Claimed to be 'the most complex, ambitious and technically challenging highway project ever undertaken in the US', Boston's £4.9bn Central Artery/Tunnel project is designed to cure a massive traffic headache and make the historic city a more pleasant place to live.
When the elevated Central Artery road opened in the 1950s its dual three lanes could handle peak traffic flows of around 75,000 vehicles a day. Four decades on, flows are touching 200,000 a day, accident levels are four times the national average and the 'rush hour' is often 10 hours long. Congestion on this scale is costing the city an estimated £300M every year. And the decaying steel structure of the Central Artery forms an awkward barrier between the centre of Boston and its reviving waterfront districts.
The problem for the project's designers is that the Central Artery is not only in the middle of a busy city, it also lies right at the centre of one of the busiest transport crossroads in America. Linking the north/south Interstate 93 to the east/west I-90 Massachusetts Turnpike, the road also takes much of the traffic heading to and from Logan International Airport to the east.
Local traffic squeezes on and off the Artery via a multiplicity of viaducts and ramps, whose close proximity is one of the major factors in the appalling accident rate. A further complication came from the two ageing tunnels beneath Boston Harbour, whose poor capacity meant that airport-bound traffic often backed up on to the Artery and turned the 1.5km journey from central Boston to Logan into a 60 minute crawl.
The latter problem was partially solved four years after the whole CA/T project was launched in 1991, when the new Ted Williams tunnel was opened. Even though plans for the new road links to the tunnel are still on the drawing board it has already helped to slow down the rate of increase in congestion on the Artery itself.
But the options facing the planners were otherwise very limited. Local geography ruled out any major re-alignment of the main interchanges. Even minor re-alignments would be extremely complicated, given the congestion of the city centre and the proximity of working surface and underground railways. And any widening of the existing elevated sections would still leave a massive barrier through the city centre.
So a radical solution was adopted. The Central Artery would be 'pushed into the ground' over a 2.5km length in the city centre. A complicated new interchange, part on viaduct, part in tunnel, would link the I-90, I-93 and the new road to the Ted Williams tunnel. To the north, a landmark cable stay bridge would carry the I-93 over the Charles river. Then the Artery would be demolished and replaced with parks and tree-lined boulevards.
By 2004 the city should have a new dual four and five lane artery capable of handling up to 250,000 cars a day with peak periods restricted to two hours morning and evening. With fewer access ramps and more separation between local and through traffic, accident rates are predicted to drop to normal levels. By then inflation will have bumped up the total project cost to a predicted £6.7bn - which, given the alternative, Bostonians are likely to agree will be money well spent.