off the bat," Morris said.

In the morning, some drivers said their trip to work was faster than before President Clinton ordered Pennsylvania closed between 15th and 17th streets. State Place and a part of South Executive Avenue behind the White House also were closed to vehicles to protect against a car or truck bombing like the one that killed 167 people at a federal building in Oklahoma City last month.

"This morning was weird, because you just didn't have the traffic you usually do down there," King said. Today could be a truer test, and King advised drivers to "try to avoid the area unless you really need to be there."

The Barry administration yesterday began openly questioning the legality and the suddenness of the action. D.C. law requires the D.C. Council and Congress to approve street closings; neither had a chance to consider the closing of Pennsylvania Avenue.

A U.S. Treasury Department spokesman, meanwhile, said the federal government would reimburse the city for police time and the cost of changing signs, street markings and traffic signals.

Lighter morning traffic than usual, the presence of police directing traffic at key intersections and the federal government's decision to allow workers in the District to arrive an hour late were among the reasons officials gave for the relatively trouble-free early rush hour.

"This morning showed that everyone gave great thought to how they were going to get to work," said Janice LaChance, a spokeswoman for the federal Office of Personnel Management. She said federal employees are expected to show up for work at the regular time today.

Metrobuses were on time, for the most part. Coffee shops sold fewer muffins and poured less rush-hour because business was down. Parking garages parked fewer cars.

Stuart MacIntosh had time to read the paper and sip a Diet Coke on a bench at 14th Street and Pennsylvania Avenue because he caught a Metrobus in Adams-Morgan a half-hour earlier than usual. Suzanne Thrift, of Annapolis, saved a few bucks because she qualified for an "early bird" parking discount by arriving earlier than she thought she would.

Bearded panhandler Andrew Wagoner scanned the cars coming off the 14th Street bridges near the Bureau of Engraving and Printing and summed up the morning nicely: "This looks just like a regular day."

The morning traffic jams predicted along 15th, 17th, H, I and K streets NW -- the roads most affected by the closing -- never materialized. Monday and Friday usually are less busy downtown, but clearly, many commuters stayed away from the White House area.

"Everyone got the word early," said Steve Eldridge, of Metro Traffic Networks, which supplies traffic information to local radio and television stations. "People had notice there was going to be a potential problem on Monday, and maybe they took a day off."

Police and Metrobus officials helped by assigning more people in the area. Forty D.C. police officers, many working on their day off, were assigned to the area from 6:30 a.m. to 8:30 a.m. As traffic increased about 8:15, the officers directed traffic at intersections along 14th, 15th, 17th and H streets NW, where most east-west traffic was diverted.

There were several glitches yesterday, some of them unavoidable. Building construction meant that some lanes were blocked, a problem on southbound 15th Street just south of H and on northbound 17th Street.

To make more room for traffic diverted from Pennsylvania Avenue, the city has banned parking along 17th and 15th streets between Constitution Avenue and K Street. As a result, King said, the city will lose $750,000 a year in revenue from parking meters.

Meanwhile, along Pennsylvania there was an eerie yet serene quiet in front of the White House, where roller bladers, bicyclists and joggers made the only sounds on the instant pedestrian mall. There were also tourists, such as Anna Bussian, of Cleveland, snapping pictures.

"It's the few who have the power to alter our freedom, and I resent that," Bussian said. "Those who would threaten the president of the United States are hemming us -- the good people -- in."

Staff writers Brian Mooar, Lena H. Sun and Debbi Wilgoren contributed to this report.