Mr. Liu, a possible candidate for mayor next year, is set to outline his concerns in a report next week that he said would focus on improving the safety of riders and pedestrians and, as a result, better insulate the city from liability.
But accidents will happen, as will lawsuits, and the $10 million insurance policy the city required Alta Bicycle Share, which is operating the program, to obtain may not be large enough, he said.
“It’s unclear; it’s untested,” he said. “What we do know is that claims against the city to date have exceeded $10 million a year, and that’s without these 10,000 rental bikes.”
The Transportation Department disagreed with Mr. Liu’s assessment. “The city has no additional exposure,” said Seth Solomonow, a department spokesman, “and we informed the comptroller of this before he put his stamp on the contract.”
The bike-share contract with Alta Bicycle Share protects the city from claims, Mr. Solomonow said, even those above the insured amount; the company has not faced claims so far over the bike-share programs it operates in Washington or Boston.
The city unveiled the program, which is sponsored by Citigroup and known as Citi Bike, after years of urging by bike groups. It will let riders rent bikes at one location and return them at another. A few other American cities, as well as Paris and cities in China, have similar programs.
The Transportation Department announced proposed locations last month for 420 stations in Manhattan, Downtown Brooklyn and a slice of Queens where the bicycles can be picked up and returned. Discussions are under way to add more.
The comptroller noted that there had been more than 100 bike-related claims filed against the city annually since 2002, with total settlement and judgment costs of about $3.6 million a year. Those payouts represent a small fraction of the roughly $500 million annually that plaintiffs in personal injury and property-damage suits against the city have collected in recent years.
In 2009, the city settled two large bike-related cases — one involving a cyclist killed by a Police Department tow truck in 2006, his office said — that pushed the total for that year to $11.5 million. More bikers would mean more claims, he said.
In addition to recommending that the city require Alta to increase its insurance coverage, Mr. Liu’s report will also call for the city to investigate all bike crashes resulting in serious injury, and to place five police officers trained in accident investigation in each precinct.
Now, 19 specialized detectives who cover the entire city investigate traffic accidents but only ones in which a person was killed or is likely to die.
New York’s program will be the country’s biggest and among the largest in the world.
When a program twice as large began five years ago in Paris, it got off to a bumpy start. Emmanuel de Lanversin, a French transportation specialist, said cycling accidents rose to their highest level in the program’s first year.
But, he said, the next year they fell to around the same number as before the program started.