Public transit riders in the San Francisco Bay Area could soon face tickets and hefty fines for hogging multiple seats – a sign of extreme overcrowding in the metropolitan area’s trains and the latest effort by a US transportation system to combat passengers’ rude behaviors, including so-called manspreading.

The Bay Area Rapid Transit (Bart) agency – which shuttles more than 420,000 customers per weekday in San Francisco and surrounding California cities and suburbs – is considering a new rule that would formally ban riders from spreading into multiple train seats, with an offense punishable by a fine of up to $500.

The controversial proposal comes as mass transit systems in dense urban centers in the US and across the globe have been forced to grapple with huge growths in ridership, which can lead to severe crowding on train cars and buses and can make poor etiquette by riders particularly bothersome.

Social media shaming has also increasingly shined a light on offensive habits, with commuters publishing photos of seat hoggers and “manspreaders”, referring to men – and women – who spread their legs wide enough to take up more than one seat.

While San Francisco’s transportation officials are considering a uniquely aggressive tactic – $100 for the first violation, $200 for a second and $500 for each additional offense – many other large transit systems have opted to raise awareness through marketing campaigns that are often humorous.

New York City’s metropolitan transit agency made headlines when it unveiled subway posters that said, “Dude … Stop The Spread, Please”, and Seattle, Philadelphia, Chicago, Boston and other cities’ transportation systems have rolled out various signs and slogans that remind riders to limit themselves to one seat. Online public shaming has also broken out in London, Toronto, Istanbul and beyond.

“We feel like this has been an effective way for us to get the word out on what good manners are in transit,” said Bruce Gray, spokesman for Sound Transit, a transportation agency in Seattle that used a cartoon octopus to remind riders of the “one body, one seat” principle. “Whether or not people who are the worst offenders are actually seeing this stuff, I don’t know.”

Joel Keller, a Bart director, said he was not aware of other transit systems that had specifically used citations and fines to fight seat hogging, though he emphasized that he expects Bart police would use this punishment as a last resort. The ordinance also contains exceptions for “medical ailments and large body types” and would only apply when a passenger using more than one seat is blocking other patrons from sitting.

“I’m not looking for ... troopers going in there looking for violations,” Keller said. “I would hope it would start and end with the warning.”

Transportation officials and riders agree that the Bart system – which has grown by more than 100,000 weekday riders over the past five years – has major capacity problems, especially during rush hour when trains can be so crowded that passengers sometimes have to let trains pass before they can squeeze in.

But not everyone agrees that a new rule and steep fines would be an effective tool to help alleviate the pains of congestion.

“I’m concerned that it’s too broad and the penalties are too high, particularly since the people who this will affect the most are homeless people sleeping on the train and also people who are traveling from out of the area and don’t know the rules,” said Rebecca Saltzman, another Bart director who said she won’t vote for the ordinance in its current form.

In the past, Bart has also faced accusations that riders use its customer complaint system to racially profile black passengers and to unfairly target the homeless – a problem that some worry could escalate with a new law.

“It’d be extremely mean for homeless people – they don’t have money and they don’t have anywhere to live,” said Anita Smith, a 56-year-old Bart rider who lives in Oakland. “It’s a little severe.”

Jordan Bruno, 24, said that when he asks a Bart seat hogger to make room for him, he never has a problem. “It’s unfortunate we have to resort to fines to enforce what should be decent human behavior.”

Ivy Tullis, a 33-year-old passenger who commutes from the suburb of Pittsburg to San Francisco, said seat hogging is a major problem. “People have actually gotten into arguments and gotten pretty aggressive about sharing seats,” she said.