Three of the nation's most heavily used multipurpose trails, the Mount Vernon Trail, the Washington and Old Dominion Railroad Regional Park and the I-66 Trail, will be connected next year, creating 68 miles of continuous trail through Northern Virginia from Mount Vernon in southern Fairfax County to Purcellville in western Loudoun County.

The $2 million project will include the first pedestrian overpass to cross the George Washington Memorial Parkway, at Rosslyn, and will allow direct access from Arlington to the Potomac River for the first time since the parkway was built in 1932.

But the linkup of the three trails, which will bring more traffic to the Mount Vernon Trail, has raised concerns among some area bicyclists, who say the Mount Vernon Trail is one of the most dangerous in the nation and that more traffic will make it worse.

According to Linda Hulvey, director of emergency nursing at Mount Vernon Hospital, five to 10 bicyclists are treated at the hospital on a typical summer weekend. "When the weather gets warm, the number of people we treat for bicycle accidents goes up significantly."

"We have estimates of a half million to 600,000 visitors a year on the {Mount Vernon} Trail, which is a tremendous volume of traffic," said John Byrne, superintendent of the parkway.

"We will be painting a yellow stripe down the middle of the trail this summer to encourage people to stay to the right . . . . We will be straightening and leveling a dangerous curve at the southernmost end of the trail" by the Mount Vernon estate, Byrne said.

In addition, Byrne said, "We are seriously considering posting 15 mph speed limit signs along the trail for bikers . . . . It's something we need to think about with deliberation." Local bicyclists say a speed limit on the trail would be one of the first in the country.

"In the opinion of {the Washington Area Bicyclist Association} the Mount Vernon Trail was not designed by people who fully understood how bicyclists ride," said Peter Harnik, vice president of the association, who works for the Rails-to-Trails Conservancy.

"The yellow stripe would help . . . . I think speed limit signs are not really realistic because bicyclists don't know how fast they are going. Fifteen miles an hour is very fast for that trail," Harnik said.

According to U.S. Park Police records, nine accidents on the trail have been reported since Jan. 1. About 10 trail accidents have been reported to the bike association in the past six weeks. Each organization says it hears about only a small proportion of such accidents.

"I think the typical problem is that bikers go too fast," Byrne said. "If we try to make {the trail} safe for a speeding biker, then we'd have to cut down trees and smooth out the curves, and that would make it unenjoyable for the other users."

One of the most publicized accidents on the trail occurred in September when Shirley Metzenbaum, wife of Sen. Howard M. Metzenbaum (D-Ohio), received severe head injuries when she fell from her bike.

"I didn't think the trail was so curvy. The last thing I remember was looking in my rear view mirror for Howard . . . . Maybe what happened is {the trail} zigged and I zagged," Shirley Metzenbaum said in an interview last week. She said she incurred blot clots to the brain and still has blurred vision and is unable to drive. She said her doctors tell her she should be fully recovered by September. She said she is not certain that the accident was the fault of the trail.

Other improvements planned for the Mount Vernon Trail include a connection at National Airport to a closed pedestrian tunnel that runs under train tracks and to Crystal City.

"It will allow walkers and bikers to safely ride from the Mount Vernon Trail onto the streets of Crystal City," explained Mark Kellogg, the public works planning supervisor for Arlington County.

Kellogg said construction of the overpass in Rosslyn, which is to be completed in February, will cost $1.3 million.

Byrne said that within a year a portion of the Mount Vernon Trail will be relocated along an access road to Fort Hunt Park by paving a small portion of trail next to the road. He added that similar improvements involving paving a trail next to roadways will be made at several points between National Airport and Memorial Bridge where the trail crosses roads.

More than 1 million people use the Washington and Old Dominion trail each year, according to Dorothy Werner, spokeswoman for the Northern Virginia Regional Park Authority, which runs the trail.