The Longfellow now carries roughly 25,000 cars each weekday - over two lanes in each direction, widening to three where it approaches Charles Circle - and a few thousand foot and bicycle travelers, despite sometimes narrow and even interrupted bike and foot paths. The T’s Red Line, which rumbles down the center of the bridge, carries about 100,000 riders on weekdays between the Kendall and Charles/MGH stations.
Engineers say that two lanes, widening to three, continue to be needed at Charles Circle on the Boston side because of the complexity of that intersection, while a single lane is sufficient to carry vehicles toward Cambridge, where bridge traffic is unimpeded. Reducing outbound traffic to a single lane - and slightly narrowing the two inbound lanes - frees up more space for bike lanes and pedestrians.
Major work on the $300 million project is likely to begin in a year, with plans calling for the bridge to be closed to Cambridge-bound traffic for much of the three years of construction. Officials expect drivers to head to the Craigie or other nearby bridges.
The Longfellow’s distinctive but tilting “spice-shaker’’ towers, which prompt some to call it the “Salt and Pepper Bridge,’’ will also disappear for part of that time, to be dismantled brick by brick and then be rebuilt around sturdy steel frames.
Weekday Red Line service will not be interrupted, though some weekend busing will be required, officials said.
The Longfellow is a centerpiece of the state’s $3 billion Accelerated Bridge Program, aimed at repairing or replacing more than 200 deteriorated spans by 2016. It was prompted by a fatal bridge collapse in Minnesota in 2007.
