This gray-to-green metamorphosis is underway or under consideration in major cities seeking ways to revive sections of their downtowns from Los Angeles and Dallas to St. Louis and Cincinnati.
Transportation departments are not opposed as long as the plans don't reduce highway capacity. In most cases, traffic is rerouted.
"It's the coming together of people wanting green space and realizing that highways are a negative to the city," says Peter Harnik, director of the Trust for Public Land's Center for City Park Excellence. "Covering them with green space gives you a wonderful place to live and work."
Groups that are not always on the same page — environmentalists and developers — are embracing the "capping" or "decking" efforts for different reasons. Environmentalists encourage more trees and grass to offset carbon emissions and promote walkable neighborhoods to reduce reliance on cars. Developers are eager for space to build on in prime downtown locations. Citizens want parks and amenities they can reach on foot.
"Highways are extremely destructive to the fabric of urban life," says Harnik, author of Urban Green: Innovative Parks for Resurgent Cities. "The noise that emanates from it, the smell."
Capping freeways dates to the 1930s. A recent example is the Rose Kennedy Greenway over Boston's "Big Dig," which created open space by putting elevated roadways underground.
The resurgence of downtowns has turned available pieces of land into hot commodities. At the same time, the drumbeat for more parks in smog-choked cities is getting louder.
"It's essentially like creating oceanfront property," says Linda Owen, president of the Woodall Rodgers Park Foundation in Dallas. "It's an economic engine."
The group leads the effort to build a 5-acre park on the eight-lane Woodall Rodgers Freeway that runs north of downtown, between U.S. 75 and Interstate 35E. Traffic will be channeled to a tunnel. It's part of a bigger plan to revitalize the city's core and connect all corners of a 68-acre cultural district, from museums, restaurants and residential towers to a new opera hall and performing arts center.
"The freeway is like our medieval wall," Owen says. "You couldn't get over it. … The park is just being created out of thin air."