Dig's next squeeze may be tightest yet

Just as motorists started getting used to the new southbound Big Dig, project officials are planning what could be the most aggravating detour in its history: a detour at the Dewey Square tunnel, which is being renovated.

The change could bring thousands of cars off Interstate 93 and onto Surface Road through Chinatown, a prospect that city officials worry will overwhelm the area.

Starting at the end of March or the beginning of April, southbound I-93 traffic will be split, with two lanes squeezed right into the west side of the Dewey Square tunnel. They will then be able to continue on I-93 south or veer off to Interstate 90 west, as they can now.

Drivers will also have the option of taking a single, left-hand lane, which will bring them to Exit 22 and onto Surface Road through Chinatown and then rejoin I-93 south at Kneeland Street. Project managers say that is a relief valve for through traffic.

"Everything the project has done in terms of detours has been like pulling wisdom teeth; it hurts, but we've managed it," said Matthew J. Amorello, chairman of the Massachusetts Turnpike Authority, which oversees the $14.6 billion Big Dig. "This may be more like a root canal."

Amorello called it the "last inconvenience" in the over-budget, behind-schedule project.

In about 10 months, when the Dewey Square tunnel is renovated and linked to the new Big Dig tunnel, the project will be almost entirely done, and drivers will have the run of five lanes through the one-third-mile stretch.

The reason for the temporary detour is that the Dewey Square tunnel, built in 1959 after officials decided to bury the Central Artery through Chinatown, is being readied for reuse. The tunnel has to be gutted and brought up to modern highway standards, which call for gentler curves and a slight, racetrack-like banking in the roadway to accommodate higher speeds.

Traffic must move through the tunnel while it is being worked on, said project director Michael Lewis. So the solution was to work on one side of the old tunnel, which had a total of six lanes, and then the other, he said. It is an $86 million job, and the contractor is Modern Continental.

Southbound I-93 traffic now is being routed in three lanes through the east side of the Dewey Square tunnel, the side formerly used for northbound traffic. Northbound drivers are using the new I-93 tunnel under Atlantic Street that opened in March.

Now that the west side of the Dewey Square tunnel has been completely refurbished, traffic is being moved there, so that the east side can be worked on.

Big Dig officials are worried that big backups will result from southbound traffic squeezing into two lanes, jams that may stretch all the way to the Leonard P. Zakim Bunker Hill Bridge, the Tobin Bridge onramps, and the lower deck of I-93.

That is why they planned "a relief valve," said Big Dig traffic manager Charles Sterling, giving drivers the option of taking the surface route through Chinatown. But they will have to decide early which way to go in the new southbound I-93 tunnel, because a Jersey barrier will extend about 700 feet before the split.

Those electing to take the surface detour will head up the existing Exit 22 offramp, travel south on Surface Road, and cross Kneeland Street to rejoin I-93 south. They can also reach the Mass. Pike (I-90) west and Albany Street, which gives them another chance to get on I-93 south via an onramp near Traveler Street.

There will be three traffic lights, but they will be synchronized so they are green at the same time, allowing traffic to pulse through, Big Dig officials said. If they hit a red light, drivers will have to wait 50 seconds at most, officials said.

James Gillooly, the city's liaison to the Big Dig project, said that he doesn't want drivers heading southbound on I-93 to use the surface detour.

"People will be better served staying in the tunnel," he said. "You will have to be a little more patient, but you will get through."

The Surface Road is primarily for local traffic or for people downtown or in the Financial District to get to I-93 south at Kneeland Street, he said.

Chinatown will have to put up with several more months of heavy use of Surface Road, Gillooly acknowledged, but the payoff will come when the project is finished and the street is made narrower, allowing for more sidewalks and park space.

In addition to increased difficulty for pedestrians trying to cross a road being used as an interstate detour, the intersection of Surface Road and Kneeland Street is another potential gridlock spot and source of confusion. The entrance ramp to I-93 will only be for drivers using the Surface Road detour; no one on Kneeland Street will be able to use it.

That means drivers on Kneeland must ignore the signs for I-93 south and instead use the entrance ramp for I-90 west and then bear left onto Albany Street and use the Traveler Street onramp to I-93 south.

Trying to devise the Dewey Square tunnel rehabilitation has been giving Big Dig project managers fits for years. One earlier plan that was abandoned called for a viaduct to be built through Chinatown along Albany Street, which would have emptied the old tunnel completely, so it could be refurbished.

When the Dewey Square tunnel is finished, sometime around February 2005, the Big Dig will be close to completion.

There will be four lanes on the Zakim Bridge and five lanes in the three-quarter-mile southbound tunnel. At Dewey Square, those headed to westbound I-90 will use a two-lane collector on the west side of the refurbished tunnel, though they still will have the option to continue south on I-93. Those headed south of the city on I-93 can use three lanes on the east side of the renovated tunnel. Project officials say the net effect will be a wide-open artery from the Zakim Bridge to the Southeast Expressway.

Anthony Flint can be reached at flint@globe.com.