Toward the end of the twentieth century, Boston had traffic problems as severe as any city in the world. When the elevated six-lane Central Artery Highway, which ran through the downtown area, was opened in 1959, it quite easily coped with 75,000 vehicles a day; by the early 1990s the traffic load had increased to 190,000—effectively more cars per lane than any other urban interstate road in the United States. Movement was slowed to a snail’s pace for over ten hours each day, and the accident rate was four times the national average for similar thoroughfares. Moreover, the urban area was divided by the elevated road so that access between the north and south sectors was greatly restricted. Naturally, the same congestion characterized the two tunnels under Boston Harbor that joined downtown Boston with East Boston and Logan Airport; the airport, only 1 mile (1.6 kilometers) from the central business district, was an hour away by road!
The $10.8 billion Central Artery/Tunnel Project (CA/T), conceived in 1981 and under construction as of 2001 by the Massachusetts Turnpike Authority, deserves a place among the engineering marvels of the modern world. Referred to by Bostonians as the “Big Dig,” it is the largest, most complex highway project ever initiated in a U.S. city—indeed, the largest public works project of any kind in the United States. Scheduled for completion in 2004, the project faces all the challenges associated with building in the heart of a busy city: that is, to meet the continuing demands of traffic capacity, to make sure that life and business are not unduly disrupted over a construction period lasting thirteen years, and to satisfy environmental and esthetic standards.
The spine of the multifaceted project is an eight-lane underground expressway directly under the existing road; in places its roof is at ground level, and at its deepest point it is 120 feet (36 meters) below ground, resting on bedrock. Tunneling was made especially difficult by the fact that there are four distinct soil types beneath Boston. Much of the downtown area is built on landfill placed at various times between the late eighteenth and the mid-twentieth centuries. Under the fill is a layer of mixed silt, sand, and peat, and below that the marine clay known as Boston blue on the bedrock. The demolition of the old elevated road releases about 27 acres (10 hectares) of open space for a linear park in the center of the downtown and for the construction of new city streets connecting North and South Stations; and existing cross streets, cut off since 1959, will be reconnected. Other advantages spring from the project, including a predicted 12 percent reduction in carbon monoxide levels and the creation (by using the spoil from excavations) of 105 acres (42 hectares) of open space at Spectacle Island in Boston Harbor and 40 acres (16 hectares) of new parks on the riverbanks below two new bridges.
The Central Artery rises to the surface at Causeway Street on the northern edge of Boston and crosses the Charles River on a 1,407-foot (42-meter), ten-lane, asymmetrical, cable-stayed bridge designed by the Swiss engineer Christian Menn. The bridge, constructed at a cost of $87 million, is the widest of its kind in the world. The Charles River Bridge links with National Route 1 and local access roads. The project also included a parallel, 830-foot (250-meter) four-lane bridge, also for local traffic, which was opened in October 1999. The Massachusetts Turnpike has been extended to Boston’s international airport via a new tunnel connected to the four-lane Ted Williams Tunnel under Boston Harbor; the new tunnel was opened to commercial traffic in December 1996. Four highway interchanges will eventually connect the new roads with the regional system.
The part of the program that caused most local concern was the crossing of the Charles River. A proposal in August 1989, after construction had started on the Central Artery North Area Project, included three bridges, with a large area of the north shore being occupied by connecting ramps. Marshaled by the press (the Boston Globe dubbed the scheme a “grotesque monstrosity”), local residents, environmentalists, and even public servants opposed this design—to the point of litigation—on the grounds that it would “overwhelm their neighborhood with visual blight, shadows, noise and air pollution.” A Bridge Design Review Committee (BDRC), appointed in January 1991, next produced an alternative plan that was not finalized until September 1992. Although this proposal won an Urban Design Award from the American Institute of Architects, the state rejected it, doubtless under political pressure. Instead, it prepared its own new plan, with two bridges and an underground ramp on the south shore; it was approved by state and federal environmental agencies in June 1994. Objections continued until 1997, when the U.S. District Court found that all necessary conservation measures had been taken. Construction began on the bridges in 1999.
The project has been under construction since late 1991. The extension of Interstate 90 through South Boston to the Ted Williams Tunnel and the airport was scheduled to open in 2001. The northbound lanes of the underground highway through downtown Boston will follow in 2002, and the southbound lanes in the following year, allowing the demolition of the elevated highway and the creation of the landscaping to be carried out by 2004. The federal government will meet about 70 percent of the cost, and the Commonwealth of Massachusetts the remainder. The Central Artery/Tunnel Project is owned and managed by the Massachusetts Turnpike Authority. Design and construction management was provided by the Bechtel-Parsons Brinckerhoff consortium.