The rebuilt Longfellow Bridge will shrink to a single traffic lane bound for Cambridge, keep two headed for Boston, and gain wider bike lanes and sidewalks in each direction as part of a new design released to the public yesterday.
For the 105-year-old Longfellow, that plan, filed with federal regulators last month, reverses some 1950s-era changes that recast one of the state’s best-known bridges to allow for faster driving. It represents a win for advocates of greener travel and shared streets.
“These are public ways, so we want to make sure we’re accommodating all public users and not just being vehicle-centric,’’ said Frank DePaola, the state’s highway administrator.
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.
The project will restore the grandeur of a dingy, rusting bridge that formally opened to fireworks in 1907 as the marquee event in Boston’s Old Home Week, several months after traffic first crossed. At the time, the bridge’s profile was the same, but the travel surface was different - the inbound and outbound lanes were cobblestone and carried cars, horses, and trolleys; the sidewalks were wider; and the center lane was empty in anticipation of the 1912 addition of rapid transit.
The 21st century reconstruction project proved initially thorny for officials torn between moving as swiftly as possible - and advancing plans to rebuild the bridge as is - and complying more fully with changing state and federal policies aimed at encouraging walking, biking, and transit use for environmental and public health reasons.
Those two goals were at odds because the Longfellow’s landmark status meant it could not be widened over the river to provide ample room for everyone.
The state pulled back on its Longfellow plans in 2010 and convened a 36-member task force that included bike, pedestrian, and environmental advocates, neighbors, and civic and business leaders, whose input contributed to the new design.
“One of the breakthroughs of the task force was to treat the inbound side of the bridge and the outbound side of the bridge differently,’’ said state Representative Martha M. Walz, a Democrat whose district includes the bridge’s Boston approach and part of its Cambridge approach.
Fellow task force member Richard A. Dimino said the plan addresses contemporary needs while respecting the history of the bridge. “They’ve made exceptional efforts to ensure that the historic character of the bridge will be preserved, and obviously it’s a landmark bridge,’’ said Dimino, president and chief executive of A Better City, which represents hospitals, universities, financial firms, and other major employers on regional transportation planning.
Longfellow vehicle traffic dropped 20 percent after the completion of the Big Dig and opening of the Leonard P. Zakim Bunker Hill Memorial Bridge improved nearby Interstate 93. By a quirk of local traffic patterns, more Longfellow vehicles head toward Boston than toward Cambridge in morning and evening.
Some advocates who had called for even more substantial improvements for bikes and pedestrians - and fewer car lanes - stopped short of celebrating.
“We’re really excited that the project is moving forward. It’s been such a long time coming. . . . But we’re not sure about the details yet,’’ said Jackie Douglas, executive director of the LivableStreets Alliance, which coordinated a meeting yesterday to begin reviewing the 150-page filing with advocates from bike, pedestrian, environmental, and disability-access groups.
The state will hold a hearing on the plans, officially known as the environmental assessment, on March 1 from 7 to 9 p.m. at the State Transportation Building in Boston.
Contractors since 2010 have completed $18 million in preliminary work, including utility relocation, masonry cleaning, and steel inspection.
In a sign of faith that the plans will receive federal approval, the state has issued a call for combined teams to finish design, engineering, and construction - instead of separate, consecutive contracts - and intends to award the bid within six months, DePaola said.
That remaining work is estimated at $270 million and will include a new connecting footbridge to the Charles over Storrow Drive.
The total exceeds $300 million when preliminary work and earlier environmental planning and design are included, he said.
Plans call for moving the Longfellow’s exterior wall a few feet on the inbound side of the Boston bank to preserve three lanes at Charles Circle while accommodating a bike lane and wheelchair-accessible sidewalk.
Eric Moskowitz can be reached at email@example.com.