Findings of the new study bolster already sturdy evidence that drivers should adhere to the bumper sticker admonishment: "Hang up and drive!" Other published research shows:

• Highway statistics suggest drivers on cell phones are four times more likely to be in an accident. Strayer's earlier research suggests the risk is 5.36 times greater.

• Hands-free cell phones are no less dangerous while driving than hand-held cell phones because the conversation itself is the major distraction.

• When young adults talk on cell phones while driving, their reaction times become as slow as reaction times for senior citizens.

• Drivers talking on cell phones are as impaired as drivers with the 0.08 percent blood alcohol level that defines drunken driving in most states.

• Cell phone users follow at greater distances, are slower to hit the brakes and are slower to regain speed after braking.

So old and new research combined means more traffic congestion, says Strayer.

"You have motorists on cell phones who tend to drive slower, their reaction times are slower, if they do hit the brakes it takes them longer to come back up to highway speed, and they are less likely to change lanes. Overall, they are more likely to gum up the highways."

Strayer says the study impacts any discussion of cell phone regulation. "They often don't factor in the cost to society associated with increased commute times, excess fuel used by stop-and-go traffic and increased air pollution, as well as hazards associated with drivers distracted by cell phone conversations," he said.

Indeed, two components — accidents and delays — are considered when looking at "user costs" associated with road travel.

"A fatal accident could cost as much as $5 million when we take into account medical, property and loss-of-income costs," says Peter Martin, director of the University of Utah Traffic Lab.

Delay is measured using a composite number representing the value of a typical American traveler's time — about $13 per hour, according to Martin.

"If we compile the millions of drivers distracted by cell phones and their small delays, and convert them to dollars, the costs are likely to be dramatic," Martin said. "Cell phones cost us dearly."

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