"At the end of the day, the average person’s commute is longer because of that person who is on the cell phone right in front of them," says University of Utah psychology Professor Dave Strayer, leader of the research team.

Cooper and Strayer conducted the study with Ivana Vladisavljevic, a doctoral student in civil and environmental engineering, and Peter Martin, an associate professor of civil and environmental engineering and director of the University of Utah Traffic Lab. Martin says that, combined with Strayer’s previous research, the new study shows "cell phones not only make driving dangerous, they cause delay too."

Previous Research on Wireless Phones and Driving

In recent years, Strayer’s research group has published studies showing that:

- 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.

Highway statistics suggest drivers on cell phones are four times more likely to be in an accident, and Strayer’s earlier research suggests the risk is 5.36 times greater. The researchers note that 50 countries have adopted laws banning handheld phones while driving. But they say hands-free phone conversations are distracting, "thus, the majority of current regulation appears to be misguided."

"Results indicated that, when drivers conversed on a cell phone, they made fewer lane changes, had a lower overall mean speed and a significant increase in travel time in the medium and high density driving conditions," the researchers wrote. In medium and high density traffic, drivers talking on cell phones were 21 percent and 19 percent, respectively, less likely to change lanes (roughly six lane changes per 9.2-mile drive versus seven or eight lane changes by drivers not on cell phones). That may seem minor, "but if you have a lot of people who are not changing lanes and driving slower, this could substantially reduce traffic flow," Cooper says.

When considered with the earlier studies, "it’s going to increase traffic congestion," says Strayer. "You have motorists on cells 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." In low, medium and high traffic density, cell phone users spent 31 percent, 16 percent and 12 percent, respectively, more time following within 200 feet of a slow lead vehicle than undistracted drivers. That meant they spent 25 to 50 more seconds following another vehicle during the 9.2-mile drive.

"If you were not distracted by talking on a cell phone, you would overtake and pass the slower vehicle and come to your destination faster," Vladisavljevic says. Strayer adds: "If you get two or three people gumming up the system, it starts to cascade and slows everybody’s commute."

He acknowledges that, "in itself, staying in a lane and not passing might be construed as being safer, just as driving slightly slower or having a greater following distance also could be considered safer. But if you are doing that so you can take your mind off the road and talk on the phone, that isn’t safer." Compared with undistracted motorists, drivers on cell phones drove an average of 2 mph slower and took 15 to 19 seconds longer to complete the 9.2 miles. That may not seem like much, but is likely to be compounded if 10 percent of all drivers are talking on wireless phones at the same time, Cooper says.

Vladisavljevic already has begun computer "microsimulations" of multiple vehicles. She tried the simulation repeatedly with the proportion of drivers on cell phones ranging from none to 25 percent. "We saw an increase in delays for all cars in a system, and the delays increased as the percentage of drivers on cell phones increased," she says.

Strayer says it is important to show how cell phone use affects traffic because "when people have tried to do cost-benefit analyses to decide whether we should regulate cell phones, 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."

Martin says transportation analysts include two components -- accidents and delay -- when they calculate the "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 Martin. "Delay is measured by a composite number representing a measure of the value of a typical American traveler’s time. Today, this number is about $ 13 per hour. While the costs associated with accidents seem high, there are so very few of them, comparatively, they actually are dwarfed by the user costs associated with delay. 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."