NHTSA's study will assess hands-free, semi-hands free and hand-held mobile phone use and assess the risk and distraction associated with each, the administration said.

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"Although it is legal to use a hands-free set at the moment, the research shows that actually, in terms of distraction, there is very little difference between a hand-held mobile phone and a hands-free one, " he said.

BBC: Mary Bowers

"There's conflicting evidence" on whether hands-free cell phone conversations would be as unsafe as those by hand-helds, Harsha said, adding that more "definitive research" is needed.

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Contract-free "prepaid" carriers, on the other hand, leave consumers to pay the full price of a phone.

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