Sen. Dianne Feinstein renewed her call Wednesday for a so-called Southern Crossing — a new east-west transbay bridge somewhere south of the always-congested Bay Bridge.

Feinstein, D-Calif., has repeatedly called since 2000 for a Southern Crossing, an idea that has been around since the 1940s, but has been rejected in recent years as too costly and environmentally problematic.

On Wednesday, joined by Rep. Mark DeSaulnier, D-Concord, she sent a letter to Steve Heminger, executive director of the Metropolitan Transportation Commission.

The letter chastised the commission for not moving forward on a new transbay span. Its authors said it would relieve Bay Bridge congestion, and that it should be funded by Regional Measure 3, a ballot measure — written, but not yet scheduled — that could raise tolls on state-owned toll bridges by as much as $3.

“The traffic demands on our streets and transit systems have become intolerable. Quality of life is suffering,” Feinstein and DeSaulnier wrote. “A second crossing would alleviate traffic through San Francisco and the East Bay, would better connect the entire Bay Area, and would provide significant benefits for toll payers.”

They added: “We have communicated in the past about the need for another bay crossing, but this has not led to any concrete action. We need to move this issue forward expeditiously, and we would very much like your assistance in doing so,” they said.

MTC spokesman Randy Rentschler acknowledged receipt of the letter. He said the commission appreciated the acknowledgment of the need for major investment in Bay Area infrastructure. The letter, he said, will be taken seriously.

“We’re grateful to Senator Feinstein and Congressman DeSaulnier for leading on the issue that the Bay Area needs to build significant new infrastructure projects,” he said. “Imagine if instead of giving millionaires a $1.5 trillion tax break, our country invested in rebuilding the nation’s infrastructure to benefit everyone.”

But Rentschler did not commit to undertaking a new bay-crossing study, pointing out that it’s not on the list of projects the Legislature approved when it agreed to let the Bay Area vote on Regional Measure 3 to raise its tolls.

The commission is scheduled to discuss the measure, including how much of a toll increase to seek and when to put it before voters, on Dec. 20. But it probably won’t make a decision on the measure until Jan. 24.

“Where the commission is focused right now is on making a decision on Regional Measure 3,” Rentschler said. “A Southern Crossing is not a new idea, but it would be something in addition to the decision before the commission right now.”

Several versions of a Southern Crossing have been proposed through the decades, none of which has been undertaken.

After Feinstein floated the idea in 2000, the commission undertook a two-year study of what it would cost. It concluded that building a bridge from Highway 238 in Hayward to the Peninsula would cost $8.2 billion, saying that was cheaper than a new BART tube between San Francisco and Oakland but still costlier than the Bay Area could probably afford.

Another two-year study, completed in 2012, estimated the cost of a bridge that would carry both cars and some form of transit, at $12.4 billion.

Both studies assumed a new crossing that would be built from Interstate 380, in San Bruno, to Interstate 238 in San Leandro. The letter from Feinstein and DeSaulnier included no specifics but did mention “the need for an additional route for both BART and vehicular traffic.”

“What I would like is a really innovative look,” he said. “Maybe you look at light rail, maybe you look at BART, maybe you look at autonomous vehicles.”

Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: mcabanatuan@sfchronicle.com Twitter: @ctuan