Traversing the Monongahela River via the busy McKeesport-Duquesne Bridge on a bicycle is difficult even for experienced riders. For a family of novice cyclists, it is frightening and dangerous.

But in a couple of weeks, pedestrians and cyclists will have a bridge of their own running parallel to the McKeesport-Duquesne Bridge, which has narrow shoulders and no sidewalk.

Today, Allegheny County Chief Executive Dan Onorato and U.S. Steel Chief Executive Officer John Surma will gather for a ribbon-cutting to mark the ceremonial opening of the Riverton Bridge, a former railroad bridge that was donated by U.S. Steel and will be retrofitted exclusively for nonmotorized traffic. The bridge is still under construction.

The opening of the Riverton Bridge is a crucial step in the completion of the Great Allegheny Passage, a network of trails that will run from Pittsburgh to Cumberland, Md. The network connects with another group of trails that run to Washington, D.C.

Only 9 miles of the route remain unfinished and all of the gaps are in Allegheny County, where cyclists are forced to ride on busy roads.

"We're building a nonmotorized, accessible trail system, so it's really important for us to be able to advertise that it is 100 percent safe," said Linda Boxx, president of the Allegheny Trail Alliance, which facilitated the Riverton Bridge project.

Funding for the $1 million project came from the Colcom Foundation and the Pennsylvania Department of Conservation and Natural Resources.

Mr. Surma, who is chairman of a county task force working to fill in the gaps of the trail, decided to donate the span to the county to be converted to a pedestrian and bike bridge, and handed it over in July.

It was "an almost impossibly wonderful gift," Ms. Boxx said.

"Without this bridge, the last 9 miles of the trail would have been on roads," she said. "Here, you have this great trail from D.C., and as you approach Pittsburgh, this last 9 miles would be less than pleasant."

Scott Bricker, co-founder of the bike advocacy group Bike Pittsburgh, said cyclists take longer routes through Braddock to avoid traffic in and around McKeesport. Some cyclists have said the path through McKeesport is the most difficult and dangerous part of the 150-mile journey from western Maryland.

"It's a huge deal when you put it all together," Mr. Bricker said. "People will be able to enjoy their bike rides without dealing with fast-moving vehicles."

The bridge is expected to be completed by Oct. 4, when about 85 cyclists will finish their ride from Washington, D.C., to celebrate Pittsburgh's 250th birthday.

The next step is to complete a trail along Route 837 that would run from the bridge to The Waterfront shopping and entertainment complex in Homestead.

Kevin Evanto, Mr. Onorato's spokesman, said the county has acquired easements for nearly all of the property required to build the remainder of the trail and anticipates the entire Great Allegheny Passage will be finished sometime next year.

First Published: September 25, 2008, 9:45 a.m.