The Big Dig: project background

Background information about the Central Artery Tunnel Project

Table of Contents


The Central Artery/Tunnel Project was the largest, most challenging highway project in the history of the United States. It reduced traffic and improved mobility in one of America's oldest, most congested major cities. It built a framework for continued growth in Massachusetts and New England. Additionally, it improved the local environment.

The project replaced Boston's deteriorating six-lane elevated Central Artery (I-93). It did so with an underground highway that is state of the art and two new bridges over the Charles River. It also extended I-90 to Boston's Logan International Airport, and Route 1A. This project created more than 300 acres of open land while reconnecting downtown Boston to the waterfront.

When planning for the CA/T Project began in 1982, experts could not have predicted the challenges that lay ahead from design to construction. Congress approved federal funding and the project's basic scope in April 1987.

Construction began in September 1991 on a bypass road through South Boston. This took truck traffic off neighborhood streets. A third tunnel to cross Boston Harbor was also begun. The first major milestone was the opening of the tunnel. Named for baseball legend Ted Williams, majority completion occurred January 13, 2006. Three major milestone openings took place in 2003.

The Problem

Boston, Massachusetts had a world-class traffic problem called the Central Artery. The Central Artery was an elevated highway running through the center of downtown. When it opened in 1959, the Central Artery carried about 75,000 vehicles a day. In the early 1990s it carried upwards of 200,000 making it one of the most congested highways in the United States.

Traffic crawled for more than 10 hours each day. The accident rate on the Central Artery was four times the national average. The two tunnels between downtown Boston and East Boston/Logan Airport had the same issue. Without major improvements, Boston expected a traffic jam for up to 16 hours a day by 2010.

The annual cost to motorists from this congestion was an estimated $500 million. Costs included a high accident rate, wasted fuel from traffic, and late deliveries.

And traffic wasn't the only problem the old Central Artery caused in Boston. The elevated highway displaced 20,000 residents. It also cut off the North End and Waterfront neighborhoods from downtown. This limited these areas' ability to take part in the city's economic life.

The Solution

This extraordinary traffic mess was a drain on the economy and quality of life of Boston and New England. The solution was the Central Artery/Tunnel Project. A massive undertaking constructed under the purview of the Massachusetts Turnpike Authority.

The project's two major components:

Replacing the six-lane elevated highway with an underground expressway beneath the existing road. This finished at its northern limit in a 14-lane, two-bridge crossing of the Charles River. After the underground highway opened to traffic, demolition of the crumbling elevated artery. Finally, in its place, open space and modest development.

The extension of I-90 (the Massachusetts Turnpike) from south of downtown Boston. It now goes through a tunnel beneath South Boston and Boston Harbor to Logan Airport. The first link in this new connection - the Ted Williams Tunnel under the harbor - finished in December 1995.

The Challenges

Putting this underground was one of the more challenging infrastructure projects ever in the United States. The project spanned 7.8 miles of highway, 161 lanes miles in all, about half in tunnels. In total, the CA/T placed 3.8 million cubic yards of concrete. This is equal to 2,350 acres, one foot thick. It also excavated more than 16 million cubic yards of soil. The larger of the two Charles River bridges is a ten-lane cable-stayed hybrid bridge named the Leonard P. Zakim Bunker Hill Bridge. It is the widest ever built and the first to use an asymmetrical design.

The project also included four major highway interchanges. These connect the new roadways with the existing regional highway system. At Logan Airport, a new interchange moves traffic from I-90 and Route 1A, as well as onto the airport road system. In South Boston, another takes traffic between I-90 and the waterfront and convention center area. At the northern limit, a new interchange connects I-93 to the Tobin Bridge, Storrow Drive, and the new underground highway.

At the southern end of the underground highway, the interchange between I-90 and I-93 was completely rebuilt. On six levels, these connect with the underground Central Artery and the Turnpike extension through South Boston. The interchange carries a total of 28 routes. These include High Occupancy Vehicle lanes, and channel traffic to and from Logan Airport to the east. A fifth interchange, at Massachusetts Avenue on I-93, has been rebuilt by the project.

The Central Artery/Tunnel Project was public works on a scale comparable to some of the great projects of the last century. Projects like the Panama Canal, the English Channel Tunnel (the "Chunnel") and the Trans-Alaska Pipeline. Each of these projects presented unique challenges. The Panama Canal confronted earth-slides, malaria, yellow fever, and Central American jungles. The Chunnel was dug from either end, 31 miles apart, meeting at a precise point under the channel floor. The Alaska Pipeline contended with vast distances, freezing temperatures, and major environmental concerns.

The Central Artery Project had a unique challenge: to construct this project in the middle of Boston without crippling the city. Work of the CA/T Project's scope was never before attempted in the heart of an urban area. Unlike any other major highway project, the CA/T worked to maintain traffic capacity and access to residents and businesses. Keeping the city open for business throughout construction was a priority. When the interstates were first built, projects gave very little thought to the communities in the path of the new roads. Disruption and dislocation were the rule of the day.

Failure to maintain the state of Boston's economy during the project would damage our competitive position for years. With that in mind, project planners worked with many organizations, groups, and leaders to create consensus on how to build the project. The process of keeping the city open and making certain that everyone sees fair treatment is mitigation. Mitigation efforts were responsible for more than a fourth of the project's budget.


With the improvements and delay reductions, total vehicle hours on project highways dropped 62 percent from 1995 to 2003. These improvements are now providing around $168 million per year in time and cost savings to travelers. Residents' average travel times from the I-90/I-93 interchange to Logan Airport during peak periods have dropped from 42 to 74 percent. A 12 percent reduction in citywide carbon monoxide levels was also achieved. Economic and transportation benefits are also detailed in this report.

Along with improved travel through downtown, neighborhoods cut off by the old highway could reconnect. This improved the quality of life in the city beyond the limited confines of the new expressway. Clay and dirt from the project went to fill and cap landfills throughout New England. This includes the former city dump at Spectacle Island.

Parks and Open Space

The Central Artery/Tunnel Project created more than 45 parks and major public plazas. It completed shoreline restoration in the Charles River Basin, Fort Point Channel, Rumney Marsh, Spectacle Island and large stretches of the Boston Harborwalk. Visitors enjoy the 100-acre Spectacle Island's park and pathways. In 2006, the City of Boston and the Department of Conservation and Recreation took charge of Spectacle Island and its facilities.

The project built a series of parks with water features and other amenities in the path of the old elevated Central Artery. From Chinatown through the Wharf District and North End, this is now known as the Rose Kennedy Greenway. Created in 2004, the non-profit Rose Fitzgerald Kennedy Greenway Conservancy is its steward.

Accessing the Greenway is a new tree-lined boulevard in Boston's downtown corridor. This boulevard includes miles of new and refurbished sidewalks, close to 900 trees with irrigation and many plazas. Other parks and new landscaping along the Charles River Basin and in East Boston have transformed the city. While most of the 30 downtown acres will remain as open space, 25% is set aside for modest development. This includes retail, commercial, and housing uses in low-rise buildings.

The Central Artery/Tunnel Project was first owned and managed by the Massachusetts Highway Department. It was then run by the Massachusetts Turnpike Authority (MTA), and was part of the Metropolitan Highway System (MHS). Bechtel/Parsons Brinckerhoff (B/PB) provides design and construction management consulting . This is a joint venture of Bechtel Corporation of San Francisco and Parsons Brinckerhoff Quade & Douglas, Inc., of New York.

Central Artery/Tunnel Project Milestones

1982 Work begins on Final Environmental Impact Statement/Report (FEIS/R).
1985 Final Environmental Impact Statement/Report (FEIS/R) filed; approved early the next year.
1986 Bechtel/Parsons Brinckerhoff begins work as management consultant.
1987 Congress approves funding and scope of project.
Building acquisition and business relocation process begins (no private homes taken).
1988 Final design process under way.
Exploratory archaeology digs begin.
1989 Preliminary/final design and environmental review continue.
1990 Congress allocates $755 million to project.
1991 Federal Highway Administration issues Record of Decision, the construction go-ahead.
Final Supplemental Environmental Impact Statement/Report (FSEIS/R) approved.
Construction contracts advertised and awarded.
Construction begins on Ted Williams Tunnel and South Boston Haul Road.
1992 More than $1 billion in design and construction contracts under way.
Dredging and blasting for the Ted Williams Tunnel ongoing.
Downtown utility relocation to clear path for Central Artery tunnel construction begins.
Archaeologists find 17th and 18th century artifacts at a North End dig.
1993 South Boston Haul Road opens.
All 12 tube sections for Ted Williams Tunnel placed and connected on harbor floor.
1994 Charles River Crossing revised design and related FSEIS/R approved.
New set of loop ramps open in Charlestown.
1995 Ted Williams Tunnel opens to commercial traffic.
1996 Downtown slurry work under way for I-93 tunnels.
1997 Utility work 80 percent complete.
1998 Enter peak construction years.
Construction begins on the Charles River Crossing.
1999 Construction 50 percent complete.
New Broadway Bridge opens.
Leverett Circle Connector Bridge opens.
2000 Close to 5,000 workers employed on the Big Dig.
2001 Construction 70 percent complete.
2002 Leonard P. Zakim Bunker Hill Bridge completed.
2003 I-90 Connector from South Boston to Rt. 1A in East Boston opens in January.
I-93 Northbound opens in March.
I-93 Southbound opens in December.
2004 Dismantling of the elevated Central Artery (I-93).
Opening of tunnel from Storrow Drive to Leverett Circle Connector, providing access to I-93 North and Tobin Bridge.
2005 Full opening of I-93 South.
Opening of Dewey Square Tunnel, including new entrance and exit ramps.
Opening of the two cantilevered lanes on Leonard P. Zakim Bunker Hill Bridge.
Opening of permanent ramps and roadways at I-90/I-93 Interchange and in other areas.
2006 Reached majority completion of the Central Artery/Tunnel Project in January.
Spectacle Island Park opens to the public.
2007 Restoration of Boston city streets.
Continued construction of the Rose Kennedy Greenway and other parks.
Construction on development parcels continues after Central Artery/Tunnel Project completes. 

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