The old bridge's paint system has failed, Williams said, and no longer effectively protects the span's steel elements from water. Moisture has also seeped into the space between the driving surface and the subsurface layer, separating them like laminate peeling off cardboard.

"As it rains, it'll get into the deck," Williams said.

From a spot near Ponca Street and a show bar called the Night Shift, a visitor can look up at the viaduct and see the stark contrast between the undersides of the roughly 55-year-old I-895 and a 25-year-old elevated section of I-95.

On I-895, the steel spans and girders are mottled with "bad" rust that eats away at the metal, Williams said. On I-95, which was built using improved construction methods and materials devised over the intervening 30 years, "good" rust has formed an even patina that protects the steel, he said.

Williams said the aim of the project is to build a new bridge that conforms to the current construction code; the code has changed over the decades to accommodate such changes as the ever-increasing size of the nation's trucks.

Once it's completed, the new bridge should be good for about 75 years, he said. Current plans call for three years of engineering, with bids to go out in spring 2014 and construction to begin late that year. Williams said the project has been pushed back a couple of times because of budgetary issues.

Among the reasons the authority's engineers recommended a full replacement, Williams said, is that building the bridge anew will cause less traffic disruption than a rehabilitation project. Replacement, he said, will take about four years, with traffic reduced to three lanes for the first two-thirds of the project and four lanes open for the remaining time.

But he said a rehab would take 5 to 51/2 years and limit the roadway to three lanes for virtually the entire duration of the job.

Williams said the authority intends to manage traffic during the replacement job so that two lanes are always open in the direction of the prevailing traffic.

The new viaduct is expected to include various safety improvements, including wider shoulders and more space for police officers to pull over trucks before they enter the tunnel.

The construction project itself will be a monumental challenge, Williams said. He noted that the viaduct crosses over more than a dozen train tracks operated by multiple railroads, all of which will expect to maintain traffic with minimal disruption.

Meanwhile, the builders will have to remove a structure that weaves though the pillars of I-95 — the busiest traffic artery along the East Coast.

No matter how complex the project, Swaim-Staley said it can't be put off too much longer.

"You don't wait until the steel girders have rusted out so much you harm the entire integrity of the structure," she said.