By the time John Meuleman noticed the Bird scooter on the ground outside the entrance of San Diego’s SDCCU Stadium, he would later tell relatives, it was too late. The 75-year-old was already writhing on the pavement after tripping over the device, his right knee throbbing in pain, he recalled.

Meuleman was taken to a hospital, where X-rays revealed his knee was shattered in four places, according to a copy of the medical report.

Unable to walk during his recovery, the formerly active retiree relocated from his home in Boston to an assisted-living facility in Florida to be closer to family. There, his health rapidly deteriorated, according to his daughter, Robin Miskel. Nearly two months after his accident, Meuleman died days after doctors discovered he had metastatic bone cancer.

Though she doesn’t blame Bird for her father’s death, Miskel said her family is considering suing the company for her father’s injuries, saying its practices “robbed him of a chance of any quality of life for his last weeks on earth.”

She added, “This accident was completely avoidable. . . . What other mode of transportation can you just leave in the middle of the sidewalk with no repercussions?”

Citing rider privacy, a Bird representative said the company does not comment on “specific incidents.”

For months, public officials, doctors and ­scooter company employees have warned about the dangers associated with riding electric scooters, which have appeared in more than 100 cities worldwide since last year. At the same time, in emergency rooms across the country, trauma doctors have reported an influx of severe injuries among users of the devices that began as soon as they appeared on city streets.

Now, many of these people are beginning to warn about the dangers the devices pose to pedestrians. There are no official numbers illustrating how frequently pedestrians are injured by scooters, but doctors interviewed in five cities say badly injured pedestrians are showing up in trauma centers multiple times a week.

In San Diego — where thousands of e-scooters have flooded the streets — the founder of one neighborhood group told the city council’s public safety committee that his elderly neighbors are afraid to set foot outside, knowing a broken hip can be a debilitating injury requiring surgery. Curt Decker, executive director of the National Disability Rights Network, said the devices are a commuting nightmare for the visually impaired and those who get around via wheelchair.

While able-bodied people can usually maneuver around ­e-scooters, the elderly and disabled can have a much harder time, said Wally Ghurabi, medical director of the Nethercutt Emergency Center at the UCLA Medical Center in Santa Monica.

“I’ve seen pedestrians injured by scooters with broken hips, multiple bone fractures, broken ribs and joint injuries and soft tissue injuries like lacerations and deep abrasions,” he said, estimating he sees several people injured by e-scooters each week.

Last month, the Centers for Disease Control and Prevention announced plans to study the health risks associated with the devices by analyzing injuries to riders and pedestrians in Austin over two months.

Charged overnight by scooter company workers, e-scooters are left on city streets during the day, where they can be accessed using an app. Because the devices are dockless, they can be left anywhere, including on crowded sidewalks, once a user has finished riding.

Though laws differ nationwide, in many cities, riding e-scooters on sidewalks is banned, with Denver being a notable exception before its city council outlawed the devices on sidewalks this week. In some cities, such as Austin and Washington, riding on sidewalks is permitted in some areas but not in others.

Two of the largest e-scooter companies, Lime and Bird, say that safety is a top priority and that they encourage riders to follow local regulations. Lime says the company is investing more than $3 million to “promote safe-riding behavior and proper etiquette.”

Bird says the company provides in-app safety information tailored to local laws. In some cities, the company also employs “Bird Watchers,” whose job it is to ensure the company’s devices are “parked and picked up correctly,” a Bird representative said. “Bird instructs riders to follow all local rules regarding e-scooter riding. We are deeply committed to the safety and well-being of the entire community, and so we make a concerted effort in every city where we operate to provide safety information to our riders that reflects their city’s rules.”

Regardless of local laws, critics say, scooter riders — often lacking access to bike lanes and hoping to avoid speeding cars — frequently take refuge on crowded walkways. The results can be deadly.

In August, a Spanish teenager riding an electric scooter while reportedly looking at his phone struck a 90-year-old woman out for her daily walk, according to the newspaper El País. The victim died due to severe head injuries several days later, becoming the first pedestrian publicly identified as being killed by an e-scooter. According to El País, prosecutors have said the teenager may face involuntary manslaughter charges.

Efforts to confirm details from the case, such as the defendant’s name, were unsuccessful.

A month earlier, Cody Daniels said, he was walking out of a parking garage in downtown Dallas when he was mowed down by a man traveling “full speed” on a Lime scooter. The rider left the scene.

The 200-pound 32-year-old was left with scrapes on his knee and face, as well as a deep gash above his right eye that required seven stitches.

He couldn’t pay for a lawyer, and with no way to track down the rider, Daniels said, he paid a $250 urgent-care medical bill himself and tried to “move on.”

Though his physical injuries healed, he said, the accident has made him anxious.

“I’m paranoid now,” said Daniels, who lives and works in an area where packs of e-scooter riders are commonplace. “Every time I turn the corner, I peek out real quick to make sure somebody isn’t coming on a scooter. I’m always looking out for them everywhere I go.”

In October, multiple pedestrians joined a class-action lawsuit in Los Angeles County Superior Court, accusing Lime and Bird, as well as other e-scooter companies, of “gross negligence” and “aiding and abetting assault.”

Responding to the allegations, Bird said cars “remain the greatest threat to commuters.”

Lime said the company is reviewing the complaint.

How will the lawsuit fare? Legal experts say the patchwork of differing rules suggests that establishing liability in cases involving e-scooters and pedestrian injuries will largely depend on where accidents take place and the circumstances. In the coming years, they say, test cases will give the industry a clearer definition of liability.

Unlike shared bicycles, they say, which tend to place liability on the user, or vehicles, which are covered by liability insurance, e-scooters operate in a gray zone in which liability is often undefined. The difference between tripping over a scooter left on the sidewalk and tripping over a random piece of trash is that it is likely a scooter’s owner or rider can be identified, one expert said.

In some situations, multibillion-dollar scooter companies may be held liable, but in others, reckless scooter riders, local governments or their insurers could be forced to compensate injured pedestrians, according to Bryant Walker Smith, a law professor at the University of South Carolina who is teaching a technology law class next semester exploring ­e-scooter regulation.

The question for courts surrounding electric scooters will be whether someone — or something — behaved unreasonably, Smith said, whether that’s an e-scooter company, a local government or someone who left a scooter on a sidewalk.

“Was that person legally required to act and did their failure to reasonably act cause a pedestrian’s injury?” Smith added. “Pretty soon, judges will face injured people with limited options, and they will begin to answer that question by creatively shaping the law.”

In Cincinnati, where riding e-scooters and bicycles on the sidewalk is illegal, the city council has forced scooter companies to create a $1 million fund covering medical costs and lost wages incurred by injured pedestrians.

It’s a step in the right direction, council member David Mann said, though he added that he’s still troubled that police are being forced to monitor sidewalk riders.

“We’re using precious police resources to deal with a problem caused by a profit-making company,” he said. “It’s just bizarre, in my opinion, that we have to deal with this. We have lots of wide streets and walking pathways.”

Tara Williams, 44, was returning to work from lunch in late August when a young man riding an e-scooter ran a red light and slammed into her, throwing her to the ground, she said. Williams said she has racked up about $1,000 in medical bills, and though Bird agreed to cover it, she said, the company’s insurance provider refused. Williams paid the bill herself, noting she had never heard of a $1 million fund for pedestrian injuries.

Bird narrowed down the suspect to three people, whose accounts were suspended, Williams said. But Bird refuses to reveal the rider’s identity, citing privacy laws, she added.

Without disputing the details of Williams’s claims — or explaining why the company wouldn’t pay her medical costs — a Bird representative declined to comment.

“We see tons of little kids on these scooters, some of them not even tall enough to see over the handlebars,” said Williams, who is convinced the person who hit her was a teenager. “They’re just whipping around not even looking for pedestrians, and there’s no repercussions for them using Bird’s property.”

Bird requires riders to upload a driver’s license to confirm they’re at least 18. A company representative said Bird also encourages people to report “irresponsible behavior” to local authorities.

“We investigate each report, cooperate with local authorities, and take appropriate next steps, which can include removing individuals from the platform,” the representative added.

But if there’s one city experiencing the greatest friction between e-scooters and pedestrians, it may be San Diego, which has a large population of retirees and e-scooter-using tourists.

Mayor Kevin Faulconer has proposed regulating scooters by restricting their speed to 8 mph in busy pedestrian-traffic zones, and California law bars e-scooters from being operated on sidewalks.

But Jonathan Freeman, the founder of Safe Walkways — a Facebook group started by concerned neighbors that seeks to keep scooters off sidewalks — wants the city to ban e-scooters until companies can ensure they are ridden only on the street.

Freeman said his elderly neighbors, terrified to walk along the city’s waterfront promenades for fear of being struck, find themselves under self-imposed house arrest.

“A 200-pound projectile traveling at 8 mph is going to do severe damage to an elderly person, a disabled person or any person traveling on foot who is hit by them,” he said. “The mayor’s proposal is a non-starter — an utterly ridiculous proposal.”