Perhaps no local issue in this election is more contentious than Proposition K, which would permanently close a 2-mile stretch of the Great Highway to cars and move it closer to becoming a park.
Supporters and opponents have sparred over the implications for traffic, with the measure’s backers arguing that drivers would have only slightly longer commutes, and detractors rejecting that assertion while claiming the closure could push drivers into residential areas, making them more dangerous for pedestrians.
In an effort to understand Proposition K’s potential impacts on traffic, the San Francisco Public Press reviewed multiple studies and consulted the researchers who wrote them, as well as experts not affiliated with the city. The examination found that many, but not all, claims and concerns had a valid basis, while some were mired in uncertainty.
Would closing the highway increase travel times for coastal commuters by a meager 3 minutes, as Proposition K’s proponents have said? There is not enough evidence to support that specific estimate, though a minor slowdown is likely, sources said. Delays for drivers would probably be heaviest through and along Golden Gate Park as travel patterns shifted inland. Buses on Sunset Boulevard also would see delays.
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No, cars diverted to Sunset Boulevard wouldn’t overwhelm the artery, which would see a slight drop in traffic speed.
Yes, the slated closure of a southern section of the thoroughfare would naturally cause traffic to thin along the coastal highway — an idea that has added momentum to arguments to close it entirely — but the effect would be slight and most drivers would continue using the road.
And would traffic and related hazards increase in residential areas? The concern is reasonable, but the city could address it with traffic-calming measures.
Read on for our full analysis.
Proposition K supporters say commutes would be only 3 minutes longer. Experts cast doubt.
Proposition K would affect the Upper Great Highway from Lincoln Way to Sloat Boulevard.
The measure’s opponents warn that the Upper Great Highway’s closure would force drivers to take other, slower routes, which would inconvenience frequent commuters and possibly discourage visitors from patronizing small businesses in the area.
Many of the measure’s supporters have suggested that it would increase average north-south commute times by merely 3 minutes. As evidence, supporters have cited a study this year by the San Francisco Municipal Transportation Agency on how the highway’s recent temporary closures have affected traffic patterns.
But while the research findings imply that the closure would slightly slow commutes, they are probably inadequate to measure by how much, due to an approach to data gathering and analysis that was not rigorous, the experts said. The transit agency did not address the experts’ criticisms, and said it stood by its methodology and findings.
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For the study, researchers checked the predicted southbound travel times for weekday commuters driving from the San Francisco Veterans Affairs Medical Center, in the Richmond District, to the Westlake Shopping Center, in Daly City. They compared travel times for drivers who used the Upper Great Highway with those for drivers using Sunset Boulevard on days when the coastal highway closed due to windblown sand or other reasons. They checked 22 traffic predictions, on 11 days, using Apple Maps data, which staffers said is more accurate than Google Maps data when one is calculating travel times for routes that include road closures. Agency staff shared the data in response to a request by the Public Press.
The study found that drivers took 3 minutes longer, on average, using Sunset Boulevard.
The study is a “great first step” toward understanding how the highway’s closure could affect traffic, said Rounaq Basu, a postdoctoral associate at MIT’s Department of Urban Studies and Planning specializing in ways to reduce car dependency.
But “we just don’t have enough data to be confident” in the 3-minute estimate, Basu said.
Transit agency researchers had checked travel predictions at a wide range of times, with few predictions taken at the same times across the 11 days. Therefore, it is difficult to rule out the possibility that travel times varied in response to fluctuations in traffic throughout the day, Basu said.
“In the morning and in the evening, obviously the times are going to be higher than in the afternoon,” Basu said.
The agency also collected most of its data on Mondays and Fridays, which are “a bit weird” in the COVID-19 pandemic’s aftermath, he said. People with hybrid work schedules tend to work remotely on those days, resulting in lighter vehicle traffic and potentially faster commutes.
Simon Tan, a transportation planner at Sound Transit, a public transit agency serving the Seattle metropolitan area, also took issue with aspects of the study’s data-collection approach. For example, agency staffers recorded traffic predictions on July 5, the day after a national holiday.
“People take July 4 off, they’re going to take July 5 off,” which could result in unusually sparse traffic and quick commutes, Tan said. Another reason traffic might have been light that day: It was a Friday.
The study’s researchers collected most of the traffic predictions during a roughly one-week period. Basu suggested that the study would have been better, with an appropriate volume of data, if it had systematically collected travel predictions at the same times each day for one month, during morning, midday and evening peak-commute periods. That would have let researchers compare predictions within each period, to understand traffic impacts at a granular level.
Tan echoed Basu, suggesting the same approach. He added that analyzing travel times between just two destinations — the medical center in San Francisco and the Westlake Shopping Center — might not have been enough to understand the closure’s potential impact.
The study’s findings are valuable “for people who care about that particular trip or around those locations,” Tan said, adding that travel to destinations like the Cliff House or Stonestown Galleria also might be important to measure. “If I was doing a different trip, I wouldn’t care about this. This wouldn’t be relevant to me.”
However, even if the study had been more rigorous, its method of analysis — checking travel predictions during the highway’s temporary closures — would be insufficient to determine the impact of permanently closing the Upper Great Highway to cars, Basu and Tan said. That would require also simulating future traffic with computer models, which would use data on current travel patterns and planned road improvements that might affect commutes. Basu said it was important to survey people, to understand their travel needs and explore alternatives to shuttering the highway.
The Public Press asked the city’s transit agency to respond to the criticisms and explain why it used its methodology instead of one that more closely resembled what Basu and Tan described.
The agency did not answer those questions. Spokesperson Michael Roccaforte said it “stands by its analysis and report,” including its use of travel predictions, and added that the research relies on travel time modeling performed for three prior reports by city and county departments.
Worst impacts at Golden Gate Park, slowing some commutes between western neighborhoods
The camps for and against Proposition K have also argued over whether closing the Upper Great Highway to cars would slow drivers commuting back and forth between the Richmond and Sunset districts.
The impacts would probably be mixed, with the greatest slowdowns for people passing through Golden Gate Park, according to a 2021 study by the San Francisco County Transportation Authority. Former District 4 Supervisor Gordon Mar called for the study, which used computer modeling to predict traffic patterns, during an earlier phase of the city’s debate over the highway’s fate.
Today, many southbound drivers using the Upper Great Highway enter it from a northern stretch of coastal road, along Golden Gate Park’s western edge.
But if Proposition K passes and the Upper Great Highway closes to cars, two-thirds of the traffic that has historically fed it would relocate eastward as drivers made their way to alternative major thoroughfares — mainly Sunset Boulevard — based on the study’s findings. That would slow inland commutes through the park. Vehicles on Chain of Lakes Drive East, which allows north-south travel, already crawl at about 13 mph and would slow to 10 or 11 mph.
Southbound commuters driving through the park would turn east onto Lincoln Way, to redirect toward Sunset Boulevard. They would converge with eastbound drivers on Lincoln Way, who would take 7 minutes longer to pass through that intersection than if the Upper Great Highway had been open. This analysis did not factor in drivers taking Martin Luther King Jr. Drive to Sunset Boulevard, which could ameliorate southbound delays.
Sunset Boulevard could easily accommodate the traffic displaced from the Upper Great Highway, and would experience little slowdown, according to the report — contrary to what many Proposition K opponents have said.
Some southbound drivers would aim for 19th Avenue rather than Sunset Boulevard. Many would take Crossover Drive, which would probably back up where it met Martin Luther King Jr. Drive.
Northbound drivers who had used the Upper Great Highway would see slowdowns in the city’s southwest, as cars made their way to Sunset Boulevard. Skyline Boulevard would experience delays of about 2 minutes at its intersection with Lake Merced Boulevard.
Upon reaching Golden Gate Park, the northbound drivers would turn west onto Lincoln Way. They would experience a 5-minute delay turning north onto Chain of Lakes Drive East.
Drivers aiming to travel north through the park, and who had approached from 19th Avenue, would experience 6-minute delays on Lincoln Way where they merged with the drivers who had taken Sunset Boulevard.
The three-year-old study’s findings are probably still accurate despite having used pre-pandemic traffic data, said Dan Tischler, who worked on it as principal transportation modeler. That’s because while traffic has thinned along the Upper Great Highway since COVID-19 touched down in San Francisco, it has generally not changed much on the city’s west side, he said.
Still, the findings could represent a “worst-case scenario,” Tischler said. And “in the real world, you might expect some of that traffic to disappear” as drivers adapted by trying other routes, he said.
Uncertain timetable for some traffic solutions
Supervisor Joel Engardio, who co-sponsored Proposition K’s placement on the ballot and has advocated for closing the Upper Great Highway to cars and creating a park there, has said the city could mitigate the impacts with road improvements. He has proposed replacing stop signs with traffic lights to increase flow in multiple areas, and has secured funding for some of that work.
Design is nearly complete for one light on Lincoln Way near Chain of Lakes Drive East, with construction likely to start in fall 2025, said Johnathan Goldberg, Engardio’s legislative aide. But it is unclear when the other lights will be installed.
Meanwhile, the agency is overdue to install a traffic light at the three-way intersection of Skyline Boulevard, Sloat Boulevard and 39th Avenue. The light would ease the impact of the Great Highway Extension’s closure; just south of the Upper Great Highway, this stretch of road is scheduled to shutter in early 2025 at soonest because it faces erosion due to sea level rise.
Delays for one bus line
Drivers aren’t the only ones who would experience delays if Proposition K passed.
With drivers displaced to Sunset Boulevard, the 29-Sunset bus line would slow, according to a 2021 environmental impact report on how the combined closures of the Upper Great Highway and Great Highway Extension would affect public transit.
Buses would arrive at least 4 minutes beyond their scheduled times, the report said. That would make them late, according to city standards, said Dan Sider, chief of staff for the San Francisco Planning Department, which created the report. Forthcoming infrastructure could reduce delays by speeding up bus boardings.
Two other major bus lines for north-south travel on the city’s west side, the 18 line running along 46th Avenue and the 29 line along 19th Avenue, would not see delays.
Some traffic would divert into residential areas, but road improvements could ease hazards
Opponents of Proposition K contend that permanently closing the Upper Great Highway would push drivers into surrounding neighborhoods, making them more dangerous.
The County Transportation Authority’s 2021 report agrees that there is a risk. It projects that about 20% of the highway’s traffic would redirect into the Sunset District west of Sunset Boulevard, increasing traffic volume and speeds which might raises the risk of vehicle-pedestrian collisions.
Even if that did happen, basic traffic-calming methods might ameliorate the problem, based on findings by the Municipal Transportation Agency. As part of their study this year, researchers analyzed the impacts of installing speed bumps and speed-limit and stop signs in the western neighborhoods, in response to concerns during the pandemic that the Upper Great Highway’s temporary closures were diverting drivers to the area.
After the improvements, average vehicle speeds in two zones were both 26 mph, compared with 32 mph and 33 mph prior to the pandemic, the report said.
Highway traffic will abate little after section closes — contrary to pro-measure argument
Two-thirds of the Upper Great Highway’s drivers use it to commute between the Richmond District and South Bay, according to the County Transportation Authority’s report. Today that route includes the Great Highway extension, which connects the Upper Great Highway to Daly City.
The extension’s future closure will add turns to the route, and Proposition K supporters have argued that this will discourage drivers from making the commute at all. That’s one more reason that a park would be a better use for the artery, they say.
But according to the county report, three-quarters of drivers would probably continue using the road after the extension closed.
The 2021 environmental impact report contains a nearly identical finding. Without the extension, 73% of car traffic would remain on the Upper Great Highway rather than divert to parallel major arteries like Sunset Boulevard or 19th Avenue, the report said.
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