New York recently became the first state to prohibit people from using hand-held mobile phones while driving. "This is going to save lives, I'm sure," said New York Assembly Speaker Sheldon Silver.

It's not clear how he can be sure. As The New York Times reported, the new law instructs state traffic officials "to analyze the causes of accidents over the next four years to determine if cell phones do cause accidents." This seems to be a case of legislate first, ask questions later.

The evidence on which the ban is based comes from opinion polls rather than research showing that cell phones are a serious road hazard. A Quinnipiac University poll published in March found that 87 percent of New York voters liked the idea of making drivers hang up their phones. By a remarkable coincidence, that was exactly the level of support for the legislation in the New York Assembly.

The motivation of Assemblyman Felix Ortiz, the bill's main champion, does not inspire confidence that his colleagues' views on this issue were better informed than the opinions of randomly selected New Yorkers. According to the Times, Ortiz decided to push for a ban "after witnessing an accident caused by a woman on a telephone." One wonders what shape Ortiz's legislation might have taken if he had seen a crash caused by a guy eating a Big Mac.

Once you get past the anecdotes, there's not much empirical support for cell phone bans, which have been considered by 42 states since 1997. That was the year two Canadian epidemiologists, Donald Redelmeier and Robert Tibshirani, published a study in The New England Journal of Medicine suggesting that using a phone quadruples a driver's chance of getting into an accident.

Redelmeier and Tibshirani examined the telephone records of about 700 Toronto drivers who owned mobile phones and had been involved in crashes that resulted in serious property damage. Accidents were especially likely to occur around the time of a phone call.

The study did not support one of the assumptions underlying New York's law, which permits the use of phones with headsets or speakers. "We observed no safety advantage to hands-free as compared to hand-held telephones," Redelmeier and Tibshirani wrote. "One possibility is that motor vehicle collisions result from a driver's limitations with regard to attention rather than dexterity."

Although provocative, this study did not include data for accidents in which people were injured, the sort that advocates of cell phone bans seem to have in mind. More important, there was no indication of whether the drivers with cell phones were at fault.

Assuming they were, the cause may not have been talking on the phone itself. A driver who is already distracted because he's lost, for instance, might use his phone to get directions.

"Our study indicates an association but not necessarily a causal relation between the use of cellular telephones while driving and a subsequent motor vehicle collision," Redelmeier and Tibshirani wrote. "For example, emotional stress may lead to both increased use of a cellular telephone and decreased driving ability. If so, individual calls do nothing to alter the chances of a collision."

Even if using a cell phone does increase the risk of a crash, the benefits of a ban might not be worth the cost of enforcement, depending upon how often such accidents occur. A study sponsored by the AAA Foundation for Traffic Safety, based on data for accidents involving more than 32,000 vehicles, found that cell phones were rarely a factor in distracted driving. Phone use was cited in only 1.5 percent of the cases where inattention contributed to crashes; by comparison, 29 percent of the drivers were distracted by things going on outside the car, 11 percent by the stereo, and 11 percent by passengers.

It's hard to imagine a law against driving around with little children, although they must cause far more accidents than little phones do. The decision to single out a particular driving hazard is partly a matter of practicality, but it may also reflect a moral judgment that has nothing to do with safety.

Laws against driving while intoxicated, which criminalize behavior that's less dangerous than driving after midnight when you've been up all day, make a statement against drinking in general. Similarly, cell phone bans suggest discomfort with a new technology that goes beyond its role in traffic accidents.