Closure of I-93 may hit rush hour

The process of shutting down Interstate 93 during the upcoming Democratic National Convention will begin up to three hours prior to the 7 p.m. start that security planners have announced, state and city officials say, and will disrupt the end-of-the-day commute more extensively than originally believed.

Boston transportation planners and mayors of cities north of Boston expressed relief last month when it was announced that I-93 would close at 7 p.m. on the four nights of the July 26-29 convention, which would allow most commuters to get home before the vital north-south artery was sealed.

But Mayor Thomas M. Menino and top state officials are now warning that closing the interstate will itself take as long as three hours, with some feeder roads and entrance and exit ramps being shut down in stages beginning at 4 p.m., in the very heart of rush hour.

"There's no drawbridge here," said state Public Safety Secretary Edward A. Flynn, who acknowledged that the closure is an elaborate task involving State Police, state highway officials, and Boston and other local police departments. "To get this roadway empty and shut, it's going to take a few hours."

Seth Gitell, a spokesman for Menino, said the mayor expects the impact of such preparations to be felt by 4:30 or 5 p.m.

"It's true that they need a couple of hours to set it up, to put up all the barriers and so forth," Gitell said.

Officials at the Massachusetts Turnpike Authority, which has had to close portions of I-93 for construction dozens of times, said police must close the road one entrance ramp at a time, a process that involves detouring traffic to open roadways and secondary roads that in turn have been clearly marked.

"It happens in stages, and it's a lot of work," said spokesman Sean O'Neill. "You can't just snap your fingers and be done."

The highway, used on a typical weekday by some 200,000 drivers, is being closed for security reasons, because of its proximity to the FleetCenter, site of the convention.

State transportation officials have refused to say how much of I-93 will be closed and thus what ramps and feeder roads will be closed or the sequence in which they will be shut down.

However, it is anticipated that northbound traffic will be diverted through the Ted Williams Tunnel or the Massachusetts Turnpike westbound. The Sumner Tunnel will almost certainly be closed, including its offramp to I-93 north, and motorists will probably use an alternative route to the Tobin Bridge heading north.

Southbound traffic may be diverted at the Leverett Connector, which is the ramp to Storrow Drive and the Tobin Bridge. It is not known where drivers will be allowed to enter I-93 southbound in downtown Boston, if at all.

All drivers and particularly commercial through-traffic will be encouraged to avoid Boston altogether and use Route 128, whether they are headed south or north.

Mayors from north of Boston are especially worried about the impact on their communities, concerned that their local roads may be overwhelmed during the interstate's closure. They were relieved when Boston Police Commissioner Kathleen M. O'Toole told them last month that the shutdown would be from 7 to 11 p.m., according to the latest information she had from the Secret Service. That time frame now is seen as overly optimistic, because it does not account for time needed to set up detours.

"It's going to be a heavy burden on us," said Mayor Joseph Curtatone of Somerville. "We have 65,000 cars a day on McGrath-O'Brien [Highway]. Chances are, if you are coming from Boston points north, you're coming through Somerville, and there's a real public safety concern."

Curtatone said that with a shutdown starting at 4 p.m., depending on which ramps and feeder roads are closed in what order, more commuters will be seeking alternate routes. Police will have to be posted at key intersections, he said, to avoid gridlock and minimize confusion that worses congestion.

"We know the closure will start rolling at a certain time, and it's going to be dynamic," he said. "When the final plan is crafted -- in conjunction with Everett, Cambridge, Medford, and Chelsea -- we'll be able to get a better grasp on it and prepare locally. But we also want some resources. We don't have the manpower, from a public safety standpoint. We don't have the money to pay for that type of overtime."

When asked if Menino was worried about people not being able to commute home from Boston on the four weekday afternoons of the convention, Gitell said a "comprehensive public awareness campaign" will assist all travelers and make the best of the situation.

"The final transportation plan will be completed soon, and as soon as that happens," the public outreach campaign will begin, Gitell said. Menino "is confident the campaign will raise awareness and prepare for this momentous event."

Jon Carlisle, spokesman for the Executive Office of Transportation and Construction, said Highway Department engineers are meeting regularly with Boston 2004 and the Secret Service, which sets the closing time and will lay out the specific road-closing plan.

"It's reasonable to assume that there will be some staging and setup time required," Carlisle said.

Meanwhile, US Representative Stephen F. Lynch, Democrat of South Boston, said he is worried the road closure process itself poses a security problem, in case there is an emergency and large numbers of people need to get out of the city. He has asked transportation and security officials for a briefing to explore the question, a spokesman said.

State officials continue to hope that commuters will stay away from the city the last week of July.

"We're making sure people know: This will be a rugged commuting week," said Flynn. Those who cannot stay away should give themselves plenty of extra time and take mass transit, he said.

The transit option for commuters north of Boston, however, is also problematic. North Station will be closed, and commuter trains will stop at points north of the city and transfer riders to the subway and special buses.

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