A map shows the proposed road tunnel under the Calgary airport. (City of Calgary)The Wildrose Alliance Party is adding its voice to those calling on the city to build a tunnel underneath a new runway slated for the Calgary International Airport.

On Monday the provincial opposition party's leader Danielle Smith waded into the debate, which is shaping up to be a key issue in Calgary's mayoralty race.

The city's next slate of leaders must work with the Alberta government to get the route constructed, Smith said.

"A city council that makes airport access a top priority in an active partnership with the provincial government is the only way we are going to unleash the full potential of Calgary international," Smith said.

The airport is building a new 4,270-metre long runway, which requires the closure of Barlow Trail between 48th Avenue and Airport Road N.E. in April 2011.

The tunnel would preserve the east-west route to the airport from the city's northeast, connecting 96th Avenue east to 36th Street N.E.

Without a tunnel, area businesses will be cut-off from airport traffic, and Deerfoot Trail — the other main route to the airport — will become too congested, according to the project's advocates. The cost of building the tunnel has been pegged at $287 million.

The Calgary Airport Authority and the city agreed in December 2009 to jointly cover $90 million, but the provincial and federal governments rejected a request to fund the remaining $197 million. In July city council rejected a suggestion to spend $150 million to rough in the basic infrastructure and build the tunnel at a later date.

Of the 17 candidates for mayor, Bob Hawkesworth has been the most vocal opponent of building a tunnel under the new runway.

The project could end up costing as much as $500 million once all the necessary interchanges and related infrastructure are paid for, he said.

The cost of boring a tunnel under a completed runway would be about $900 million, the city calculated earlier this year.