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Road rage, applying makeup, eating, scrambling for items on the back seat, reading, daydreams, punching buttons on the radio, checking the GPS, talking - the litany of risky on-road antics that wreaks havoc on the freeways of Los Angeles is a moveable feast.

But on the Richter scale of driver distraction, mobile phones score a perfect 10.

Everyone has a hellish tale about being almost wiped out by someone talking on a mobile. According to a 2006 study by the Virginia Tech Transportation Institute, inattentive motorists cause 80 per cent of all US car accidents, with mobiles the worst offender. But could a new state law, due to take effect on July 1, vanquish this threat in California?

On that date California will ban the use of hand-held cellphones while driving. Offenders face a fine of US$20 ($25.50) for the first offence and US$50 for every offence thereafter. Genuine emergencies, when drivers call 911, are exempt. Hands-free phones remain legal.

"You don't have to stop talking on your cellphone, but use a headset or use a speaker system and you will be fine," said Governor Arnold Schwarzenegger, in a bid to mollify California's legions of cellphone junkies, when he signed the bill in 2006.

Whether this law will have any real impact is unsure. Questions remain about enforcement and effectiveness, especially given claims that hands-free devices are as distracting as hand-held ones.

Finally, the modest amount of the fines in a city where parking fines start at US$55 hardly acts as a deterrent. "It will be just one more thing we'll be out there looking for. Because we believe cellphones are a distraction," said a California Highway Patrol spokesman, who noted that court costs for contested fines could triple the total.

Nonetheless, since 2001, when New York led the trend towards some sort of prohibition on mobile phone use while driving, that state has issued around 1 million tickets.

California joins New York, New Jersey, Washington DC, Utah, Connecticut and Washington state, all of which ban hand-held mobiles. California has also banned the use of all mobiles, as well as pagers and other electronic communications devices, by drivers under 18. Sixteen other states, plus Washington DC, have similar teen laws on the books. Washington state and New Jersey also prohibit text messaging while driving.

"At least 21 other states are considering legislation to ban text-messaging," says Matt Sundeen, a transport analyst with the National Conference of State Legislators.

The trend is creeping into corporate thinking, with Chevron, Shell, ExxonMobil and DuPont forbidding their employees to use their mobiles while driving on the job.

To date, the federal Government has not weighed in. But growing evidence cites mobile use as a factor in traffic accidents. In 2003 the US National Highway Traffic Safety Administration concluded 955 people died in 2002 from crashes linked to their use. In a letter they warned fatalities would rise as mobiles became more ubiquitous.

"Overwhelmingly, research worldwide indicates that both hand-held and hands-free phones increase the risk of a crash," the letter warns. The research was never made public. The letter, obtained by the Los Angeles Times, continues: "We are convinced that legislation forbidding the use of hand-held cellphones will not be effective."

Worse, by implying hands-free devices are safe, a limited ban might encourage more frequent and longer phone use, raising driver distraction.

"It appears that something different goes on when you talk on a cellphone, as opposed to talking to someone next to you," says Arthur Goodwin, a researcher with the University of North Carolina's Highway Safety Research Centre. "A passenger can see the driving environment and can point out any hazards. But a person on the other end of the phone has no idea what's happening."

The suppressed North Carolina report is echoed by other recent studies. In 2003 the Harvard Centre for Risk Management estimated the annual death toll at 2600, with 12,000 serious injuries. Studies in Canada and Australia calculated a fourfold rise in crashes in incidents involving mobile use, whether hand-held or hands-free.

And a 2006 University of Utah study that tested drivers on simulators concluded that the likelihood of their having an accident while using a mobile was statistically the same as if they had been driving while drunk.

A further 95 bills are being considered in 28 states, suggesting that the drive for limited bans - although none of the bills wants an outright ban- has reached a critical mass.

Is teen prohibition the thin end of a wedge that will end in a total ban? "I wouldn't be surprised if 10 or 15 years down the road we reach a point where any kind of cellphone use is restricted for all drivers," volunteers Goodwin.

Perhaps. But Americans spend a lot of time behind the wheel and mobile use has soared in the past decade. According to CITA: The Wireless Association, an industry lobby group, there were 255.4 million US mobile subscribers last December. Annual revenue from mobile charges came to US$138.9 billion, with Americans chatting for 2.1 trillion minutes.

And whereas the industry appears to support teen bans, they are noticeably unenthusiastic about adult prohibitions, suggesting education, not legislation, is the way to go.

Eventually, society may toughen its stance on mobile use by drivers, following safety trends set by bans on drunk driving and mandatory seatbelts. But it could be a while.

"For a lot of legislators the jury's still out on this, because they don't have a wide body of statistical data to indicate they should pass some sort of prohibition," says Sundeen.

"I think they're reluctant to take away all sorts of cellphone devices from people."

And there's the rub. As much as some people worry about mobile use, they also find it hard to give them up.