Shutdowns set for convention

Interstate 93 will be shut down in both directions for all four evenings of the Democratic National Convention, and North Station will be closed to all commuter rail and subway service, officials involved with security planning said.

The closures are being done at the insistence of the Secret Service, which is in charge of security for the party gathering and will unveil the detailed plan at a press conference today.

State and city officials had hoped for more limited disruption to commuters and had argued that shutting down both the highway and the rail and subway service would create a transportation nightmare the last week of July. But the closure of North Station commuter rail, in particular, became nonnegotiable after the March 11 train bombings in Madrid, according to one of the officials.

The highway, at the Leonard P. Zakim Bunker Hill Bridge and at the northern portal of the Big Dig tunnels, will be shut down because it comes within 40 feet of the FleetCenter. Security planners were concerned that there was no other way to prevent a truck bomb from traveling over the bridge and coming dangerously close to the FleetCenter while the convention was in session.

City and state officials declined to comment on the plans, but one person who has sat in on the security planning discussions over the last several months said, "We're just going to have to discourage people from coming into the city that week."

The Secret Service and Boston police will appear at a briefing on the security and transportation arrangements set for 10 a.m. today. The major precautions that are expected to be unveiled include:

Shutting down I-93 near the FleetCenter for the evening sessions of the convention, from July 26 to July 29. It was not clear last night what time the shutdown would begin. One official said the road would be closed in the late afternoon, but another said that commuters would be allowed to get home in the early evening rush hour, and then the highway would be closed.

Keeping commuter rail trains away from North Station during the convention and requiring approximately 25,000 commuters to disembark north of the city and get on buses or subway lines to finish the trip into the city. They would have to do the same thing in reverse to get home at the end of the day. The Associated Press, citing a source who spoke on condition of anonymity, reported yesterday that North Station will be closed beginning the Friday before the convention and continuing until July 29.

Closing the new North Station subway station at the corner of Causeway and Canal streets to Orange and Green Line service. Orange Line cars will go through without stopping; Green Line trolleys will empty at Haymarket, and passengers bound for North Station, Science Park, and Lechmere will board buses.

The closure of I-93, part of the Big Dig, is expected to be highly disruptive. State officials showed the Secret Service the complicated detours that would result from the highway closure. Traffic may be diverted into the Ted Williams Tunnel to Route 1-A, as well as down the Leverett Connector north of the city to Storrow Drive. The use of Route 128 will also be encouraged.

A compromise that was "in play" was to close I-93 only during the final night of the convention, when the presumptive nominee, Massachusetts Senator John F. Kerry, addresses the convention. At one point, city officials also argued that I-93 should remain open except when Kerry and the vice presidential nominee were in the building.

But the Secret Service considers the event itself to be as likely a target as the candidate, a person involved with convention planning said, and is concerned about the security around the FleetCenter whenever the building is full, typically late afternoon through the evening in convention programming.

The highway is within the jurisdiction of the Massachusetts Turnpike Authority. Officials there declined to comment.

Jon Carlisle, spokesman for state Transportation Secretary Daniel A. Grabauskas, would say only: "This is the Secret Service's show at this point. We have to defer to the folks with the earpieces and the microphones in their sleeves." Joe Pesaturo, spokesman for the Massachusetts Bay Transportation Authority, declined comment.

Seth Gitell, spokesman for Mayor Thomas M. Menino, would not comment on any of the plans, and directed all questions to Boston police. Police Commissioner Kathleen M. O'Toole met yesterday with Secret Service representatives.

The transportation security plans, as well as a flurry of transit and highway construction projects that will be going on through July, have raised concerns that Boston will be a traffic and transit nightmare during the convention week. Governor Mitt Romney suggested switching the venue to the new convention center in South Boston, but that plan was rebuffed as unworkable by Democratic convention planners.

Mac Daniel and Rick Klein of the Globe staff contributed to this report. Anthony Flint can be reached at flint@globe.com.