His project, called UnderGardiner, was unveiled on Tuesday along with a $25-million donation from philanthropist Judy Matthews to make it happen.
The plan is to create a park under the elevated highway using the five stories of space to build the green space up instead of out. Greenberg calls the unused space "hidden in plain sight." He said the public space will be less like Toronto's existing parks — such as neighbouring Little Norway park — and more like a "great public living room."
"It's a real breakthrough in the way we think about public space in Toronto and how we build it," said Greenberg on Metro Morning on Tuesday.
The space stretches 1.75 kilometres and spans seven different city neighbourhoods. Greenberg plans to have 55 "civic rooms" in that space, all connected through a multi-use trail for pedestrians and bicycles.
The rooms would be formed by the columns and beams of the Gardiner, which are called bents. The rooms would house events staged by cultural producers in the city. Artist renditions show an ice rink in winter, and open benches and street performers in the summer.
Due to the shelter from the expressway overhead, the space would be intended for use year round.
"You could never afford to build this great civic roof if you were doing it from scratch. But because it exists and because the city is spending $150 million to restore this part of the Gardiner," he said, "we have an opportunity to bring into the space visual arts, performing arts, specialty markets, recreational and children's activities."
Greenberg said there would be a programmer to help plan events in the space.
The project is being built with public and private funds. Judy and her husband Wil Matthews, the City of Toronto and Waterfront Toronto are all invested. Greenberg is working with urban landscape company Public Work.
The project hopes to attract more donations throughout its planning and construction. In fact, Waterfront Toronto is in the process of changing its legal status so that it can accept more donations, Greenberg said.