• Researchers used computers to simulate numerous cell phone users' driving behavior and thus overall traffic.

• The study used a PatrolSim driving simulator. A person sits in a front seat equipped with gas pedal, brakes, steering and displays from a Ford Crown Victoria patrol car. Realistic traffic scenes are projected on three screens around the driver.

• The study involved 36 U. of U. psychology undergraduates. Each student drove through six, 9.2 mile-long freeway scenarios, two each in low, medium and high density traffic, corresponding to freeway speeds of 70 mph to 40 mph. Each 9.2-mile drive included 3.9 miles with two lanes in each direction and 5.3 miles with three lanes each way. Traffic speed and flow mimicked I-15 in Salt Lake City.

• Each student spoke on a hands-free cell phone during one drive at each level of traffic density and did not use a cell phone during the other three drives. A volunteer on the other end of the phone was told to maintain a constant exchange of conversation.

• The drivers were told to obey the 65 mph speed limit and use turn signals. Participants decided their own speeds, following distances and lane changes.

• Researchers designed the study so traffic would periodically slow in one lane and the other lane would periodically free up, creating a situation where progress down the road was clearly impeded by slower-moving vehicles, and a driver would benefit by moving to the faster lane.

• Researchers 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," they wrote.

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Source: University of Utah research team led by David Strayer, professor of psychology