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.