It sounds absolutely dire.

Proponents of a Bay Area transit sales tax measure warn that rejecting it on Nov. 3 could unleash a traffic apocalypse, with rush hour delays on the Bay Bridge potentially doubling.

A new analysis by the Chronicle and UC Berkeley’s Mobiliti Labs found that the added congestion may be more modest, though it would hinge on the depths of cuts to BART service and riders’ ability to adapt. Currently, it takes about 13 minutes to cross the bridge westbound during peak morning hours. That figure could rise if BART were to close 15 stations, a concept officials presented earlier this year to illustrate how they could possibly slash service if the rail system were to enter a death spiral.

Just how much it would change depends on what displaced riders do next.

If every rider from a mothballed station switched to driving today, the average bridge crossing time could stretch to almost 24 minutes. Should half of them instead opt to carpool, depart from a different BART station, take the bus or work from home, the crossing would take about 14 minutes, roughly a minute and a half longer than it does today.

A driver going all the way from Antioch to San Francisco, who withstands a 65-minute commute today, would see that trip expand to 74 minutes — or to 86 minutes if the stations closed and every transit rider drove.

In the unlikely event that all public transit in the Bay Area stopped running, the model estimates that it would take a whopping 32 minutes to cross the bridge with all commuters driving.

Those minutes of lost productivity would of course be consequential when multiplied over a large driving population. Traffic models convey little of the human toll for people who have to fritter away hours on jammed freeways, search for parking or spend more time lingering on train platforms because there are fewer trains running. Erosion of transit would drag down real estate prices and dampen job opportunities.

“The real story,” said Mobiliti founder Jane Macfarlane, could be about low-income residents who face a much longer and more agonizing journey to jobs in downtown San Francisco.

For BART and other transit operators, the pressure is mounting. COVID emergency funds have largely dried up. Ridership has seen limited gains in the era of remote work. BART, Caltrain, AC Transit and Muni have struggled with a deepening financial predicament that advocates say can only be solved with a five-county sales tax which the public will decide at the polls on Nov. 3.

Absent that economic lifeboat, officials project annual budget gaps of $400 million, which could force BART to lay off hundreds of people, pare back service to trains running every half-hour, shutter stations and scrap the connector to the Oakland airport.

Supporters of the regional transit tax argue these reductions would translate into crippling backup, an assertion that so far has been difficult to verify.

San Francisco nonprofit SPUR, whose staff helped steer the measure campaign, published a report in September that looked at service contractions being contemplated by BART, Caltrain and Muni. It said Bay Bridge travel times would swell from 30 minutes to more than an hour during the peak morning commute. SPUR’s analysis included the bottlenecks at freeway approaches as part of the bridge drive time, and factored in additional vehicles if BART dials back service at every station.

Opponents of the measure accuse agencies of overstating their prospective cuts and cast doubt on BART’s station closures, which were not approved by the board. They contend that transit should strive to operate more efficiently before appealing to taxpayers.

To better understand how various permutations of cuts could impact regional traffic, the Chronicle teamed up with researchers at Mobiliti — a startup spun out of the Lawrence Berkeley National Laboratory — to simulate three possible scenarios for the morning commute. At the most extreme, we modeled a traffic forecast with all regional transit shut down and all riders shifting to cars.

We also tested the ramifications of BART’s “phase 2” deficit proposal, which would shut down 15 stations, one of which is the airport connector rather than a stand-alone facility. Finally, we evaluated a case in which 15 stations close and half of displaced riders drive or take ride-hails, while the other half remain on transit.

The models suggest that those circumstances would produce a slight uptick in freeway volume rather than a full-on catastrophe.

“I think initially you will see a pretty significant increase in traffic on the bridge, and in the longer term people will adjust,” said Jamey Volker, a faculty researcher at the Institute of Transportation Studies at UC Davis. “Look at the example of COVID. People adjusted. They didn’t get to work riding transit, and they also weren’t driving.”

He believes that in the long-term, roads and bridges would see a small net increase in vehicle volume as people retool their daily routines.

Teasing out the many effects of a regional transit meltdown is complicated. Our simulations offer a glimpse into the complexity of commuter behavior and decision-making, while also showing that traffic does not build up in a linear fashion.

Mobiliti used supercomputers and agentic AI to build a scaled orchestration of the Bay Area’s transportation network, with a “baseline” level of eight minutes of morning delay on the Bay Bridge.

Were the Bay Area to completely abandon public transportation and every commuter to become a driver, the results would be drastic. The Antioch-to- San Francisco trip serves as a harrowing example. Current drivers, who at present spend a little more than an hour on roads and freeways each morning, could be stuck in their cars for a soul-crushing 2½ hours.

This most extreme outcome is unlikely, though, as many commuters would carpool or work from home.

These numbers demonstrate how critical transit is to the basic function of the Bay Area economy and residents’ quality of life. But they are the product of an intellectual exercise, rather than a realistic forecast.

The trick, for transportation planners, is to decide which future is most plausible. Analysts often sort transit riders into two categories: those who ride by “choice,” since they have the schedule flexibility and disposable income to drive if they wanted to, and those who are completely “dependent” on transit.

These classifications are slippery. School drop-off schedules and proximity to a local BART stop could have more bearing on people’s travel habits than other demographic attributes.

It’s also hard to predict how the world might change if BART puts more than a dozen stations on death-watch. Buses would likely re-route to drop people off at the next open station. People who drive and park at stations farther away from their homes would pack more traffic onto local streets and roads.

If BART parking lots filled up, cars would spill onto neighboring streets and potentially create a nuisance for residents. Eviscerating BART would discourage casual riders from catching trains to shows or sporting events, Volker noted, adding that people might start disassociating from transit and be less likely to fund it in the future.

Not to mention that even a moderate influx of vehicle traffic could put disproportionate strain on freeways.

A Chronicle breakdown of BART data found that during the morning peak, fewer than 1,400 people ride each hour from the 11 threatened East Bay stations through the Transbay Tube. Theoretically, the Bay Bridge has enough capacity for all of them.

On an average morning, about 7,300 drivers cross the westbound span each hour. Its five lanes should be able to move 10,000 vehicles an hour under ideal conditions, if every car were evenly spaced and moving at 35 miles per hour. ( John Goodwin of the Metropolitan Transportation Commission said bridge traffic “rarely tops 9,000” an hour, with metering lights controlling the flow of incoming cars.)

Yet perfection is elusive on freeways full of harried drivers. Many get distracted by their phones, merge lanes at the last minute and aggravate bottlenecks at key interchanges or off-ramps. Fender-benders take longer to clear if a whole freeway is packed. Predicting all the cascading problems is an art rather than a science.

At the same time, it’s too simplistic to assume that everyone who loses their home BART station would automatically drive to work. More people might work remotely. A few could carpool. Many will leave for the office later or drive and park at a different train stop. Travel modeling expert Ben Stabler described every commute as a series of “nested” decisions.

Take Brittany Taylor, a mother of three watching for a San Francisco-bound train at Orinda Station — one of the 15 slated to close in BART’s worst-case scenarios. Like many working parents, Taylor pulls a high-wire act each morning, dropping two of her children off at preschool around 7:30 a.m. before catching a train and bus to her office.

That journey takes an hour and 15 minutes, comparable to driving the whole way, she said. If Orinda were to close, she would contemplate taking a 5-mile detour up Highway 24 to Lafayette Station or west to Rockridge Station in Oakland. Then she would have to circle a parking lot that would almost certainly already be full.

“That’s the tradeoff,” Taylor said, grappling with two unappealing choices: A roundabout park-and-ride commute with several transfers, or a long slog across the bridge. “I want to keep taking BART because my mental state is so much better when I’m not sitting in gridlock. But I’m also really curious how we would absorb all these cars driving and parking at other stations.”

For others, the situation is even more daunting.

Daunte McFadden is a construction worker who lives in San Francisco but frequently stays with family members in east Contra Costa County. Since he has no driver’s license, McFadden relies on transit to get to job sites all over the Bay Area, often with a 6 a.m. call time. Getting to work that early is difficult even with a functioning transit system, he said. If BART were to eliminate the last four stations on the yellow line in Contra Costa County, he may resort to ride-hails.

“I might take an Uber,” McFadden said, mulling the possibility as he stood in the Pittsburg/Bay Point Station concourse.

That station could be on the chopping block if BART hits a fiscal cliff, as could its eastward neighbors Pittsburg Center and Antioch. The next station downstream, North Concord/ Martinez, is 6 miles away on Highway 4. Catching a ride-hail there from Pittsburg might cost $18 to $25.

Nearly three-quarters of the people who commute from the 15 at-risk stations said in BART customer surveys that they are headed to work. Yet those who ride transit to recreational outings said they would also be left scrambling if their local stations were to shut down.

On a recent Tuesday evening, Pranav Vasavada sat with his wife and two sons at the West Dublin Pleasanton Station — another stop that could close — waiting for a blue line train to San Francisco. They were en route to a Warriors preseason game at Chase Center, which meant transferring downtown from BART to Muni Metro, another transit system wrestling with a financial crisis.

Should West Dublin close, Vasavada said he would probably grit his teeth and drive to Mission Bay for any future Warriors games he attends, knowing he would pay up to $80 for parking.

Farther down the platform, Ping Li leaned against a tiled wall, checking destination signs. She regularly takes BART from South Hayward out to Dublin, then catches a bus to San Ramon to care for her infant granddaughter, allowing her daughter and son-in-law to work. If BART knocks out stations on the blue line, the family might have to devise a new arrangement. Either the parents could pay for childcare, or Li could shell out $50 one way for Uber rides.

The price concerns her more than the traffic. After all, she said, “I don’t drive.”

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