Building a subway underneath parts of 19th Avenue would speed up the M-Line by as much as 35 to 40 percent, according to the San Francisco County Transportation Authority.
A final feasibility study last month said the M currently travels an average of 8.5 miles per hour in that corridor.
Liz Brisson, a senior project manager with the authority, said the feasibility study recommended building a subway south of St. Francis Circle through Stonestown. The M would then return to the surface as it approaches San Francisco State University.
Brisson said the M would still make the same stops at Stonestown and the university. The stop at Stonestown would be underground and the university stop would be relocated to the west side of 19th Avenue.
The M would then enter Parkmerced at a new station planned by developers for the residential area. A bridge would then bring the M back to Randolph Street.
Improving the speed and reliability of the M were just two of the benefits of removing the light rail line from street level along 19th Avenue. Brisson said sidewalks and crosswalks could be widened to make it safer for pedestrians crossing 19th Avenue.
The new stations on the west side of 19th Avenue would make things safer for university students and mall shoppers, would not need to cross three lanes of traffic to get to those destinations. Currently those stations are on raised medians along the busy corridor.
The new stations will also fit trains longer than the two-car trains Muni currently runs on the M.
The San Francisco Municipal Transportation Agency will continue the study by working on a pre-environmental report. A Caltrans report on the study will also be needed because 19th Avenue is part of state Highway 1.
The earliest possible completion of the project is 2022 if the transit agency moves forward with the project.
Funds for the project are still not identified, but Parkmerced has roughly committed $70 million in design and construction for its site. SF State has also committed $1.8 million toward building the Holloway Avenue station.
The cost of project is estimated at $420 million to $780 million, with the feasibility study pegging costs near $520 million.