Drivers, beware: The next two days are expected to bring traffic jams that could make even the most grizzled veterans of Washington’s notorious congestion wince.

Transportation officials are advising motorists to avoid western Maryland on Friday morning, Northern Virginia on Friday afternoon and Maryland’s northbound Baltimore-Washington Parkway and Interstate 95 on Saturday morning as police escort nearly 1,800 motorcycles on a ride remembering the victims of the Sept. 11 terrorist attacks.

The group, whose procession is predicted to stretch 12 to 15 miles, is scheduled to ride Aug. 19 to 21.

The ride’s sponsor, the nonprofit America’s 911 Foundation, said riders will visit the three crash sites: Shanksville, Pa., the Pentagon and the World Trade Center site. Some of the money raised will fund college scholarships for children of police and fire-rescue workers, organizers said.

Police said they will do rolling road closures ahead of the riders. Major roads affected Friday morning in western Maryland include Interstate 68 east, Interstate 70 east and U.S. 15 south. Friday afternoon will bring closures on U.S. 15 through Leesburg, the Dulles Greenway, the Dulles Toll Road, Interstate 66 and Route 110. On Saturday morning, traffic will be affected on Interstate 395 north, Interstate 295, Pennsylvania Avenue, the Baltimore-Washington Parkway and I-95 north. Significant delays also could occur in the opposite directions because of rubbernecking, officials said.

The ride is also expected to affect access to Dulles International and Reagan National airports on Friday and Baltimore-Washington International Marshall Airport on Saturday.

Motorists can call 511 in Maryland and Virginia for road conditions or visit for route details.