Dark and neglected'that's the kind of derelict space that lurks below most highways and elevated roadways. Cities in the process of densifying, however, can no longer afford to ignore such concrete underbellies. Toronto, which has been busy completing 70,000 residential units between 2008 and 2012'mostly condominium apartments'recently opened Underpass Park, a gutsy template that repairs a previously marginalized urban zone in the city's East End neighborhood.

During a sudden rain shower this spring, about 20 young men hustled to Underpass Park to shoot hoops, glad for the roadway over their heads. A few teenagers on skateboards rode the park's undulating metal railings and concrete ledges, while others sipped from the water fountains. Softening the edges of all the hard surfaces, more than four dozen Kentucky coffee and black locust trees rise in the gap separating the elevated Richmond and Adelaide ramps running off the Don Valley Parkway.

Designed by the Vancouver-based landscape architecture firm Phillips Farevaag Smallenberg (PFS) in collaboration with The Planning Partnership, a Toronto-based planning, urban design, and landscape architecture firm, the $9 million Underpass Park is part of an ongoing effort by the publicly funded agency Waterfront Toronto to reimagine public space below and around major transportation links.

'The park is really about the everyday,' says PFS's Greg Smallenberg. It helps to bind together two large development parcels in the new West Don Lands neighborhood: to the north, the stunning black-glass mid-rises by Saucier + Perrotte Architectes and, directly south, the 18-acre Corktown Common, which opened last month; it was designed by Michael Van Valkenburgh Associates as a recreational park that doubles as a flood-protection zone.

'The thing about all of this transportation infrastructure littering our cities is that it isn't going away,' says Smallenberg. 'As open-space resources continue to dwindle in our cities and urban populations and densities increase, we have to take advantage of whatever open spaces we have.'

Transforming a squat and grim space requires imagination'and inventive illumination. Mirage'an artpiece by Toronto's Paul Raff Studios consisting of octagonal panels of reflective metal suspended from one portion of the overhead highway'adds life to the park by bending light and creating watery reflections of people passing below. And lights projected onto the highway's concrete columns help animate the space as they cycle through the color wheel.

Activating the city's in-between areas is a tall order, especially where the floor-to-ceiling height is less than 20 feet (as it is at Underpass Park). The east section of the park, with the hoops and skateboard amenities, is the most popular and best suited to the hard edge of the gritty environment. Less successful is the playground, its equipment looking bereft and difficult to love on concrete paving. Not surprisingly, children rarely gather there. The colored lights on the columns are subtle to read, even at night. Some daylight does bounce off Raff's artwork, but a larger reflective array would have incited a deeper transformation of the space.

Waterfront Toronto is charged with the revitalization of 2,000 acres of the city along the edge of Lake Ontario. With the Pan American Games opening in June 2015, the agency is under pressure to complete as many of its public spaces and housing developments as possible. Compared to high-profile waterfront parks in Toronto such as Sugar Beach or Sherbourne Commons, Underpass Park is a sleeper, except to those with skateboards or basketballs.

What to do with the Gardiner Expressway, another elevated highway that cuts through downtown Toronto, is currently provoking debate. Six design teams'including Diller Scofidio + Renfro'recently unveiled proposals for reinventing the Gardiner, above and below its massive ramps. While this discussion will probably rage for years, possibly decades, architects and planners might want to check out Underpass Park to see what works and what doesn't.

Lisa Rochon is the architecture critic for The Globe and Mail.