Driving

They're calling it the new rush hour -- the sudden flood of vehicles expected to gush out of Boston between 2 and 3 p.m. during the Democrats' convention in anxious anticipation of the 4 p.m. shutdown of Interstate 93.

For a city in love with its autos and its routines, the early exit is one in a series of changes and challenges anticipated during the party gathering, as commuters confront detours, lane closures, and the potential for blockbuster gridlock.

Transportation planners have said they need to reduce traffic to Boston during convention week by at least 50 percent and have been spreading the word that significant delays should be expected for anyone who insists on driving into or out of the city.

While much of the attention has been on the planned afternoon shutdown of I-93, the morning rush hour will be tricky, too, in part because of numerous lane restrictions.

On I-93 and the Massachusetts Turnpike, lanes will be designated for emergency vehicles and buses only, leaving less room for other traffic.

The morning commute on the Southeast Expressway, for example, is predicted to be tough starting the Friday before the convention when the Interstate drops from five lanes to three to create the dedicated lanes. The lane drop is scheduled to end Friday, July 30.

During that same period, I-93 south from Woburn to Medford will drop from four to three lanes, and there will be no HOV lane on I-93 north or south.

In the afternoon, the ride home could be more complicated, with the lack of certainty about exactly when I-93 will be shut down.

Mariellen Burns, the convention's planning committee spokeswoman, said traffic reporters at local television and radio stations will be told when the road shutdown has begun, but there will be no advance notice.

The closures will begin first with the shutting off of onramps in Braintree, Milton, Quincy, and Woburn and move slowly towards the city. During the shutdown, the number of travel lanes also will be reduced.

Drivers should try to avoid the final exits that will remain open -- Exit 20 (to the Massachusetts Turnpike and Logan Airport) on I-93 north and Exit 32 (Route 60 in Medford) traveling south are expected to be major bottlenecks.

Alternate routes to avoid the interstates or the Pike are also expected to be clogged.

Somerville and Medford officials have said they will shut down access to their local roads if traffic gets too heavy. Route 28 in Stoneham and Medford, Route 60 in Medford, and Route 16 in Medford, Everett, and Chelsea are expected to be hotspots.

Drivers are encouraged to monitor traffic reports closely. Updated information can be found at www.smartroutes.com or by dialing *1 on a cellphone.

Traffic planners urge motorists to avoid Boston as much as possible, and to use the loop roads -- Route 128, Interstate 95, and Interstate 495 -- to get around the city.

Those diversions carry risks, too. A rollover accident on Route 128 south is one thing on a normal day, quite another when thousands more cars are using the loop.

A rollover on I-93 south at 3 p.m., on the other hand, could be a potential nightmare.

''The problems that will come up is very honestly stuff . . . that you can't plan for that happen everyday," said Burns. ''Those are going to be the things that are toughest to deal with."

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