Hands on the wheel, eyes on the road ...mind on something simple.

New research shows that performing complex mental tasks can reduce a driver's ability to detect visual targets by as much as 30 percent. According to the research, performing complex mental tasks--feeding back concrete or abstract information as opposed to simply listening--significantly reduced drivers' ability to detect visual targets (in this case, flashing lights), discriminate among them and respond correctly. And, in an experimental variation that examined the impact of hands-free telephone use, task complexity again compromised concentration.

Psychologists had long known that environmental distractions--a car accident or a baby crying in the back seat--can compromise a driver's concentration. In fact, distractions are a top cause of traffic accidents. These new findings suggest that distractions inside one's own head can be just as disruptive. The new data suggest that higher-level mental tasks draw attentional resources away from the road, resulting in those rueful post-accident reports: "I didn't expect it" or "I saw it too late."

Driver error appears to stem from a reduced ability to perceive or identify targets, rather than from a problem with making decisions or responding skillfully, the researchers say. In other words, distracted drivers still know how to drive, but they don't see things well or fast enough to safely use their skills.

Traffic lights

Psychologists Miguel Angel Recarte Goldarecena, PhD, of the Universidad Complutense in Madrid, and Luis Miguel Nunes González, PhD, of Spain's Administration for Traffic Safety, studied 12 adults who drove a specially outfitted Citröen for about four hours on a highway near Madrid. Drivers performed a sequence of cognitive tasks (listening to different types of information and then telling what they heard), and at the same time had to press buttons corresponding to lights flashing in their field of vision. The researchers recorded how much drivers' eyes focused, where they looked and how they responded to the flashing lights. The resulting "within person" data revealed how various degrees of mental distraction affected driver behavior.

Here's how it worked: During the drive, an experimenter riding as a passenger asked drivers to listen to recorded information and keep it in mind. Changing the order around by participant, tapes presented a sequence of tasks, presenting concrete information (such as objects' size, shape and color), abstract information (pieces on justice and human rights, or the measurement of human intelligence), or no information at all--the control condition. Drivers were then asked to reproduce whatever information they just heard.

To illuminate whether and how the mental tasks of receiving and producing interfered with responses, drivers pressed a left- or right-hand button mounted near the steering wheel whenever they saw a flashing light inside the car. Specifically, when special equipment beamed flashes to light-emitting diodes mounted at 10 different spots within a driver's visual field, he or she pressed left or right depending on whether lights flashed fast or slow.

Using unobtrusive eye-tracking equipment, Recarte and Nunes measured how often the drivers glanced at the spotlights before responding. Later, they analyzed response accuracy. As further measures of attention and distraction, they recorded the direction of drivers' gazes as well as their ocular fixation, since pupil size reflects the effort given to mental load.

The data revealed that listening to either concrete or abstract information had little or no effect on performance. However, when drivers had to reproduce the content of either audio message, nearly all the measures of attention changed significantly. When generating information, drivers not only glanced at the targets less often, but they also more often responded without looking at the targets at all. When drivers did look at targets, they detected them later and glanced at them for a shorter time, which reduced accuracy and increased errors.

In the cell-phone variation, drivers had two different mental tasks: to convert foreign currency and to recount in detail their actions on a given day and time. The calculation task proved to be particularly distracting, and so the authors say that complex conversations--whether by phone or with a passenger--are dangerous for road safety.

"This work adds to the growing body of scientific evidence, establishing that cell phones and other in-vehicle devices can often draw attention away from processing the information necessary for the safe operation of a motor vehicle," says University of Utah psychologist David Strayer, PhD, who conducts extensive research on driving and cell-phone use.

Driver's ed

Although most driving studies are conducted in the lab using simulators, this study could be especially relevant because it took participants on the road, say some researchers. "The study took advantage of a nice ensemble of high-tech measuring devices, allowing considerable control and measurement precision in a real-world situation," explains Frank Durso, PhD, director of the cognitive ergonomics lab at Texas Tech University.

The researchers say their findings will aid in evaluating the potential impact of in-car devices, such as cell phones and navigational systems, and in designing safer, less distracting interfaces. The findings can also shape legislation and standards governing emerging "intelligent" transport systems. With more naturalistic research, psychologists can make a stronger case in influencing new laws.

"If experimental psychologists are to influence real-world applications, they must bring their knowledge of methodology, their quantitative skills, and their talent at design out of the lab," says Durso. "This study is a nice demonstration that claims about causality can also be made in the real world."

In sum, internal distraction can cause at least as many mistakes in driving as external distraction. "This work is consistent with the observation that a basic form of driver error is that drivers look at, but fail to see, roadway traffic and other imperative information in the driving environment," explains Strayer.

Nunes notes that safety experts must not only come up with better engineering, but also educate drivers about the role of mental activity--perhaps cautioning them to keep thoughts and conversation light.

"A driver can be perfectly awake and yet have his or her mind elsewhere," he says. "And for the moment, I'm afraid there is no device for countering distraction."

Rachel Adelson is a science and technology writer in Raleigh, N.C.