As California joins five other states in requiring drivers to use hands-free devices when talking on cellphones, an increasing body of research suggests the legislation will accomplish little, the New York Times reports.

The risk doesn't stem from whether one or both hands are on the wheel, the research suggests. It's whether the driver's mind is somewhere else.

The biggest danger is "cognitive capture" — or being blind to driving cues because one is absorbed in conversations, especially emotional ones.

California motorists will be required to use a hands-free device to talk on a cellphone starting July 1 under a new traffic safety law. Such laws are already in effect in New York, New Jersey, Connecticut, Utah, Washington state and the District of Columbia.

Hands-free laws have come to be seen as the most politically feasible way to address the dangers of driver distraction because of cellphone use.

Some scientists say that hands-free laws could actually make things worse by encouraging drivers to make more or longer calls.

Indeed, federal highway safety officials drafted a letter from then-Transportation Secretary Norman Y. Mineta to the nation's governors in 2003 to warn against laws like California's that allow hands-free calling. For reasons never fully explained, the letter was neither signed by Mineta nor sent. According to the bluntly worded letter, obtained by The Times, "overwhelmingly, research worldwide indicates that both hand-held and hands-free phones increase the risk of a crash."

After a June, 2003 meeting with Department of Transportation authorities, the letter was drafted but then spiked. The fatality estimate was never made public.

Two widely cited studies found a fourfold greater crash risk for drivers using cellphones than for normal driving — with nearly identical risks for hand-held and hands-free phones. The studies looked at drivers and collisions in Canada and Australia, where cellphone records were available for analysis, unlike the U.S.

There are some skeptics. A 2006 paper co-authored by James E. Prieger, a professor of public policy at Pepperdine University in Malibu, Calif.

found that the link between cellphones and collisions was less conclusive, and the crash risks probably lower, than indicated in some of the most prominent studies.

But, Prieger said, "if you've ever used a cellphone in a car and you're honest with yourself, it's hard to doubt that at some level it doesn't make you a riskier driver."

Of 95 bills pending in 28 states that relate to cellular use by drivers, none would impose an all-out ban, according to the National Conference of State Legislatures. Typically, the bills would prohibit talking by teenagers or school bus drivers, or require hands-free devices — all measures the multibillion-dollar cellphone industry no longer opposes.