Any number of obstacles can kill a high-rise project in Philadelphia. Usually it's money. Construction costs can run as high as New York's, yet rents remain much lower here, the margins smaller. Tall towers frequently get ensnared in politics, especially when a neighborhood group is nervous about having more people around.

But the forces that have delayed construction of a much-praised skyscraper design at Second and Race Streets are of a different sort entirely.

This time, the building is being held up by a billboard.

It is, to be sure, a very lucrative billboard. Owned by Keystone Outdoor Advertising of Cheltenham, the three-sided display rises 150 feet above a traffic island in Old City at the Race Street entrance to I-95. Two of the billboards are positioned so they are visible for roughly eight seconds to every motorist crossing the Ben Franklin Bridge.

Sure, the apartment tower could shave some valuable time off their visibility. It might even block them entirely from certain angles. On the other hand, the project would make Old City a better place to live. The building would occupy a large lot alongside the bridge that has been empty far too long. It would restore the streetscape with ground-floor retail and greatly strengthen connections to the developing Delaware waterfront.

It's not even a contest. No one owns a view. Not homeowners, and certainly not companies competing for our eyeballs.

But for Keystone, advertisements come first, and so it has marshaled a small army of white-shoe law firms to fight the project.

It is now six months since the developer, Brown/Hill, presented its plans for the tower, designed by noted New York architects Peter Gluck & Partners. Although some Old City residents felt the building was too tall for the neighborhood, the design received an enthusiastic endorsement from city planners in September.

Known as 205 Race, it looked set to sail through its variance hearing at the Zoning Board of Adjustment. With demand for rentals soaring in Philadelphia, the proposal was on a fast track to construction.

Then nothing.

The developers say they were forced to put the project on hold after Keystone threatened to sue. Fearing the project would be tied up in court for years, partner Greg Hill told me, his company decided to deal. They offered a redesign to preserve views of the billboards.

At Keystone's request, Gluck slimmed down the tower, which would sit atop a 65-foot-high podium. His designers angled the north facade and reshaped the corners to improve sight lines, losing about two apartments per floor. To make up for the lost units, Brown/Hill had its architects raise the height of the building two floors, from 197 to 222 feet.

Keystone still wasn't happy, according to Hill's account. Because the billboard company could still sue, the developers decided the only way to move the project forward was to seek legislation making it legal to build a tall tower on the site. Brown/Hill hopes the bill would be harder than a variance to challenge in court.

Councilman Mark Squilla introduced the ordinance last week, and Council likely will approve it. To avoid charges of spot zoning, the bill creates a new "bridge approach district," between Second and Fourth Streets, from Race to New Streets.

Saying the dispute is an "ongoing matter," attorney Steve Pollock of Montgomery, McCracken, Walker & Rhoads - one of three firms working for Keystone - declined to comment. Keystone owner Dominick A. Cipollini did not return phone calls. But the city's chief planner, Alan Greenberger, who helped negotiate the deal, said he believed the design changes should satisfy Keystone.

Squilla's bill may be the best solution to a bad situation, yet it is unfortunate the city had to resort to the tactic. The new zoning code was intended to limit tinkering, especially by Council members. The Old City Civic Association, which represents the neighborhood and objects to the tower's height, is now furious. Its zoning chair, Rich Thom, complained in a letter to the Planning Commission that "special interests have commandeered zoning - again."

He's right in theory, wrong in practice. The code, as Greenberger noted, is a "coarse tool" that can't reflect every situation perfectly.

The tower (which I reviewed positively in a September column) wasn't too tall before, and it's not too tall now. Though buildings taller than 65 feet are not currently allowed in Old City - an area of historic 19th-century warehouses - the site at Second and Race sits hard by the bridge, which rises to 190 feet. It's also on the very edge of the historic district.

So, even though the redesigned tower would be taller than the bridge, its slimmer profile should improve its appearance. The activity it brings to that desolate corner will also help support two key waterfront anchors on Columbus Boulevard, the Race Street Pier Park and the Fringe Arts Festival's new headquarters.

Instead of focusing solely on building height, we ought to be asking why a deep-pocketed billboard company gets to call the shots. Philadelphia exists as a place to live and work, not as a backdrop for advertisers.