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Until his bike slid out of control while he was going 35 miles an hour downhill around a sharp turn, Dr. Harold Schwartz thought cycling accidents were something that happened to other people. Now, after recovering from a fractured pelvis, Dr. Schwartz, 65, the vice president for behavioral health at Hartford Hospital in Connecticut, has changed his mind.

“No one is immune,” he said in an interview. Like many avid cyclists, he is convinced that it is not if you crash. It’s when.

But Rob Coppolillo, 43, who was an elite level amateur bicycle racer for 10 years, led cycling tours in Italy and regularly rides in his town, Boulder, Colo., begs to differ. He’s never had an injury more serious than a little road rash, he says.

“For the vast majority of us, it’s a pretty safe sport,” he said.

ROOM FOR DEBATE Rules of the Road Should cyclists obey the laws written for cars, or do we need different infrastructure and rules?

Who is right? Although many cyclists have strong opinions on the safety of their sport, the answer is that no one really knows how safe it is, or whether its safety has changed over the years.

It’s not that there is a lack of data. Instead, it is that the data are inadequate to answer the questions. No one has good statistics, for example, on crashes per mile ridden. Nor do the data distinguish road cycling on a fast, light, bike with thin tires from mountain biking down dirt paths filled with obstacles or recreational cycling on what the industry calls a comfort bike. Yet they are very different sports.

What remain are often counterintuitive statistics on the waxing and waning of cycling in the United States, along with some injury studies that could give cyclists pause.

For instance, although there is a widespread perception that bicycling is becoming more popular, data from the National Sporting Goods Association show that the sport’s peak — as measured by the number of people who say they ride — was in 2005, when it reached 43.1 million Americans. Last year, the number was 39.3 million.

Those data go back to 2003. But the National Bicycle Dealers Association has sales figures that go back decades. Consistent with the ridership survey, 2005 was a good year, with 14 million adult-size bikes sold. Last year, that number was 13 million. But the record year, never surpassed, was 1973, when sales reached 15.2 million.

The Centers for Disease Control and Prevention keeps statistics on deaths and emergency room visits resulting from bicycle accidents. The yearly death rate has ranged from 0.26 to 0.35 per 100,000 population, with no particular pattern; in 2010, the agency says, there were 800 bicycle fatalities, about one-fortieth of all road deaths.

“There is no trend,” said Linda Degutis, the director of the agency’s National Center for Injury Prevention and Control, who added that bicycling seemed no more dangerous than other sports.

Dr. Rochelle Dicker, a trauma surgeon at the University of California, San Francisco, does not see it that way. She cares for victims of the worst bicycle injuries, people who might need surgery and often end up in intensive care. So she decided to investigate those crashes.

She and her colleagues reviewed hospital and police records for 2,504 bicyclists who had been treated at San Francisco General Hospital. She expected that most of these serious injuries would involve cars; to her surprise, nearly half did not. She suspects that many cyclists with severe injuries were swerving to avoid a pedestrian or got their bike wheels caught in light-rail tracks, for example. Cyclists wounded in crashes that did not involve a car were more than four times as likely to be hurt so badly that they were admitted to the hospital. Yet these injuries often did not result in police reports — a frequent source of injury data — and appeared only in the hospital trauma registry.

Dr. Dicker is not a cyclist, but she said, “Lots of my colleagues do not want to ride after seeing these injuries.”

Her study seems to give credence to the “not if, but when” camp, or at least justify a fear of bicycle crashes. Still, if the statistics show cycling to be relatively safe, why do so many people know (or know of) cyclists who have had serious injuries? Why does the sport seem so dangerous?

George Loewenstein, a professor of economics and psychology at Carnegie Mellon University, wonders if part of the problem is that official statistics miss most of what is happening.

“There are all sorts of reasons why bike accidents are likely to be dramatically underreported,” he said. Unlike auto accidents, they rarely involve either an insurance claim or a police report. And injured cyclists may not go to emergency rooms. Even those with a broken collarbone may see an orthopedist instead.

Dr. Loewenstein’s own accident was typical. He cut through a parking lot and hit a huge patch of ice. He went down, injuring his shoulder. But he never went to an emergency room and was not counted in accident statistics.

Another reason for the perception that cycling is uniquely dangerous is the very nature of the injuries, Dr. Loewenstein said.

Unlike injuries in other sports — a stress fracture from running or a rotator cuff tear from swimming — cycling injuries do not come on gradually. There is a before and an after, and people tend to dwell on that one moment when everything changed.

“There is a focal moment,” Dr. Loewenstein said, “and it is easy to replay it in a way that undoes it. ‘If only I was at the intersection 10 seconds before or 10 seconds later.’ These accidents haunt especially because the ‘if only’ is so intense.”

Still, many cycling injuries are nowhere nearly as bad as overuse injuries in other sports.

“I no longer play soccer,” Mr. Coppolillo, the Boulder cyclist, said. “I sprained one of my ankles so many times it does not work anymore.”

But he added: “I rarely meet a person who had to stop cycling because of injuries. People fall off their bike, but for the most part injuries are niggling things like skin abrasions.”

When a bone does break in a cycling crash, it is often the collarbone. And compared with a stress fracture or a torn rotator cuff, a broken collarbone is not so bad.

“You are back riding indoors on your trainer in a week and riding outside in a month,” Mr. Coppolillo said.

Andy Pruitt, the founder of the Boulder Center for Sports Medicine and an avid lifelong cyclist, agrees.

He also understands the impressions people have of cycling’s dangers.

“If you went into my waiting room, you would be convinced we are all going to die of cycling injuries,” he said. “But that is just not true.”

Dr. Pruitt cites his own example. Now 62, he was a bicycle racer and has been riding for the past four decades. He covers 5,000 to 10,000 miles a year.

In all that time, he has had four serious crashes. He broke his collarbone twice while racing and had two crashes on a mountain bike, breaking a hip one time and spraining a wrist the other.

Considering all the miles he’s ridden and all the risks he’s taken racing and crashing down trails on a mountain bike, he thinks that injury rate is not so bad.

“I’ll take it,” he said.