The bait bike program is not unique to San Francisco; various cities and college campuses have been busy adopting them. The programs are meant both to get thieves off the street and to act as a deterrent, making prospective criminals unsure which bike might be bait and not bounty.

At the University of Wisconsin, Madison, a pioneer in the bait bike movement, the police department saw a 40 percent drop in reported bike thefts on campus in 2008, its first year using the tactic. The police there also use social media, but mainly to put out the word about the bait bikes, not to shame the perpetrators by posting their photographs, said Marc Lovicott, a spokesman for the department. But he sounded impressed by the tactic. “Interesting — we haven’t gone that far,” he said, adding that they might.

More often Officer Friedman’s postings on Twitter are pedestrian, explaining how to lock a bike properly or encouraging people to put a sticker on their bike that says, “Is this a bait bike?” He also recommends that people register their bicycle’s serial number with the police — surprisingly few do — so that officers can match a stolen bike with its owner.

Many bike thefts happen at people’s homes, often from the garage, rather than on the streets, hence Officer Friedman’s Twitter post this month: “If anyone is willing to let us camp out in their garage for the evening, contact me at Park Station.”

He promises to sit quietly in the garage, catch a bike thief, then tweet and repeat.