I-93 loses 3d lane to convention

Transportation planners, worried about gridlock during the Democratic National Convention, plan new restrictions on Interstate 93 as part of an effort to persuade drivers to use Route 128 instead.

Planners had already announced that the 6-mile stretch of I-93 that snakes through downtown past the FleetCenter will be closed from approximately 4 p.m. to 1 a.m. each day during the convention. Under the restrictions detailed yesterday, only two lanes of traffic will be allowed on the portions of the interstate north and south of the city that will remain open.

In addition, if the congestion gets too heavy, a longer stretch of the interstate will be shut down altogether.

"We're restricting it down to two lanes because if, God forbid, nobody pays attention to us and that road turns into a parking lot, we'll have fewer cars to worry about," said State Police Major Michael Mucci, who is coordinating the convention-related road closures that have been prompted by security concerns.

The restrictions could set off more anxiety, as commuters and planners alike worry about logjams that could result during the four-day party gala, which begins July 26. The traffic closures are among the most controversial of the security measures adopted for the convention, with some residents calling them an overreaction and others confused by the detours and changes and fearful that traffic in the region will be overwhelming.

Transportation planners say their goal is to reduce by half the volume of cars and trucks on I-93, which typically carries some 8,500 vehicles an hour. But officials are also trying to prevent local roads, especially around Medford, from being overwhelmed by traffic exiting I-93 at the last possible departure points closest to Boston. Mayors in the close-in communities have expressed concern that their streets will be so clogged by drivers exiting I-93 that emergency vehicles won't be able to get through.

Mariellen Burns, spokeswoman for the convention planning group coordinating security and transportation, said, "We're not going to let any community get gridlocked."

Travel was already going to be more difficult for commuters on I-93 during the convention, because one lane north and south of the city will be limited to emergency and bus travel.

In addition, from about 4 p.m. to 1 a.m. on convention days, State Police said yesterday, they will reduce traffic on I-93 southbound from three lanes to two along a 5-mile stretch from Route 128 in Woburn to Medford -- the last operative exit, where southbound traffic will be diverted from the highway.

During those same hours, traffic on I-93 northbound from Route 128 in Braintree to the Frontage Road-Massachusetts Turnpike exit in Boston will also be squeezed from three lanes into two. If bottlenecks develop at the final exits, State Police may force cars to exit I-93 farther from Boston.

"The volume of the traffic is the key to the whole thing," Mucci said. "We have the basic structure of the plan, but there's a lot of fluidity to it. If people think it's just another Monday, and they do everything they normally do on any Monday, we're in a lot of trouble."

South of the city, Mucci said, State Police will be monitoring traffic flow at Exit 20, where drivers can go either east or west on the Massachusetts Turnpike (Interstate 90), or take Frontage Road to downtown. If gridlock develops, troopers will divert traffic off I-93 further south, possibly at Columbia Road, where drivers can be turned around and sent back southbound on I-93, Mucci said.

North of the city, planners are setting up another release valve, so that Route 60 in Medford is not overrun. Officials have long planned to divert all traffic at Exit 32 in Medford, where drivers have the option of going east or west on Route 60. To relieve some pressure there, local traffic will be allowed to travel in one lane to Exit 31 at Route 16, a modification made at the request of Medford officials.

Similarly, if Route 60 east or west becomes gridlocked and local emergency vehicles can't circulate, Medford officials will be authorized to shut down the diversion at Exit 32. In that case, traffic will be forced off I-93 one exit north, at Exit 33 at Roosevelt Circle.

The traffic will be carefully monitored by the Operations Control Center for the Big Dig in South Boston. Police will use cameras, but will also count on hundreds of state troopers, "who will be able to see one another, they will be so evenly spaced," Mucci said. "We will have instant information to make all decisions."

Mayor Michael J. McGlynn of Medford said he was relieved there would be lane restrictions on I-93 and flexibility built into the traffic-diversion plans.

"What this does is, it eliminates the gridlock, which is the big fear that I have had," said McGlynn, who met with State Police yesterday. "With gridlock, you have problems dispatching public safety vehicles."

McGlynn said he was confident that State Police, in coordination with the Massachusetts Highway Department, would monitor traffic carefully starting at 4 p.m., and close exits and options if the situation deterioates.

"They're going to have a command center, and they will have the ability to shut down a ramp that's creating gridlock, and that's what we have sought all along," he said. "They are going to have 165 troopers from other states and put them 400 yards apart, and additional troopers will be posted throughout the city. Now we have a plan that will be successful for all of us."

Mucci said the ultimate impact of the revised plan is twofold: to give drivers more options, but to take those options away if there are huge traffic jams. Allowing local access to Route 16, for example, relieves pressure on Route 60. But if that option gets overused, it will be taken away.

The plan calls for Medford officials to watch for gridlock in Medford Square, as drivers take Route 60 west. If an unmanageable traffic jam develops, drivers will be barred from traveling west on Route 60 and will have to take Route 60 east. If Route 60 east backs up, drivers will be sent back onto I-93 north.

"If both ways on Route 60 get gridlocked, we'll close off [I-93] at Roosevelt Circle," Mucci said.

Mucci said he was hoping traffic volume during convention week would be about 50 percent of the usual volume and that 40 percent would be even better.

He said police will monitor the road closings very carefully, and if less time is needed to clear the highway in front of the FleetCenter by 7 p.m., the closures may all start a little later in the day.

"We're going to hold off the closures for as long as possible," he said.

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