STATE TRANSPORTATION Secretary Stephanie Pollack embraced a new all at-grade proposal for the so-called throat section of the I-90 Allston interchange that locates a bike and pedestrian path and a tiny portion of Soldiers Field Road in the Charles River.

The latest at-grade proposal, backed by the city of Boston and the business group A Better City, cleared a major hurdle when Pollack approved it as one of three alternatives to be reviewed by state and federal regulators. One of the three alternatives will be selected as the preferred alternative in the coming weeks.

Backers of the at-grade approach say momentum is building for their design, largely because it doesn’t erect an elevated highway as a wall between BU and the river and provides a large bike and pedestrian path and a refurbished river bank.

“Both A Better City and the city of Boston see this as a victory,” said Rick Dimino, the president and CEO of A Better City. “We see no reason why it shouldn’t move forward as the preferred alternative.”

The throat is the narrow section of land between Boston University and the Charles River where all of the deteriorating transportation infrastructure – four rail tracks, eight lanes of the Massachusetts Turnpike, and four lanes of Soldiers Field Road – needs to be rebuilt. The throat is part of a $1 billion project to straighten the Turnpike and clear the way for a new neighborhood being built by Harvard University.

The supports for the Massachusetts Turnpike viaduct are in tough shape.

The options under consideration include basically a redo of what’s there now – an elevated Massachusetts Turnpike along with Soldiers Field Road and the rail tracks at ground level. A second option would elevate Soldiers Field Road and put the Turnpike and rail tracks at ground level.

There was an existing at-grade option, but Pollack upgraded it to include most of the elements sought by Boston and A Better City, including a 20-foot-wide path (22 feet with railings) out in the river incorporating separate bike and pedestrian lanes and a refurbished river bank with trees and shrubs. The new design accommodates those elements by reducing the shoulders on the Turnpike by a total of 4 feet, using 5 feet of land owned by BU, and putting about three feet of Soldiers Field Road in the realm of the river at the mean annual flood level.

Pollack, backed by a strongly worded letter from Katie Theoharides, the state secretary of energy and environmental affairs, had earlier indicated she wanted to avoid any incursion into the Charles River. But her acceptance of the Boston/A Better City at-grade proposal suggests she’s now willing to make some incursions into the Charles River. (Theoharides also appeared to soften her stance in August.)

Pollack’s spokeswoman declined comment, but Dimino said the transportation secretary’s acceptance of the new at-grade proposal means she and her staff think it can be built and permitted with the incursions into the river.

In a September 28 letter to the Federal Highway Administration, Pollack said the new “living shoreline” and the wider bike and pedestrian path make the proposal more desirable than the initial at-grade design.

Dimino said the environmental benefits of the path and the new shoreline outweigh any impact on the Charles River. He said if the small section of Soldiers Field Road intruding into or near the river is a problem, the road can be moved away from the river by adding additional space, either by reducing the lane widths on Soldiers Field Road from the proposed 11 feet to either 10 ½ feet or 10 feet. He said additional BU land could also be acquired.

BU President Bob Brown on Friday sent a letter to Pollack endorsing the approach put forward by the city of Boston and A Better City. He also agreed to make land available on the BU side of the project to the extent it can be done without interfering with the operation of the school’s buildings.

“In an area where feet, and perhaps inches, may be the difference between a workable and an unworkable solution, Boston University is committed to doing the utmost to accommodate these infrastructure improvements,” he wrote.