A screech of tires lacerated the air at Howard Street and South Van Ness Avenue. In a split second, 56-year-old Russell Franklin and his bicycle went flying.

The moment haunts Taylor Ahlgren, a passerby who ran to help as Franklin lay on the asphalt that September afternoon, blood gushing from his right ear. Franklin was the third cyclist to die on San Francisco streets last year — the beginning of a surge in road mayhem that grabbed news headlines and prompted rallies at City Hall.

“It’s needless,” said Ahlgren, who is also an avid cyclist. “We don’t need this level of violence on our streets.”

Like many San Francisco residents, he’s alarmed by the spike in traffic fatalities despite the city’s Vision Zero goal to eliminate them by 2024. Though the political will exists, it’s not showing in the numbers. Fourteen people have died this year, including eight pedestrians, a skateboarder and a cyclist. At this rate, the city could see as many as 37 deaths by the end of December, the highest number in 12 years.

Severe injuries also have increased, from 492 in 2011 to 574 in 2017 — the most recent year for which San Francisco General Hospital provided data. Part of that rise could be attributed to a change in triage practice — in late 2013, paramedics started sending crash victims to S.F. General instead of other area hospitals, and part of it may owe to a 2014 guideline that all head injuries be classified as severe. Even so, the numbers show no sign of declining.

“Lately, I just kind of despair,” said Anthony Ryan, a teacher in the Mission District who frequently commutes by bicycle. He and other cyclists — including Ahlgren — gathered Wednesday night for a Ride of Silence. They pedaled for 9 miles through a lashing rain, dropping flowers at 12 memorial sites.

Nobody can quite explain the traffic death toll, but many have theories. Some point to the growing number of cars and impatient or distracted drivers. Others blame the glacial pace of safety improvements, which meet resistance from residents who don’t want to see a traffic lane stripped out or parking spaces removed. Still others say that San Francisco is hobbled by state laws that restrict cities from lowering speed limits, installing automated cameras or imposing tolls on busy roads.

“There’s a combination of factors that have intensified things,” said Marta Lindsey, spokeswoman for the pedestrian advocacy group Walk San Francisco. After holding two vigils for pedestrians killed by vehicles in the first two weeks of May, the group was ready to declare a state of emergency.

“Everybody’s feeling this crisis,” Lindsey said, “whether you’re an angry bicyclist or a senior citizen or a parent pushing a stroller, practically having a heart attack at every intersection.”

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In some senses, it’s a replay of 2014, the year that City Hall adopted Vision Zero, a strategy that cities throughout the U.S. use to track collisions and prioritize safety measures. To that end, various departments in San Francisco began collecting data on where the worst wrecks occur. Records show that the high-injury corridors zigzag through low-income neighborhoods, accounting for more than 75% of the city’s crash injuries and fatalities but only 13% of its streets.

San Francisco isn’t the only city confronting the issue: San Jose saw 50 people die in 2016, and Los Angeles suffers more than 200 fatalities each year. Even so, residents seem to expect more from a city that’s known for progressive values, bicycle infrastructure and a menagerie of two-wheeled transportation devices. And city leaders hold the same ideals.

“I believe ‘zero’ is achievable,” said transportation chief Ed Reiskin, who frequently shows up to news conferences on a bike. He came under fire for the recent spate of crashes.

Officials have tried many solutions, and each one hits obstacles. Transportation planners come up with designs for new bike lanes, wider curbs and shorter crosswalks, all of which run through a gantlet of community meetings. Many ideas get chewed up or diluted by the time they reach the SFMTA Board of Directors, and some projects face additional challenges after they’re approved.

One example is Safer Taylor Street, a proposed set of safety improvements along a busy strip in the Tenderloin. After the board signed off last fall, MTA staff discovered a complication: The street is lined with sidewalk basements, making construction more difficult. Now they’re scrambling to modify the design.

Engineering against tragedy is complex, and it won’t work unless drivers also change their behavior, Reiskin said.

But that’s proved even tougher. Speed is the No. 1 factor in serious and fatal collisions, yet police officers have limited ability to control it. Two years ago, Assemblyman David Chiu, D-San Francisco, collaborated with then-Mayor Ed Lee and San Jose Mayor Sam Liccardo on a bill to install automated speeding cameras throughout the two cities. It died in committee, dogged by criticism from law enforcement unions that thought the cameras would take away jobs, as well as automobile clubs and civil libertarians who worried about the invasion of privacy.

“The level of opposition was really unfortunate,” said Chiu, who may revive the legislation.

Leah Shahum, executive director of the national Vision Zero network, views San Francisco as a sobering illustration of how hard it is to end traffic fatalities — even with safety advocates picketing on dangerous street corners and Mayor London Breed directing the SFMTA to double the pace of protected bike lane construction.

“We’re talking about transformative change to the city’s policies and the built environment,” Shahum said. “We know progress isn’t going to be linear.”

She hopes that Breed will help reverse the trend by persistently nudging the police and transportation departments.

Others are calling for sharper interventions: congestion pricing on busy streets; tightening regulation of Uber and Lyft vehicles, adding officers to cite motorists who speed or text while driving, and banning cars along the eastern stretch of Market Street, from the Ferry Building to Hayes Valley.

These ideas are starting to gain steam in San Francisco, where transportation — like housing — is an issue that excites people. Angered by aggressive drivers and road hazards, activists have staged Occupy-style protests to make their plight more visible. A group called People Protected forms human chains to separate bike lanes from traffic on busy downtown corridors, and in recent weeks, demonstrators began turning parking spaces into ad hoc offices to show that streets aren’t just for cars.

“We all have a right to be out in the streets on our bicycles,” cyclist Devon Warner told a group of about 20 participants who gathered Wednesday for the Ride of Silence. “We’re going to demonstrate our humanity here tonight.”

At 7 p.m. the group set off, starting on Florida Street in the Mission District and rambling north toward Howard Street and South Van Ness Avenue, to honor Russell Franklin. Ahlgren led the way.

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