"Assuming it's not a 4-year-old having a temper tantrum in the back," a passenger is also monitoring traffic and modulating conversation to match, as well as pointing out problems, he said.

In an earlier study, the researchers found that people conversing with passengers had about a 90 percent success rate at following driving directions. The group on the cell phone hit less than 50 percent.

Two pairs of eyes are better than one. "There's no impairment in terms of performance when talking to a passenger," Drews said.

Last week, Drews said, his own family was "nearly killed, and I'm not exaggerating," by someone who was driving while using a cell phone. The cars, side by side, were on a curve on the highway. But the distracted driver was going straight. When Drews confronted the driver on the exit, he found the man, still on the phone, was completely unaware that there had been a near-miss, Drews said.

Driving and talking on the phone, Drews says, "is dangerous. People are trying to minimize it, but it's really, really risky."

The study was partially funded by a grant from the Federal Aviation Administration, and Strayer and Drews paid the balance. The Utah Highway Patrol lent the researchers a device to measure blood-alcohol levels.

The National Highway Transportation Safety Administration estimates that at any given moment during daylight hours, 8 percent of drivers are talking on a cell phone.

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E-mail: lois@desnews.com