By Alan Charles Johnson

An old axiom among city planning professionals says transportation is the glue that holds cities together. This is especially true of the Philadelphia waterfront, which is now lacking glue.

To make the area the economic engine that waterfronts have become in other cities, three kinds of transportation are required:

Simple, direct links to Center City and adjacent neighborhoods.

Seamless connections among existing and potential waterfront attractions.

Convenient links between the Philadelphia and Camden waterfronts, because Camden has attractions of interest to the Philadelphia population, and vice versa.

Fortunately, the glue shortage might be ending. The Delaware River Port Authority has embarked on an ambitious transportation plan for several miles of the Philadelphia waterfront.

The Inquirer's recent coverage of the plan has noted several pitfalls. For example, a proposed Market Street leg of a T-shaped waterfront rail line would be very difficult to build and would duplicate the Market subway line. And, although the Obama administration may change the funding guidelines, it will be difficult to get federal money for the project based on the projected ridership and $500 million cost.

But none of these issues undermines the basic worthiness of the idea. Many of them can be easily resolved by simple revisions in the plans during the next phase, due to begin soon.

The easiest, cheapest, and fastest way to link the subway with a waterfront trolley line is to create an interchange at the Second Street subway station by extending the existing platform to Columbus Boulevard. The subway platform and Columbus are at the same level, allowing for a direct passage under I-95. Passengers getting off the subway would then see the Delaware River directly ahead. This could save $150 million and, once and for all, make I-95 effectively disappear.

Other savings could be achieved by reducing the proposed two-track line along Columbus to a single line, using the existing track. One track is the norm for most cities' trolley lines, and the end points of this one are only 10 minutes apart, so there's no need for two. This would also preserve the beautiful existing landscaping in the median, while saving about $200 million.

These savings would make the project a better candidate for federal funding, but they would also reduce the Pennsylvania project's parity with the DRPA's proposed $1.5 billion River Line extension in New Jersey. This disparity would be politically untenable for the bistate DRPA.

The most prudent course for the DRPA would be to balance the spending by finishing several projects on the Pennsylvania side, including:

Covering more of I-95. The highway was depressed to allow covering, and foundations are already in place, but only parts of the cover have been built.

Finishing an interstate tram at Market Street. Now that London has its Eye and other cities have their own distinctive landmarks, our waterfront would be a good home for a similar project. About $17 million has already been spent on the sophisticated foundations of the tram, and the completion budget is $57 million, so one-quarter of the project is paid for.

Adding a second Riverlink ferry. The docking facilities are already in place, and this would reduce wait times, creating more ridership.

Using the Delaware River Waterfront Corp.'s small fleet of water taxis, which sit idle. With the addition of docking facilities at key points, the taxis could be an asset to waterfront transportation.

These new projects, costing a total of about $350 million, would bring the New Jersey-Pennsylvania balance closer to the DRPA's original goal. To achieve even greater parity, the trolley route in Pennsylvania could be extended south to the Sports Complex and north beyond Penn Treaty Park. Certainly, the present DRPA route, which heads inland, should be altered to serve the waterfront.

Without waterfront "glue" of many different types - land, water, and air - all of the current planning efforts will be in jeopardy. The Port Authority is in a great position to make things happen. It should be encouraged but also guided toward helping to create the vibrant waterfront Philadelphia deserves.

Alan Charles Johnson is the director of Alley Friends Architects in Philadelphia.