In early 1990s Boston, traffic on the city's Central Artery -- the main highway through the city -- backed up 10 hours a day and cost the local economy $500 million annually [source: Mass. DOT]. In response, the Central Artery/Tunnel Project -- or Big Dig -- was launched in 1991 to replace the six-lane highway with an underground road of eight to 10 lanes.
The project, the most expensive construction scheme in U.S. history, involved the erection of several other major bridges, roads and tunnels, one of them going under Boston Harbor. It was originally supposed to be finished in 1998 for $2.6 billion but wasn't complete until 2007. By then, the price tag had ballooned to $14.8 billion [source: LeBlanc]. But with the interest due on borrowed funds -- which will be paid through 2038 -- the Big Dig's real cost has been estimated at $22 billion [source: Murphy].
While traffic has indeed sped up in downtown Boston, and the city itself looks more attractive, critics say the high cost (borne by taxpayers) has meant little money to repair other aging road and bridges. Moreover, traffic itself has now increased in areas outside the urban core [source: Murphy].