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Phone conversations distract drivers more than passengers

A study that tracked drivers through a simulated task finds that not all …

Human beings have a finite capacity for working memory, which tracks a limited number of items for the short periods of time it takes to complete the task at hand. So it's no surprise that many studies have found that cell phone conversations have a serious impact on driving skills, in some ways comparable to that of alcohol. A cursory analysis would suggest that all conversations should be equally distracting, but real-world experience shows that this isn't the case—it appears that passengers actually improve the performance of the people driving them about. A new study gets into the reasons for this discrepancy.

The introduction to the study describes a variety of previous work that describe at least three aspects of driving that appear to be negatively impacted by cell phone use. The first is basic control; drivers on cell phones have a pronounced tendency to drift around in (and potentially out of) the lane they're occupying. The second is tactical, which involves adjusting to the cars around you on the road through activities like maintaining a braking distance. Finally, studies have shown that drivers engaged in cell phone conversations are bad at strategic or task-focused processes, such as actually getting to their intended destination.

In the view of the new study's authors, there are two major issues with this past work. The first is that the prior studies simulate both driving and conversation with varying degrees of accuracy. But the second is that real-world data suggests that not all conversations are created equal. Epidemiological studies of auto accidents suggest that cell phone usage quadruples the risk of getting involved in a crash; in contrast, having an adult passenger in a vehicle actually reduces the chances of being involved in an accident.

The new study attempts to improve on some of the limited approximations of driving and conversation that were used in past studies while focusing on the differences between live and phone conversations. To start with, the authors obtained a highly sophisticated driving system called PatrolSim that's apparently designed for training police officers. (On the downside, this means that everyone was tested while driving an unrealistic vehicle: a Ford Crown Victoria with a automatic transmission.) The tests were run on a simulated freeway, where the participants had to exit to a rest area.

For conversation, the authors recruited actual pairs of friends and asked them to discuss a "close call" situation, where one of them narrowly averted a life-threatening situation. Participants were tested both while driving alone and then while engaged in conversation; the order of these tests, whether the driver initiated conversation, and other conditions were randomized. Overall, these conditions should more accurately reflect real-world driving and conversations.

The authors noted that compared to having a passenger, cell phone use made drivers' lane discipline worse. It also increased their following distance although, in the absence of any speed/braking distance information, it's hard to interpret this result. Most significantly, however, fielding a phone call roughly doubled the rate at which drivers missed the intended exit.

When the authors examined the conversation videos, some reasons for this difference became clear. Passengers frequently turned the conversation towards a discussion of traffic conditions and, in many cases, noted when the relevant exit sign appeared. Obviously, someone who was only present via cell phone was unable to do any of that. When a passenger was present, the pace of conversation dropped as traffic increased, and participants often started using less complex language. In contrast, drivers actually accelerated the pace of conversation when they faced more challenging driving conditions.

Although the study was small and didn't comprehensively analyze every possible facet of conversation, the results suggest that passengers effectively increase the working memory of the drivers and steer conversation such that it focuses the driver's attention to tactical and strategic issues. In the process, it's possible that passengers have a greater impact than these data suggest—a lot of careless and reckless driving takes place to compensate for simple errors, like missing an exit.

Journal of Experimental Psychology: Applied, 2008. DOI: 10.1037/a0013119

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