Bridges are the most vulnerable sections of a highway, and their support structures can be undermined by activities, installations, or accidents, engineering experts say.
The Pennsylvania Department of Transportation prohibits large storage areas, flammable materials, or big installations beneath bridges, said Charles H. Davies, the assistant district executive for design for PennDot's Southeastern Pennsylvania district.
PennDot typically owns the land beneath its bridges or has "aerial easements" that permit it to control what is built or placed under the spans.
"Bridge inspectors need to report on any use under the highway," Davies said. Bridges are required to be inspected every two years.
PennDot restrictions were tightened after the 1996 arson under I-95 in Port Richmond, where thousands of tires had been allowed to accumulate illegally for years.
In the current closure of the I-495 bridge in Wilmington, investigators are looking at a contractor's stockpile of dirt that was legally placed just east of several support pillars that are now leaning in that direction, tilted as much as four degrees out of vertical alignment.
The contractor and state highway crews were removing the hundreds of feet of dirt last week.
The tilting of the bridge near the Port of Wilmington forced the closure Monday of the span that carried about 90,000 vehicles a day, disrupting travel throughout the region.
The closure caused backups of as much as seven miles on I-95 south of Philadelphia.
By week's end, though, motorists were finding alternate routes and travel times, and rush-hour delays on I-95 were down to three-tenths of a mile Friday morning, said PennDot spokesman Eugene Blaum.
In the spaces beneath bridges, there can also be harmless, even beneficial, uses.
Think of the Rizzo Rink ice-skating facility under I-95 at Front Street and Washington Avenue in South Philadelphia.
Or the nearby public parking lots under the interstate that serve the Mummers Museum and the Gloria Dei (Old Swedes') Church.
Now, as PennDot crews rebuild I-95 and its bridges through Philadelphia, planners are rethinking how under-bridge spaces can be used.
And there are lots of them.
There are 210 bridges on I-95 in Southeastern Pennsylvania. Their spans total 10 million square feet, about 40 percent of all the PennDot bridges in the five-county region.
Incorporating the once-isolated areas under I-95 into local communities is now part of the plan for reconstruction that will last at least another decade.
Trails, bike paths, and parking lots under a bridge can "make it part of the neighborhood instead of an uninhabited area where things happen and no one is looking," Davies said.