The Secret Service and security officials are considering rerouting Interstate 93 traffic near the FleetCenter during next July's Democratic National Convention, and they might even shut down the new underground southbound lanes.
Convention organizers stressed that no decisions have been made and said that arrangements would be worked out to accommodate commuters and minimize the disruption of the convention. Jean Mitchell, a spokeswoman for the Secret Service, said the proximity of I-93 to the FleetCenter is a concern for the agency, which is coordinating security with Boston police.
"We're trying to secure that area for not only the protectees, but also for the residents of the city," she said. "There will be traffic flow concerns. But we know that the people of Boston have to conduct their lives, so we try not to disrupt them to the extent that's possible."
The southbound I-93 underground lanes, scheduled to open before Christmas, pass just 40 feet from the FleetCenter, where the convention will be held July 26-29.
Earlier this month, authorities who are planning the event said that trucks and other large vehicles would probably be diverted from the Leonard P. Zakim Bunker Hill Bridge before they reach the roadway near the arena.
One convention official who spoke on condition of anonymity said that a total shutdown of I-93 southbound is highly unlikely. It is more likely, the official said, that traffic would be reduced to one lane, with no traffic allowed in the lanes closest to the arena. Another possibility is for traffic to be shut down entirely in the tunnel when the Democratic nominee for president is in the FleetCenter.
Massive traffic adjustments on I-93 would be the second major transportation change in downtown Boston ordered up for security reasons, as planners prepare for the first national political event since the Sept. 11, 2001, terrorist attacks. Security officials are also planning to shut down the North Station MBTA station, which is adjacent to the arena, and have commuter rail trains stop at a temporary platform short of the station.
Jeff Larson, general manager of SmartRoute Systems, a travel and information service, said a massive advertising campaign would be needed to avert chaos in the area of the tunnel if the underground roadway is shut down. At that portion of the interstate, traffic from the north comes off the bridge near where eastbound traffic from Storrow Drive meets I-93.
"It's not just alternate routes," Larson said. "When you have a situation like that, you have to look at other means people will have to come into the city, like commuter rail. It will be warm weather, so maybe some people can pedal in."
Security specialists recommend a 150-foot buffer zone between the building being protected and the first point where an explosives-laden vehicle can have access. Federal buildings have been designed with this in mind.
The northbound lanes of I-93 also fall within that 150-foot radius, but they're not as close as the southbound lanes, and convention officials say they're less of a concern for security officials.
Mitchell, the Secret Service spokeswoman, said no shutdown of I-93 would be ordered without extensive consultations with Boston officials.
Seth Gitell, a spokesman for Mayor Thomas M. Menino, said the mayor could not comment because he had not been briefed on the possibilities.
Julie Burns, executive director of the convention host committee, said she could not comment on any plans because they involve security matters.
She said no proposals for traffic changes have been offered. "No one's come to anyone for a decision or input," Burns aid.
It is not yet clear how traffic would be rerouted. Sean O'Neill -- a spokesman for the Massachusetts Turnpike Authority, which oversees the Big Dig -- said he could not discuss specific plans.
"We would obviously prepare and offer any assistance we could for traffic management, but the details -- we can't discuss at this time," O'Neill said. "We work with the security advisers and act accordingly, if anything is recommended."
Rick Klein can be reached at email@example.com.