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.
"There's a common misperception that hands-free phones are safer when the research clearly suggests that they they're both equally risky," said Arthur Goodwin, a researcher at the University of North Carolina Highway Safety Research Center.
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.
Gov. Arnold Schwarzenegger sought to reassure drivers that they need not hang up their phones when he spoke at a signing ceremony for the California law in 2006. "You don't have to stop talking on your cellphone, but use a headset or use a speaker system, and you will be fine."
If hands-free is the path of least resistance, it was still a long, hard slog for Sen. Joe Simitian, the Palo Alto Democrat who sponsored the bill. Simitian tried but failed to win passage for five years before breaking through. He said he persisted because he was sure the law would save lives.
"There isn't a study in the world that says you're safer driving with a cellphone clutched to your ear than when you are driving with both hands on the wheel," he said.
But Goodwin and other 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."
"We are convinced that legislation forbidding the use of hand-held cellphones. . . . will not be effective," the letter said. Such laws "may erroneously imply that hands-free phones are safe to use while driving."
The letter was based on a lengthy review of worldwide research on driver distraction conducted at the National Highway Traffic Safety Administration, a branch of the Department of Transportation. In that 2003 review, the agency's researchers for the first time estimated fatalities linked to cellphone use by drivers, putting the toll at 955 deaths in 2002. They predicted that it would only rise because of the growing use of cellphones and especially such activities as text messaging, former agency officials said.
After a June, 2003 meeting with Department of Transportation authorities, the letter was drafted but then spiked. The fatality estimate was never made public.
"They don't put the numbers out there because the numbers make it a lot harder to explain why you haven't been more active," said Bill Walsh, former senior associate administrator of the agency.
An agency spokesman, Rae Tyson, declined to comment.
Other published research, however, has resulted in similar findings. A 2003 study by the Harvard Center for Risk Analysis estimated that there are about 2,600 deaths and 12,000 serious to critical injuries a year in crashes involving drivers using cellphones.
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.
A 2006 study by David L. Strayer and colleagues at the University of Utah found that drivers tested on simulators performed about the same when they used cellphones as when they had a blood alcohol-level of 0.08%, which made them legally drunk. The drivers actually did better in braking and avoiding rear-end collisions when alcohol-impaired than when they were talking on hand-held or hands-free phones.
There are some skeptics. A 2006 paper co-authored by James E. Prieger, a professor of public policy at Pepperdine University in Malibu, 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."
Supporters of California's "hands-free" law cite Highway Patrol statistics showing more accidents involving hand-held phones than hands-free, but the data are limited and not adjusted for the number of hand-held or hands-free phones in use.
Some of the largest U.S. corporations bar employees from using cellphones when driving during work hours, making no exception for hands-free calling. DuPont, Chevron, ExxonMobil, Shell, and its parent, Royal Dutch Shell PLC, with collectively hundreds of thousands of employees, are among those with cellphone bans.
AMEC, a large engineering concern, also prohibits its 7,000 employees in the U.S. and Canada from using cellphones while driving. "There is no better way to proactively boost safety for a mobile, white-collar workforce," company spokesman John Kageorge said.
But with cellular use exploding -- up to 73% of Americans at least occasionally use cellphones while driving, according to one survey -- what companies can do by fiat may be politically impossible for state legislatures.
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.
Joe Farrin, assistant vice president for public affairs at CTIA-The Wireless Assn., an industry trade group, said: "Generally, our view has been the issue of driver distraction is . . . bigger than one distraction, but at the same time we are not going to oppose a hands-free only bill, we are not going to oppose restrictions on young drivers."
Total prohibition is "off the table," said Matt Sundeen of the conference of state legislatures.
"I think a number of sponsors of the hands-free legislation would tell you . . . that they're interested in prohibiting more than just the hand-held device, but it's just not something that's politically feasible," he said.
Under the law, a first offense will bring a $20 ticket; subsequent violations will cost $50, with no points against a driver's insurance.
A companion law sponsored by Simitian will bar drivers younger than 18 from using any type of cellphone, similar to restrictions in 16 other states.