The cost to cross the Golden Gate Bridge could rise to nearly $10 in the next five years, a more than 20 percent increase that officials say is vital to keep buses and ferries running, paint suspension cables and otherwise maintain the aging Art Deco span.

Board directors at the Golden Gate Bridge, Highway and Transportation District are contemplating five proposals to increase tolls over the next five years, in hopes of filling a $75 million budget deficit. Bridge tolls pay for three-quarters of the district’s operating costs; and with the price of goods and services rising by about 4 percent each year, they say, revenue hasn’t kept up.

“So we’re under some pressures,” said Denis Mulligan, general manager of the district. He pointed to other budget-straining factors, like rising demand for transit service and decaying infrastructure, which have required the board to set aside more money for capital projects.

On Thursday, the district’s Finance-Auditing Committee will vote on whether to start a months-long public discussion of the five proposals to raise tolls annually beginning in July, with the final increase in July 2023. Motorists with FasTrak transponders who currently pay $7 to cross the span would see hikes to as much as $8.75 in five years. People who pay by invoice, who are now charged $8 for every crossing, would pay up to $9.80 in 2023. One-time payments, also $8 today, could go up to $9.75.

Such increases likely will see resistance from some drivers, given that the 81-year-old Golden Gate is already the most expensive crossing in the Bay Area.

“It’s outrageous,” said David Schonbrunn, president of the San Rafael-based Transportation Solutions Defense and Education Fund, a watchdog group that opposes higher tolls. He called on the bridge district to release a line-by-line spending report before it digs into consumers’ wallets.

“I call this ‘hiding the ball,’” Schonbrunn said. “Rather than submit to public scrutiny about the decisions they’re making, they just want to present the public with, ‘Well, this is what we had to do; we had no choice.’ It’s preposterous.”

The bridge district publishes annual financial reports on its website that lay out expenses, debts and pension liability, and break down toll revenues in detail.

When officials introduced its last set of toll increases in 2013, officials received 140 comments from the public, with just half in support of the new fares. Two board directors — then-San Francisco Supervisors David Campos and London Breed — also objected to the plan, saying the district should demonstrate it had done everything in its power to cut costs first.

Despite that dissension, Mulligan said the number of critics was “surprisingly low.” When the district released a strategic financial plan a year later that included options of charging cyclists and pedestrians to cross the span, the outcry was much greater — 4,400 comments in three days.

The passage of Regional Measure 3 in June — which did not include the Golden Gate — also signals that voters have an appetite to boost bridge tolls if it helps relieve congestion and improve mass transit. Tolls during peak commute hours on the Bay Bridge, for instance, are set to rise by $1 — to $7 — next month. Further increases are planned in January 2022 and January 2025.

Buses and ferries account for a fifth of the trips made from the North Bay to San Francisco on weekday mornings between 5 and 9 a.m., which helps remove thousands of cars from Highway 101, the bridge, and city streets.

The ferry boats from Larkspur reached capacity in 2016 , and are now so full on weekdays that passengers often get left on the dock. Officials are considering measures to beef up service next year, but it would require more money — and likely a more aggressive toll package.

“I see this as a virtuous cycle,” said bridge district spokeswoman Priya Clemens. “What we pay in bridge tolls goes back to benefit buses and the ferry system, which in turn makes for less congestion on the Golden Gate Bridge. And that leads to a benefit for the drivers.”

But the toll hikes aren’t a done deal. Bridge district officials are sensitive to community feedback, Clemens said. Last year, when commuters recoiled at a proposal to raise bus fares by 4 percent, the district rolled back the increase to 3 percent, she said.

“Clearly, we could reduce services if that’s what the community desires,” Mulligan said. Yet he said he doubts people would like the consequences: jammed buses, long lines at the ferry terminal and a deteriorating bridge burdened with more cars.

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com

Twitter: @rachelswan