A once-radical idea to purge all private cars from Market Street becomes reality on Wednesday.

New turn lanes will stripe the roadway, diverting traffic from a transit spine that’s intended to be a grand promenade, rife with bicycles and clattering streetcars. The city’s new “quick-build” procedures allowed transit officials to banish automobiles east of 10th Street within three months of approving the project. Meanwhile, a more substantive redesign is inching forward.

Eventually the old “lollipop” traffic signals will get a full overhaul, gray pavers will replace the red sidewalk bricks, and officials will swap the green bunker toilets for a more futuristic model.

Already, transit backers and city officials are contemplating what comes next. Shift the 14-Mission over to Market? Ban cars on a nearby alleyway in the South of Market area? Anxious motorists grit their teeth, preparing for an even more circuitous drive downtown.

The long-term effects of this transformation remain unknown. Boosters hope to reduce vehicle collisions and create a more inviting city core. But other goals seem elusive: No one can quite predict the future of Mid-Market, a troubled stretch that draws theatergoers as well as transients and drug dealers.

For now, here’s what you need to know:

Municipal Transportation Agency crew member Ron Aquito (right) uses a blowtorch to melt down red paneling on a bus-only lane while Rene Menjivar (left) goes over it with another melting machine along Second Street leading to Market Street. Photo: Jessica Christian / The Chronicle

Municipal Transportation Agency crew member Ron Aquito (right) uses a blowtorch to melt down red paneling on a bus-only lane while Rene Menjivar (left) goes over it with another melting machine along Second Street leading to Market Street.

(Jessica Christian / The Chronicle | San Francisco Chronicle)

How many drivers use Market Street, anyway? Not as many as you might think. According to the San Francisco Municipal Transportation Agency, 200 to 400 cars travel in each direction on Market Street every hour during the peak commute. That’s roughly half the number on Mission Street, which sees 500 to 900 cars per hour in each direction.

People wait for the bus on Market Street. On Wednesday, private cars will be prohibited in favor of public transit and bicyclists. Photo: Gabrielle Lurie / The Chronicle 2019

People wait for the bus on Market Street. On Wednesday, private cars will be prohibited in favor of public transit and bicyclists.

(Gabrielle Lurie / The Chronicle 2019 | San Francisco Chronicle)

Traffic wasn’t always that sparse. The city gradually steered cars off the thoroughfare by installing forced detours in 2009, adding more turn restrictions over the years to stymie east-west travel. Many of the vehicles that currently use Market are Uber and Lyft cars, and those drivers will face the biggest shock. As of Wednesday, they will be relegated to loading zones on side streets.

Is this going to speed up the buses? With rapid, subway-like service in the center lane, Muni estimates that 464,000 riders who board buses on Market Street every day will save lots of time. Faster service unimpeded by automobiles would shorten each trip by 15% to 25%, according to transportation planners.

Graphics: Changes to SF’s Market Street, block by block

From a transit perspective, Market is the beating heart of the city, with 200 buses and streetcars running down the strip per hour at the busiest times, while BART and Muni Metro subway trains rattle underneath. Roughly 500,000 people walk along Market each day, and 650 cyclists roll through each hour during the peak commute.

What about parking? Parking on Market, already limited to six metered spaces east of Spear Street, will disappear entirely with the redesign. The more drastic change will occur on cross streets and side streets, where 227 spaces will be converted to commercial loading.

“Once this is done, people will see it was a great idea,” said Donald Shoup, a professor of urban planning at UCLA who specializes in the economics of parking. He pointed to studies showing that streets are safer, and more productive, when curb space is used for short-term loading instead of car storage. It saves Uber and Lyft passengers from having to step out in the middle of traffic, and opens space for many more people to arrive and leave.

“If you look at Paris, Vienna, or Madrid, all great cities are moving in this direction,” Shoup said. “San Francisco will be very proud of this once it’s finished.”

Won’t pedestrians wander into the bike lanes? Distracted people, absorbed with cell phones or earbuds, pose a risk — mostly to themselves — on any bustling artery. So do rule-flouting cyclists — a wide buffer between the bike lane and the sidewalk will include bike racks, benches, trees and historic lampposts.

If cars are pushed off Market, will they start flooding Mission? Not according to the transportation agency’s preliminary traffic analysis, said department chief Jeffrey Tumlin. Yet it’s hard to predict motorist behavior — in the real world, drivers don’t always follow traffic analysis models.

“We’re going to be watching carefully and collecting data,” Tumlin said. “And we’re ready to make adjustments.”

Commuters ride their bikes down Market at Sixth Street. About 650 cyclists per hour pedal along Market during the peak commute times. Photo: Gabrielle Lurie / The Chronicle

Commuters ride their bikes down Market at Sixth Street. About 650 cyclists per hour pedal along Market during the peak commute times.

(Gabrielle Lurie / The Chronicle | San Francisco Chronicle)

That could include shifting Muni’s 14-Mission bus to Market Street, which will have more transit capacity once the private cars are gone.

Should cars be banned on side streets, too? Why stop at Market Street when we could turn the whole downtown core into a free-flowing utopia for bikes and buses? The prospect makes some advocates giddy, but Tumlin said it might cause problems.

“Market Street itself is not essential for the through-movement of car traffic, because it doesn’t actually go anywhere,” Tumlin said. “But closing the cross streets would have a significant impact on the overall mobility system, and would tend to push traffic from one street to a parallel street. So that’s not something we’re looking at now.”

He noted, however, that he would happily entertain proposals from merchants or community groups that want to close their streets to automobile traffic.

What longer-term changes are coming to Market Street? It’s too early to say whether Market will become the urban utopia that captivated planners and politicians for years. But changes are afoot: The sidewalks will be widened from 35 to 37 feet and resurfaced with concrete pavers. Crews will repave the street with concrete instead of asphalt. A new loop near United Nations Plaza will allow streetcars to shuttle back and forth, carrying more tourists.

Once complete, the project will revamp a 2.2-mile stretch from the Embarcadero to Octavia Boulevard, at an estimated cost of $604 million.

Separately, BART plans to install glass-and-steel canopies over its station entrances, protecting the escalators from downpours and giving the street a cleaner feel.

Which streets are next? Transit advocates already want to extend the car-free ideal to other San Francisco arteries. The most immediate and desirable target is John F. Kennedy Drive in Golden Gate Park, a road where cars mix uneasily with cyclists, skateboarders and rollerbladers. It’s among the city’s “high-injury” roadways — the 13% of streets that account for 75% of traffic injuries and fatalities. In 2016, cyclist Heather Miller was killed by a hit-and-run driver on John F. Kennedy Drive near 30th Avenue, a crash that jolted the city.

“This is a place that people come for recreation and fresh air,” said Olivia Gamboa, a doctor in the Richmond District who helped organize a ride along JFK this Sunday, to press for the removal of automobiles. It’s ultimately up to Phil Ginsburg, general manager of the Recreation and Park Department, to decide whether to pursue a ban.

Advocates have also eyed the Tenderloin — another area known for road mayhem and fatalities — as a potential car-free zone. They have the ear of Supervisor Matt Haney, who invited the transportation agency to study that part of his district.

Rachel Swan is a San Francisco Chronicle staff writer. Email: [email protected] Twitter: @rachelswan

Timeline: How the car ban finally happened