Motorists should avoid Western Maryland and Northern Virginia on Friday, Aug. 19, and Interstate 95 North in Maryland on Saturday morning, Aug. 20. Police expect to escort 1,800 motorcycles on a Sept. 11 tribute ride that will require closing some of the region’s most congested highways.

The potential for gridlock Friday afternoon is of such concern that the U.S. Office of Personnel Management “strongly urges” federal employees to telework or take leave. A Virginia highway spokeswoman advised all Northern Virginia commuters to treat Friday as a snow day and work from home.

Maryland officials say motorists in Western Maryland who drive certain routes Friday morning should add two hours to their travel times. While the rolling lane closures should last about an hour as the procession of motorcycles — expected to stretch 12 to 15 miles — passes, officials said they expect backups to ripple throughout the area for hours.

“Friday afternoon is a day when you’ll want to be out of town or telecommuting or on the Metro,” said Joan Morris of the Virginia Department of Transportation. “We certainly hope it’s not as bad as we’re planning for.”

Major delays are expected in Northern Virginia between noon and 7 p.m. Friday. Though the ride will take place mostly on major highways — the Dulles Greenway, the Dulles Toll Road, Interstate 66 and Route 110 South — backups from the closures are likely to spill over onto the Capital Beltway, the George Washington Parkway and many local roads, officials said.

Travelers are also likely to face major delays in reaching Reagan National Airport and leaving Dulles International Airport. If forecast thunderstorms occur, motorcycle riders would have to lower their speeds, causing more backups on the three-day ride, which begins Friday in Shanksville, Pa., travels to the Pentagon and ends Sunday in downtown Manhattan.

Ted Sjurseth, a Leesburg resident and president of the nonprofit group America’s 911 Foundation, which organized the ride, said it must pass through the Washington region Friday afternoon and Saturday morning in order to reach all three plane-crash sites within three days.

He said the group understands its effect on traffic but hopes those caught in the backups will remember their cause.

“We’re here breathing today and trying to honor those who gave their life that day,” Sjurseth said. “There’s no great time to do it. . . . We’re not here to party. It’s a remembrance ride.”

Sjurseth said the group had 2,800 participants as of Wednesday, close to its goal of 2,977 participants — one for every person killed in the terrorist attacks. Most will be riding on 1,800 motorcycles, he said, and about 60 will assist with breakdowns and any injuries. He said the group raises money for $2,000 college scholarships awarded to children of police officers and fire and rescue workers.

Motorists whose patience will be tested first will be those on eastbound highways driving Friday morning through Western Maryland. The riders — and the lane closures that will accompany them — are scheduled to reach Cumberland around 8 a.m. and affect highways near Hagerstown, Frederick and Point of Rocks for the rest of the morning.

Riders are expected to enter Loudoun County about 2 p.m. Friday and head to Crystal City, where they will spend the night. They are scheduled to leave the Pentagon at 6:45 a.m. Saturday and head to New York using Interstate 395 North, Pennsylvania Avenue, the Baltimore-Washington Parkway and I-95. Maryland State Police have advised motorists to avoid northbound B/W Parkway and I-95 from 6:45 a.m. to 1 p.m. Saturday. The group plans to ride to the World Trade Center site Sunday morning.

John Townsend, a spokesman for AAA Mid-Atlantic, said he understands the “sacrosanct” nature of a Sept. 11 commemorative event but said motorists should have been given more warning.

“It’s right smack dab in the middle of rush hour,” Townsend said. “Whoever gave the permission for this needs to have his head examined.”

Lt. James E. DeFord, a spokesman for the Virginia State Police, said: “It’s really not a matter of ‘allowing’ it. It’s a public highway. We can’t throw up a gate and say, ‘You can’t come into Virginia.’ ”

He said the lane closures are required to keep the riders together as a safety precaution. Crashes are less likely to occur, he said, if riders unfamiliar with the area don’t have to weave through traffic to follow each other.

While the ride has taken place through Northern Virginia on summer Friday afternoons during the past nine years, officials said, it usually has been several hundred motorcycles, causing short-lived disruptions. This year’s event, in the lead-up to the 10th anniversary of the attacks, is expected to draw more than four times as many participants.