From a Congressional Quarterly publication:
U.S. cities are increasingly putting freeway segments underground and covering them with parkland. Whether called a lid, deck, bridge or tunnel, there are already some 20 highway parks in the country, several under construction - most notably, the Rose Kennedy Greenway park atop Boston’s Big Dig - and at least a dozen more in the planning pipeline. As urban auto impacts become less welcome, these decks have moved from the novel to the expected. Despite the sometimes considerable cost - as much as $500 per square foot - they are no longer classified as porkbarrel. They’ve been redefined as amenity investment with high economic payback.
While construction costs for deck parks can be wincingly high, there is also an upside: The land itself is generally free, made available as air rights by state transportation agencies. In center-city locations, this can amount to a multimillion-dollar gift. Land near the Santa Ana Freeway by Los Angeles City Hall, for instance, goes for between $2 million and $3 million an acre. In near-downtown San Diego by Balboa Park, an acre is worth up to $13 million.
Regardless of cost, the actual force driving the trend is the opportunity for private development and redevelopment around the parks. In Trenton, for instance, the New Jersey Department of Transportation spent $150 million on the new 6.5-acre Riverwalk deck over U.S. 29, linking the city to the Delaware River. In response, there was a significant spike in prices of nearby property. One lot, worth $120,000 pre-construction, was developed with six housing units that sold for $200,000 each. The park’s existence also helped recruit a new 82-unit market rate residential building.
Projects where freeways are already below grade are much more feasible than others, and there are four particularly high-prospect opportunities in major downtowns. In St. Louis, one of Mayor Francis Slay’s top priorities is the “three-block solution,” a plan to cover a portion of I-70 between center city and the Gateway Arch so that visitors to the Arch - there are about 3 million a year - can get into downtown St. Louis easily, while making it easier for those downtown to reach the Arch and Mississippi waterfront. An early rough estimate put the cost at a minimum of $40 million.