Interstate 95 must change. Simple as that.

After three days of intensive workshops designed to generate fresh ideas for Philadelphia's languishing Delaware waterfront, five dozen bleary-eyed planners and architects put aside their maps, satellite photos and sketches on Saturday evening, and jointly called on city and state officials to deal with the great highway canyon that cuts off the city from the river of its birth.

Bury it. Narrow it. Put a deck over it. Just get it out of our sight.

Although the recommendations varied in their specifics, the experts who participated in the weekend brainstorming exercise agreed that I-95 was the biggest impediment to redeveloping the seven miles of waterfront property central to Philadelphia.

"Unless we solve this problem, our waterfront will never be a success," Richard Bartholomew, a planner at Philadelphia's Wallace Roberts & Todd, told a crowd of more than 500 Philadelphians who attended Saturday's presentation at the Independence Seaport Museum.

The idea of reconfiguring I-95 is hardly new, but the blue-sky proposals were greeted with an enthusiasm not seen in earlier public forums on the waterfront. The crowd included a large number of casino opponents, who hope to enlist the planners as allies.

Essentially, the planners were proposing a Philadelphia version of Boston's famously expensive and lengthy Big Dig project, which involved building a platform over a highway to create new land for development. One planner, describing his proposal for Philadelphia, called it "a half-Boston."

In the past, the Pennsylvania Department of Transportation has dismissed such schemes as pie-in-the-sky. But after a spirited debate,the agency's deputy secretary for administration, Rina Cutler, concluded that a realignment of I-95 was worth exploring. Since PennDot will need to reconstruct the road in about 10 years time, she noted, "that would be the time to make changes."

"You'd be making a huge mistake if you don't bury I-95," responded Ed Uhlir, the design director for Chicago's Millennium Park, which was also built atop a complex of roadways. While Millennium Park cost a whopping $350 million to build, he said it had paid Chicago back by attracting $4 billion in private investment.

The workshop, which drew together an international battery of planners, architects and engineers, was organized by Penn Praxis, a nonprofit planning group from the University of Pennsylvania that is preparing a waterfront master plan for the city.

The goal of such intense brainstorming sessions, known as "charettes," is "to stoke the flames of our imaginations," said Praxis director Harris Steinberg.

So, along with calls to minimize the impact of I-95 on the waterfront, the five design teams produced a laundry list of suggestions, from the familiar to the fanciful:

Turn Delaware Avenue into a gracious boulevard with wide sidewalks for pedestrians, a bicycle path and generous landscaping.

Improve connections between Center City's business district and the waterfront by running a trolley line down Market Street, from 30th Street Station to the river. The line would then loop north and south along the improved waterfront boulevard.

Rather than impose a continuous waterfront park on the river's saw-toothed coastline, build a necklace of green spaces similar to Penn Treaty Park. There would be one park for each river neighborhood, located every 2,000 feet or so. Each park would have an easy pedestrian connection back to the neighborhood.

Create an industrial park on the northern portion of the Delaware. This unexpected idea came from Peter Latz, a German landscape architect whose work celebrates the industrial past. As an important demonstration project, he advised the city to save Peco's derelict, but beautiful, Art Deco electric station at Richmond Street and convert it to a biogas generation plant. It would be a statement that industry is still possible in the 21st century.

Work with the port to reclaim the natural marshland along the river's edge south of Home Depot.

But just as the wide canyon of I-95 dominates the river vista, the highway dominated the three days of discussion for knitting the city back to the Delaware.

Two distinct scenarios for pushing the city eastward emerged. The first approach, presented by Bartholomew, was dubbed the stacking scheme because it would place Columbus Boulevard on top of I-95. By creating a two-tier road system, Philadelphia could reduce the width of the highway chasm by half, to 200 feet.

The advantage of stacking the highways is that it would raise Columbus Boulevard to the level of Front Street. Rather than having to navigate a daunting overpass to get to the river, people would cross an ordinary city street, albeit a wide one. The city's east-west streets - Market, Chestnut, Walnut - could be extended across the boulevard. Once they reached the east side, the streets would slope gently down to the river, just as they did in the days before the interstate.

But the stacking approach was too tame for the team led by Gary Hack, dean of Penn's School of Design. He argued that Philadelphia should cover both I-95 and Columbus Boulevard with an enormous platform, starting at Spring Garden Street and moving south. The concrete deck would create acres of new land for development.

It would also allow Philadelphia to extend its street grid east to the river. Because the platform would sit more than 35 feet above the water, he suggested using the space below on Penn's Landing as a parking garage. Although some might criticize parking as a poor use of river frontage, Hack argued that the garage could be used to generate revenue and to keep cars out of Center City. A variety of intermodal transit systems, from ferries to trollies, would shuttle people into town.

"What you'd gain is a city," Hack said.

The first step, Hack said, is for PennDot to do a basic feasibility study to identify the engineering challenges, estimate the potential cost, and identify sources of funding.

Nearly everyone involved in the charette agreed that the city and state must move quickly before the vast stretches of vacant riverfront are locked in by development proposals.

The recent approval of two riverfront casinos by the state has already constrained the planners' flexibility. Since the Praxis study began last fall, Steinberg has insisted that planners must accept the casinos as "part of the facts on the ground."

On Saturday, however, he noted diplomatically "that the possibility of two casinos being sited along the river are felt by many to conflict with [our] values. We acknowledge these tensions."

Despite such constraints, the Praxis study marks a major policy shift in Philadelphia. Since the Rendell era, city leaders have resisted attempts at waterfront planning, arguing that it would hamstring developers. But the hands-off approach led to the riverfront's becoming a dumping ground for big-box uses.

Steinberg said he hoped that the master plan, which will be ready in October, would provide a development road map for the next mayor. Several candidates have already declared that urban planning is an important campaign issue.

Unfortunately, it isn't important enough yet for any of the mayoral candidates to have attended Saturday's presentation.

Some of the Top Recommendations

Make Delaware Avenue a broad boulevard, with room to stroll and bike.

Have a trolley run on Market Street to connect the Center City business district with the waterfront.

Make space for parks along the waterfront, one for each river neighborhood, that connect back to the neighborhood.

Create an industrial park at the north end of the waterfront, turning Peco's derelict Art Deco building at Richmond Street into a biogas plant.

Reclaim marshland at the south end.

Find out more about visions for Philadelphia's riverfront via http://go.philly.com/95waterfront.