For decades, the Gardiner Expressway was considered a blot on the landscape and an unbridgeable barrier between the city and its neglected waterfront.

But in the past few years, something remarkable has happened: the Gardiner has begun to disappear. The thickets of tall buildings that have been shooting up on either side have obscured it from view from many vantage points, making the aging grey structure much less of a visible barrier and disproving the idea that it prevents development near the waterfront.

Even more interesting than what has been happening around and above the Gardiner is what has been happening under it. With buildings rising within metres of the expressway, architects and developers have started to make creative use of what was once some of the most barren and forbidding space in the city.

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The attractive new Fort York visitors centre, with its weathered steel panels, sits right up against the pillars of the Gardiner. People can approach it from underneath the cement and girders of the expressway. Further east, a new glassed-in footbridge passes right under the elevated highway – and right over Lakeshore Boulevard – connecting new buildings to the south with the underground PATH system that takes pedestrians around the downtown core.

The latest move to enliven the underside of the Gardiner comes from the developers of a long-dormant site at Bathurst and Lakeshore. Their proposal, which went before a city-council committee on Wednesday, would convert a historic 1927 warehouse on Lakeshore into a Loblaws grocery store with office space on top. Two residential towers of 40 and 44 storeys would stand behind it, next to the Gardiner. The plaza of the buildings would extend right under the expressway, which would be illuminated with new lighting to make it more inviting.

Architect Peter Clewes calls it "found space." What used to be an inhospitable wasteland should become a sheltered open area that will act as a forecourt to the new complex.

In effect, says Mr. Clewes, the Gardiner is becoming irrelevant. "The question is not, Is the Gardiner as a piece of infrastructure terrible for the city?" he says. "The question is, What can we do with it? How can we inhabit it – beside it and underneath it?"

That is a sharp about-turn in the way planners and architects view the unloved creation of Frederick "Big Daddy" Gardiner. Conventional wisdom was that nothing much could happen on the waterfront or its approaches as long as the expressway stood in the way.

For decades, parking lots and vacant land filled much of the space between Union Station and the harbour. The only way to spark development of that land and open up the waterfront to the city, many argued, was to tear down the expressway.

That seems unnecessary now. A whole new district, the South Core, is rising south of Union. Office buildings and condo towers reach for the sky. The streets below are starting to fill up with pedestrians, giving the area a bustling urban feel. The pace of redevelopment on the water's edge is picking up.

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It's still not much fun to cross the Lakeshore under the Gardiner on foot. But projects like the Fort York centre and the Bathurst-Lakeshore buildings show that the underside of an elevated road need not be a no-go zone. Just look at how the designers of Underpass Park near the Don River filled the blank areas under and beside the Eastern Avenue, Adelaide and Richmond overpasses with playgrounds, basketball courts and other public space.

As development around the Gardiner accelerates, and hundreds of millions of dollars in repairs to the roadway proceed, the city should be encouraging new ways to use the space underneath it.