The former head of the highway safety agency said he was urged to withhold the research to avoid antagonizing members of Congress who had warned the agency to stick to its mission of gathering safety data but not to lobby states.

Critics say that rationale and the failure of the Transportation Department, which oversees the highway agency, to more vigorously pursue distracted driving has cost lives and allowed to blossom a culture of behind-the-wheel multitasking.

"We're looking at a problem that could be as bad as drunk driving, and the government has covered it up," said Clarence Ditlow, director of the Center for Auto Safety.

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The highway safety researchers estimated that cellphone use by drivers caused around 955 fatalities and 240,000 accidents over all in 2002.

The researchers also shelved a draft letter they had prepared for Transportation Secretary Norman Y. Mineta to send, warning states that hands-free laws might not solve the problem.

That letter said that hands-free headsets did not eliminate the serious accident risk. The reason: a cellphone conversation itself, not just holding the phone, takes drivers' focus off the road, studies showed.

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At the time, Congress had warned the agency not to use its research to lobby states. Dr. Runge said transit officials told him he could jeopardize billions of dollars of its financing if Congress perceived the agency had crossed the line into lobbying.