Many of the speakers showed pictures of depressing urban landscapes where large highway walls cut off access to rivers, as I-95 does along the Delaware.
"We make smart cars but stupid places," said Peter Park, who has served as planning director of Milwaukee and Denver. In Milwaukee, the city knocked down a highway to create park space.
Diana Lind, executive director of Next American City, a nonprofit based here that organized the discussion, believes Philadelphia has a huge opportunity to remake I-95 because much of the highway is due to be rebuilt in the next 30 years.
"There is an opportunity here to say . . . is the future of Philadelphia one with I-95 in it, or could it be something else?" she asked. When she asked the crowd of about 200 people how many thought building I-95 was a mistake, most raised their hands.
She noted the region has plentiful highways that could handle some of the traffic that now travels on the three-mile stretch of I-95 between the Ben Franklin and Walt Whitman Bridges.
Knocking down or redesigning the highway so it no longer blocks access to the Delaware could be an economic boon, she added, if it encouraged development and new tax revenue. She said she could imagine a university using new riverfront space for a downtown campus, for example.
"I-95 is really sitting on about 60 acres of prime land in Philadelphia," she said.
Andrew Stober, chief of staff in Mayor Nutter's Office of Transportation and Utilities, said the city must weigh competing interests and projects.
Part of the Nutter administration's vision is to improve the port on the Delaware to increase the number of high-paying jobs there. That will require easy access for trucks going in and out of the port, and it is not clear that surface roads could handle that volume of traffic.
The federal government has become far less willing to pay for large infrastructure projects, he said, adding that he has to beg for a few million dollars when he goes to Washington to ask for help adding bike paths in Philadelphia.
In Boston, where the Big Dig buried a highway to create park space, the cost has grown so great that fares on public transportation have increased dramatically. Some residents there also worry that the project will drown future generations in debt, Stober said.
But Thomas Deller, director of the Providence (R.I.) Department of Planning and Development, said improving access to waterfronts could generate revenue for cities.
Decades ago, Providence buried one of its rivers and covered it with parking lots where no one wanted to go. The city moved a highway and uncovered the river that had been underground. Now, what had been dusty parking lots are inviting waterfront parks, and Deller believes the city will be able to sell the parcels around them for $60 million.
"We started creating a community that recognized our water," he said, "to bring people back."
Contact staff writer Miriam Hill at 215-854-5520, firstname.lastname@example.org, or @miriamhill on Twitter.