Transit officials are looking into carving out carpool lanes along both Highway 101 and Interstate 280 that could double as toll lanes to allow drivers willing to pay a bit extra for a faster commute in and out of the city.

The southbound commute lane would extend along I-280 from King Street, near AT&T Park, down to where Highway 101 meets Interstate 380 near San Francisco International Airport.

The northbound lane would run along Highway 101 — from I-380 up to the San Francisco County line — then pick up again on I-280 from Mariposa to King streets.

The San Francisco lanes could eventually hook up with other express lanes being planned in San Mateo — and those that are already in operation in Santa Clara County. In time, they could stretch all the way to Morgan Hill.

“The goal is to create a more reliable commute in and out of the city and to encourage more carpooling and bus ridership,” said San Francisco County Transportation Authority Executive Director Tilly Chang.

In some sections, the new lanes would take the place of the highway’s existing shoulder. In other sections, adding a carpool lane would mean taking out a lane of traffic.

The estimated cost would be $60 million to $100 million.

But it’s not the cost of the lanes that is raising questions — it’s the politics.

“Just like anything in San Francisco, we are all about community process,” said Supervisor Ahsha Safaí, whose district includes portions of the highways.

An estimated 300,000 people a day travel the heavily congested Peninsula corridor, including drivers and passengers in single cars, passengers on Muni and SamTrans buses, and techies on the fleets of private commuter shuttles that serve Silicon Valley campuses.

There are also an increasing number of ride-hailing services such as Lyft and Uber, ferrying people to and from the airport.

It’s estimated the carpool lanes could shave five to nine minutes off the trip. At the same time, losing regular lanes to express lanes could add up to five minutes for people not in a carpool.

One question: Who benefits the most from the lanes?

“Are we looking at it in terms of San Francisco residents or in terms of people coming up from the Peninsula to work in San Francisco?” Safaí said.

There is also the question of whether the carpool lanes should be open for a minimum of two people in a car — or three.

“From what we have seen elsewhere in the Bay Area, if it’s two people per car, the lane fills up right away and does little to help traffic,” said Metropolitan Transportation Commission spokesman Randy Rentschler. “It should be three per car.”

The even bigger question is whether drivers who are willing to pay should be allowed onto the lanes, as they are in other parts of the Bay Area.

After all, this is San Francisco, and letting the affluent buy their way out of traffic is not necessarily the ideal way to promote use of public transit. The price paid depends on the time of day and length of the trip.

“For some people, it’s a question of equity,” Supervisor Aaron Peskin said.

“Are we trying to get people out of their cars, or are we encouraging those willing to pay to stay in their cars?” Safaí asked.

The debate isn’t limited to San Francisco. In neighboring San Mateo County, there are also questions about using new carpool lanes as express lanes.

“It’s a money grab,” said San Mateo County Supervisor David J. Canepa. “It’s something the voters should decide.”

Caltrans will make the ultimate call. Meanwhile, as the debate continues, Assemblyman David Chiu, at the request of the County Transportation Authority, has introduced a bill to allow either the Santa Clara Valley Transportation Authority or the Metropolitan Transportation Commission to manage the lanes — just in case.

On tap: One of London Breed’s first big decisions after being sworn in as San Francisco’s mayor Wednesday will be naming her District Five replacement on the Board of Supervisors, and it could shape up as a family affair.

Topping the list of contenders is Judson True, the longtime aide to Assemblyman David Chiu. True is married to Andrea Bruss, a legislative aide to Breed who will follow her to the mayor’s office.

Others being mentioned include Mawuli Tugbenyoh, a legislative aide to Breed, City College trustee Shanell Williams and Vallie Brown, a former Breed aide and Lower Haight activist who works for the Mayor’s Office of Workforce and Economic Development.

Whoever gets the nod will have the work cut out for her or him.

Breed only narrowly won re-election to the seat two years ago, defeating upstart progressive candidate Dean Preston by fewer than 2,000 votes — 52 percent to 48 percent. Preston, who was behind last month’s passage of Proposition F to provide free lawyers to tenants facing eviction, took out papers Friday to run again. This time he’s promoting himself as a member of the Democratic Socialists of America, newly popular because of Alexandria Ocasio-Cortez’s stunning upset of Rep. Joseph Crowley in New York last week.

Which may explain why speculation also continues over the potential appointment of a dark horse progressive, such as school board member Stevon Cook or Sunny Angulo, a legislative aide to progressive Supervisor Aaron Peskin. That type of pick that could make it tougher for somebody like Preston to mount a challenge.

By the way, we’re told there’s still plenty of hand-wringing in Breed’s circle over Supervisor Katy Tang’s abrupt — and last-minute — decision not to seek re-election and instead to back her aide Jennifer Ho for the Sunset District supervisor post.

Ho isn’t particularly well known and until recently had been living in Los Angeles. While Breed quickly endorsed her, Ho is expected to face plenty of competition, including from former Supervisor Eric Mar’s brother, Gordon.

If Ho loses, Breed loyalists fear, the new mayor will be pinned with the defeat of a candidate she had no say in picking.

San Francisco Chronicle columnists Phillip Matier and Andrew Ross appear Sundays, Mondays and Wednesdays. Matier can be seen on the KPIX-TV morning and evening news. He can also be heard on KCBS radio Monday through Friday at 7:50 a.m. and 5:50 p.m. Got a tip? Call 415-777-8815, or email matierandross@ sfchronicle.com. Twitter: @matierandross