January 27, 2003 -- Motorists are more accident-prone and slower to react when they talk on cellular telephones - even hands-free models - because "inattention blindness" makes the drivers less able to process visual information, University of Utah researchers found.

"Even when participants [drivers] are directing their gaze at objects in the driving environment, they may fail to 'see' them because attention is directed elsewhere," says the new study by psychologists David Strayer, Frank Drews and William A. Johnston. "Phone conversations impair driving performance by withdrawing attention from the visual scene, yielding a form of inattention blindness."

The study concludes that that inattention blindness explains the researchers' widely publicized 2001 findings that users of hands-free and hand-held cell phones are equally impaired, missing more traffic signals and reacting to signals more slowly than motorists who do not use cell phones.

The new study is being published in the March 2003 issue of the Journal of Experimental Psychology: Applied. A portion of the study also is featured in the February-March 2003 issue of Injury Insights, published by the National Safety Council.

In 2001, Strayer and colleagues received worldwide publicity when they used a joystick-equipped computer display to show that people talking on cell phones were more likely to miss or react slowly to simulated traffic signals than people who were not conversing on cell phones. Driving impairment was just as bad regardless of whether participants used hands-free or hand-held cell phones. That suggested the phone conversation itself was a distraction for motorists in addition to the distraction of handling the phone.

The earlier study also found there was no impairment of drivers who either conversed with a passenger or who listened to the radio or to books on tape.

The new study included four experiments aimed at explaining why cell-phone conversations are more distracting than those activities, and why hands-free and hand-held cell phone users were equally impaired when driving.

The experiments involved 110 University of Utah undergraduates who sat in a $100,000 PatrolSim II+ Driver Training Simulator manufactured by GE Capital I-Sim in Salt Lake City. Such simulators are used by law enforcement agencies.

A person using the simulator sits in a replica of the driver's portion of a Ford Crown Victoria patrol car - including a steering wheel, ignition key, accelerator and brake pedals and dashboard displays - and is surrounded by screens that show a realistic driving environment.

Sometimes students who drove the simulator also spoke on a cell phone, conversing with another student who was instructed to keep a balance between making the driver talk and listen. Only hands-free cell phones were used in the study so the researchers could avoid any distracting effects from handling phones, and focus on distracting effects of conversation.

Experiment 1 found cell phone use impaired motorists' ability to respond to a vehicle braking in front of them.

Forty students drove 40 miles on a simulated freeway, staying in the right lane and responding to brake lights from a pace car in front of them. The simulation included light and heavy traffic. Sometimes students talked on a hands-free cell phone; other times they did not.

There were no accidents in light traffic or among those not using cell phones. But three cell phone users in heavy traffic rear-ended the simulated pace car.

Drivers who talked on a cell phone reacted sluggishly, and compensated by increasing their distance behind the pace car. But when the pace car braked in heavy traffic, cell phone users took longer to brake, rode the brakes longer and took longer to accelerate again.

Experiment 1 shows "conversing on a hands-free cell phone impaired driving performance, and this impairment became more pronounced as traffic density increased," the researchers said.
Cell phone use by drivers "increases traffic congestion, it probably increases road rage and it increases air pollution because cell phone users are decreasing the volume of traffic that can flow on a freeway at any point in time," Drews said.

Experiment 2 indicated cell phone conversation inhibits attention to the driving environment rather than simply slowing a motorist's reaction time.

Twenty students in this experiment drove in a simulated city scene that included billboards. Each student drove six 1.2-mile sections of suburban streets, making an average of two left turns and two right turns. Each motorist saw 15 billboards while using a cell phone and 15 while not using the phone. Then each student left the simulator and sat in front of a computer that displayed, one at a time, 45 billboards, including 15 the student did not see in the simulator. Students were asked to say "old" if they had seen the billboard previously during the simulated drives and "new" if they had not. Participants were less accurate in remembering which billboards they had seen while driving the simulator using a cell phone.

"Conversing on a cell phone impairs the recognition memory for objects presented in the driving scene … consistent with the hypothesis that the cell phone conversation disrupts performance by diverting attention from the external environment," the researchers said.

Experiment 3 ruled out the possibility that cell phone users simply are less likely to move their eyes and look at what's around them. The results bolstered the researchers' theory that cell phone users look but don't really "see" or pay attention to their surroundings.

Twenty more undergraduates took the same simulated drives as in Experiment 2 while researchers tracked their eye movements.

The researchers found the drivers looked directly at about two-thirds of the billboards, and they looked for the same length of time, regardless of whether or not they used a cell phone. But cell phone users were worse at remembering later which billboards they had seen.

"When people look at a billboard, they are less likely to see it if they are on a cell phone," Strayer said. "We are showing is that looking and seeing are not one and the same. Directing your eyes doesn't mean you are bringing in the information and able to act on it."

Experiment 4 showed cell phone users had worse "implicit perceptual memory" - or subconscious memory of things not necessarily remembered consciously - compared with people not talking on cell phones.

Instead of the driving simulator, 30 students used a joystick to control a computer-screen cursor so that it closely tracked a moving target. Sometimes they talked on a hands-free cell phone; other times they did not. Every 10 to 20 seconds, a word would appear on the computer screen directly ahead of the student for a half second, for a total of 200 words. Later, without tracking the target, another 100 words were presented on the screen one at a time, some of them "old" words also seen during the target-tracking phase. Each word appeared gradually as dots blocking the word disappeared.

The researchers found that cell phone users were slower when they later tried to identify words they had seen during the target-tracking phase.

"Even though you're eyes are looking right at something, when you are on the cell phone, you are not as likely to see it," Strayer explained.

The researchers said the overall study supports the inattention blindness hypothesis that "the disruptive effects of cell phone conversations on driving are due in large part to the diversion of attention from driving to the phone conversation."

The findings also suggest cell-phone conversations may interfere with the ability to react to sudden events like a pedestrian darting into traffic or a car running a red light.

The researchers said their studies and others suggesting cell phone quadruples the risk of a traffic accident "provide converging evidence indicating that conversing on a cell phone while driving poses significant risks both to the driver and the general public."

"Our data further suggest that legislative initiatives to restrict hand-held devices but permit hands-free devices are not likely to eliminate problems associated with using cell phones while driving because these problems are attributed in large part to the distracting effects of the phone conversations themselves."

The researchers also noted half the students in the study "reported they have observed other drivers driving erratically while using a cell phone, but rarely if ever thought that their own driving was impaired when they used the cell phone. … A consequence of using a cell phone is that it may make drivers insensitive to their own impaired driving behavior."