The debate over the proposed expansion of Billy Bishop, Toronto's island airport, is gaining momentum. The proposal will be considered at a special meeting of the City Council's executive committee on March 25.

Torontonians have heard a great deal about the supposed economic benefits that will accompany the airport's expansion. We are writing to dispel these self-serving projections. Let's look at the facts:

First: The island airport is a proven asset. We all agree that airports generally boost the economies of the cities where they are located, and that the island airport plays an important regional role in connecting Toronto to other cities, such as Ottawa, New York, Boston and Washington. But these benefits of convenience and connectedness already exist. It is hard to see how the proposed expansion – with a focus on long-haul flights and leisure destinations – will measurably add to the economic benefits realized through the airport's current strategy.

Story continues below advertisement

Moreover, the island airport's existing facilities are not yet even close to being maximized. Billy Bishop served a total of 2.3 million passengers in 2013. It has the capacity to serve as many as 3.8 million passengers without expanding the runway or introducing jets, as Porter Airlines has requested. So there is ample opportunity to ramp up business-related traffic, if demand requires, without transforming it into a major international airport. Of course, such an expansion would invite competition for slots, which could be accommodated only by demands for even further expansion. Porter's competitors have already publicly declared their intentions in this regard.

Second: The promoted estimate that the airport generates $1.9-billion in annual economic output is not based on a valid measure of economic benefit. The number counts gross revenue from all goods and services and double counts products bought and sold between firms in the supply chain. A more relevant measure of economic output would be net value added.

The numbers matter because estimates of additional impact are based on the measure of output. If the inflated number is used, all growth estimates will be inflated. Moreover, a more careful analysis of economic and employment projections needs to be done; the promoted figures discount or ignore the impact of the Union Station-Pearson International Airport rail link opening next year, as well as possible alternative expenditures on kindred public assets, such as transit.

Third: A recent and far-fetched argument to appear is that fare reductions to some destinations constitute an economic justification for expansion of the island airport. Lower fares are the result of Porter's entrance into the market, not that the flights originate and land at the island. To support a mammoth expansion of the airport to facilitate more competition on long-haul flights while ignoring major economic, social and environmental costs is absurd.

Fourth: Whatever economic benefits do accrue will not be enough to offset expansion's negative consequences on waterfront infrastructure, communities and planning. These expected adverse effects have been well documented: intrusive runway extensions into the harbour; health and environment issues; enormous traffic increases in an already congested area; and erosion of investment in existing public assets such as parks, education and community facilities.

The key to the successful revitalization under way on Toronto's waterfront is that one activity isn't allowed to dominate the others. To put at risk this huge effort and investment, public and private, would be counterproductive and unwarranted. Dubious economic claims must not be allowed to trump the realization of an outstanding, livable, multiuse waterfront for all Torontonians.

Paul Bedford was chief planner of Toronto from 1996 to 2004. David Crombie is a former mayor of Toronto. Jack Diamond is a Toronto-based international architect. Anne Golden is chair of the Transit Investment Advisory Panel. Ken Greenberg is former head of urban design in the Toronto planning department.