A pack of vigilantes that typically does its deeds in the dark has been hard at work on the streets of San Francisco for the past few months trying to make them safer for bicyclists and pedestrians by surreptitiously plunking down orange traffic cones or planting white plastic posts.

The city’s Municipal Transportation Agency hasn’t denounced the group but has been quickly dismantling much of its work, saying it hadn’t been through the proper processes and, in some cases, violates traffic codes.

The group, which has adopted the name San Francisco Municipal Transformation Agency in an obvious play on the name of the MTA, has been placing the cones and posts to keep cars out of bike lanes and crosswalks that drivers often roll through.

An early member of the group, who wanted to remain anonymous because the actions could be construed as illegal, said SF Transformation was born in July after two women on bicycles were hit and killed by motorists in San Francisco in separate June 22 accidents.

The group believes the city isn’t working fast enough to make streets safer for cyclists and pedestrians.

“We’re fed up with how slow the SFMTA is,” said the man, who identified himself as “Copenhagen,” the Danish city known as a bicycling haven. “The reason we’re doing this is that people are dying because they’re not working fast enough.”

According to the Vision Zero Coalition, which aims to eliminate traffic deaths in the city by 2024, 20 people died during the first eight months of this year in traffic collisions, 10 of them pedestrians despite the coalition’s efforts.

The group started by buying orange cones, emblazoned with the letters SFMTrA, and then placing them along bike lanes separated from traffic by painted lines only. The cones seemed to do the trick by keeping vehicles out of the bike lanes, he said, but were easily removed. So the group ordered white plastic traffic posts, which can be glued to the streets. They are reportedly the same posts the MTA uses.

As of Friday, the group had managed to place the more permanent posts, officially known as soft-strike barriers, at the eastern entrance to Golden Gate Park on John F. Kennedy Drive and on Crossover and Transverse drives, where they narrowed a two-lane road to a single lane in the park to slow traffic approaching a crosswalk.

They have also placed them on Folsom and Division streets, at the start of a bike lane to protect a left-turn lane for bikes on Fell Street on the famous Wiggle bike lanes to Golden Gate Park, and in the crosswalk at Geary and Leavenworth streets, where they protected part of the crossing from turning vehicles.

Bicyclists riding down the busy Valencia Street bike lane Monday discovered that the SFMTrA crews had installed plastic posts between 14th and 17th streets earlier in the morning.

The posts at Crossover and Transverse, which narrowed the roadway at a crosswalk, were quickly removed, Copenhagen said, as were those at Geary and Leavenworth. At last check, he said, the Folsom Street and Fell Street posts remained. The MTA has said it won’t touch the JFK posts because it has its own plans to install plastic barriers at that location, he said.

Ben Jose, an MTA spokesman, confirmed that the group’s posts will stay until the city replaces them. But he said the agency has removed other barriers because they haven’t been properly planned or approved.

“Generally, we have no choice but to remove the cones and posts that don’t go through an official process because it is a code violation to place objects in the roadway, and it can create confusion,” he said.

The posts can also interfere with other street uses, he said, as they did on Geary Street, where they blocked a towaway zone used for parking during the day but opened to traffic during commute hours.

Copenhagen said the group has grown rapidly, attracting dozens of members and raising thousands of dollars via its website, at www.sfmtra.org. The money has enabled the renegades to buy the posts. Most of the installations take place at night and are often chronicled on Twitter, @sfmtra.

The posts are an inexpensive and simple way to attempt to give bikes and pedestrians more safety than a painted line, island or buffer, Copenhagen said. Drivers sometimes fail to see the painted lines and often choose to ignore them, he said. Uber drivers, double parkers and delivery trucks frequently use bike lanes to pull over or park.

“Paint doesn’t stop cars. They just ignore it,” he said. “It’s incredibly simple to do things like put up a white post and stop an Uber car from parking and blocking a bike lane.”

MTA officials appreciate the intent of the bike lane vigilantes, Jose said, and have engaged them in email conversations in an attempt to work with them. The MTA, a key player in the city’s Vision Zero campaign, has made many safety improvements at the city’s deadliest intersections, and has installed 27 miles of bike lanes that are separated either by various kinds of barriers or by a 2-foot-wide buffer zone painted into the asphalt.

As part of the Vision Zero effort, the Recreation and Park Department plans to install speed humps along JFK Drive through Golden Gate Park.

“We certainly understand that people are passionate about safety,” Jose said. “We are, too.”

He said the agency would like Transformation’s suggestions of where it should install official plastic stakes or other barriers. Through an interactive map on its website, the vigilante group solicits ideas from its followers for more of its makeshift improvements.

“We’d like to hear where people are suggesting them and where they would like us to put them,” Jose said.

Copenhagen confirmed discussions with the MTA, and said the group has recommended spots that would benefit from plastic posts — official or unofficial. One of those places, an infamous spot among cyclists, is on Market Street at Gough Street.

The MTA, he said, told the group it would install posts there by early next week to protect cyclists.

“If they don’t,” Copenhagen said, “we’ll have to.”

Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: mcabanatuan@sfchronicle.com Twitter: @ctuan