On a day that seemed perfect for sightseeing, four tourists climbed onto their rented Segways for some final practice maneuvers before a guided trip around Washington.

As their young tour leader and a reporter watched, the riders circled the plaza outside City Segway Tours as if trying to lead an awkward partner on a dance floor, their confidence growing little by little with each slow turn.

Then one crashed.

Megan Jennings, a tourist from New Zealand, went over a small bump, nicked the building’s wall with the handlebars, and toppled onto the plaza. She clutched her left knee in pain. Other riders gathered around her. Then suddenly the group of four became a group of two, as an ambulance was summoned to transport her and her husband to a hospital.

Grayham Jennings said in an email later that his wife suffered a broken leg that may require surgery when she returns home. He added that her injury forced them to cut their trip short — they had intended to be in the United States until June 29 — and they were making their way back to New Zealand via Houston and 20 hours of flight.

“Just a silly accident, and unfortunately she twisted her knee as she fell,” he wrote.

Mike Cook, city operations manager for City Segway Tours, said in an interview shortly after Tuesday’s crash that such mishaps are exceedingly rare.

“I would say most accidents happen — as accidents when driving a car happen — when people aren’t paying attention. If you’re not looking where you’re going, you’re going to have an issue,” he said.

Others — including a personal injury lawyer and emergency room physicians at nearby George Washington Hospital — are not so sure.

“These are much more complicated to operate than they appear,” said attorney Kenneth M. Trombly, whose firm is handling a $6 million lawsuit on behalf of an Illinois woman who broke her leg on one of the devices four years ago during a tour in Washington. “We look in the movies — and they’re sort of these nerdy devices [that] the ‘Mall Cop’ is on — and they look kind of benign. But they really aren’t.”

With the unofficial start of summer now underway, the nation’s capital is once more jammed with tourists. Many of them sign up for guided Segway rides with agencies such City Segway Tours, which according to Cook is the largest in town. Day and night, these mounted tourists set off in geeky-looking packs to cruise the White House, the Capitol, the Washington Monument and other popular attractions. The price is $65 for a two-hour tour, $75 for three hours.

Cook said a Segway tour is more fun and dynamic than a tour bus, because the groups are smaller, and the personal transporters allow people to cover a lot of ground.

“For one thing, it’s just different — you don’t normally get to ride a Segway in your home town,” Cook said. “Plus, you can get a lot closer to the monuments. I think they’re a blast.”

Riders must wear helmets and sign waivers against possible injury claims. Training lasts about 30 minutes, including hands-on assistance from guides. There’s also an orientation video that lasts five minutes or so.

Elizabeth Trotter, a family court mediator from St. Louis, said she thought the machines seemed like a good way to see the city. Although she had been assigned to a group with the Jenningses, and the accident gave her pause, it wasn’t enough to keep her from taking the tour.

“It’s a little scary at the beginning, but it seems like something you can get after a few minutes,” Trotter, 47, said.

The battery-powered, two-wheeled Segway Personal Transporter, as it’s formally known, relies on “dynamic stabilization” technology to move around, according to its owner’s manual. The machine balances, travels and turns depending on how a person stands on it. If you lean forward, the Segway leans forward and moves in that direction. If you lean back, the Segway rolls backward. If you pivot, so does the device. Their top speed is 12.5 mph, but D.C. traffic regulations impose a 10-mph limit.

Police officers, parking enforcement officials, and airport security have made them commonplace. George Oscar Bluth, Jr. — better known as Gob or G.O.B., the goofy son (Will Arnett) of a real estate king in “Arrested Development” — made them immortal sight gags.

But some critics say tourists and other first-time riders don’t understand the machine’s risks. In 2010, physicians from George Washington University Hospital’s emergency room published a paper in the Annals of Emergency Medicine because of the surprising number of cases they were seeing. What they found was also alarming in severity.

From April 2005 to November 2008, doctors found 41 people had been injured. More than 70 percent were visiting the Washington metropolitan area for the summer tourist season. Of the injured, about 1 in 4 had to be admitted for treatment, including some who had traumatic brain injuries. The median cost of their hospital stay was $25,733.

Trombly said 30 minutes is not enough time to train first-time riders, especially for a trip that lasts at least two hours on city streets and sidewalks in all sorts of weather. In his case, the client from Illinois had called before her ill-fated trip to find out if the machines were safe to ride in the rain and whether the tour would go ahead, according to court papers filed in U.S. District Court. The lawsuit says the company was negligent to take the tour out in such conditions. Toward the end of the tour, the woman broke her leg.

The company denies  any wrongdoing. Gregg E. Viola, an attorney for City Segway Tours, said in court papers that the operators were in no way negligent, arguing among other things that bicycles also have more difficulty gaining traction in wet weather but no one would say it’s too dangerous to ride them in the rain. Viola didn’t return a call seeking comment.

Hours after Jennings’s mishap this week, new groups were learning how to ride the machines outside City Segway Tours. Many first-timers appeared leery at first, then cheerful as they got the hang of it.

“Now you got it!” someone shouted. ” Woo!”

“You just need to relax,” a woman said.

The more daring performed mini-slalom runs between the supports of a canopy above the plaza’s walkway.

Then the groups rolled out. Three hours later, the groups rolled back in, including one whose tour guide had a rider in tow. The customer needed extra help after tipping over on the Mall’s gravel paths, according to Don White and Justin Reynoso, who were also in the group.

“It’s a little nerve-racking to start,” said White, 52, a Fannie Mae employee visiting from Atlanta. “But once you get on it, and get used to it, it’s kind of easy.”

White said the only time it was a little unnerving for him was when he started rolling downhill on Pennsylvania Avenue faster than he wanted to, and the guide had to intervene.

“It’s counterintuitive a little bit,” said Reynoso, 36, a Dallas resident who also works for Fannie Mae. “To me it was very similar to skiing.”

“Or skateboarding,” White said.

But both men said the trip, despite a few white-knuckle moments, had been fun.

“You don’t want to get off,” Reynoso said. “It was great.”

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