The use of cellular phones has skyrocketed in recent years, with more than 117 million subscribers in the United States as of July 1, 2001. This increase has been accompanied by an increase in the number of individuals concurrently driving and talking on the cell phone. Recent estimates suggest that cell phone users spend 60% of their cell phone time while driving. The precise effects of cell phone use on public safety are unknown; however, because of the possible increase in risks associated with the use of cell phones while driving, several legislative efforts have been made to restrict cell phone use on the road. In most cases, the legislation regarding cell phones and driving makes the tacit assumption that the source of any interference from cell phone use is due to peripheral factors such as dialing and holding the phone while conversing. Among other things, our research evaluates the validity of this assumption.

Prior research has established that the manual manipulation of equipment (e.g., dialing the phone, answering the phone, etc.) has a negative impact on driving. However, the effects of the phone conversation on driving are not as well understood, despite the fact that the duration of a typical phone conversation may be up to two orders of magnitude greater than the time required to dial or answer the phone. One study found that simple conversations did not adversely affect the ability to maintain road position. On the other hand, studies have found that working memory tasks, mental arithmetic tasks, and reasoning tasks disrupt simulated driving performance.

Our research focused on the cell phone conversation, because it comprises the bulk of the time engaged in this dual-task pairing. We sought to determine the extent to which cell phone conversations interfere with driving and, if so, the precise nature of the interference. In particular, the "peripheral interferences" hypothesis attributes interference from cell phones to peripheral factors such as holding the phone while conversing. By contrast, the "attentional hypothesis" attributes interference to the diversion of attention from driving to the phone conversation itself. The study described here is part of a larger research project that will be detailed in a forthcoming issue of Psychological Science.

This study was designed to contrast the effects of hand-held and hands-free cell phone conversations on responses to traffic signals in a simulated driving task. We also included control groups who either listened to the radio or listened to a book on tape while performing the simulated driving task. As study participants (SP) performed the simulated driving task, occasional red and green lights were flashed on the computer display. If SPs saw a green light, they were instructed to continue as normal. However, if a red light was presented they were to make a braking response as quickly as possible. This manipulation was included to determine how quickly subjects could react to the red light as well as to determine the likelihood of detecting these simulated traffic signals.

Sixty-four study participants (32 male, 32 female undergraduate students), who had normal or corrected-to-normal vision and perfect color vision, were randomly assigned to one of the radio control, book-on-tape control, hand-held cell phone, or hands-free cell phone groups. SPs performed a pursuit tracking task in which they used a joystick to maneuver the cursor on a computer display to keep it aligned as closely as possible to a moving target. The target flashed red or green and SPs were instructed to press a "brake button" located in the thumb position on top of the joystick as rapidly as possible when they detected the red light. Red and green lights were equiprobable and were presented in an unpredictable order. An experimental session consisted of three phases: (a) a warm-up interval that lasted 7 minutes, (b) two single-task segments each lasting 7.5 minutes that immediately preceded and then followed the dual-task third phase, and (c) a dual-task segment that lasted 15 minutes. The dual-task condition required the SPs to engage in a conversation with a confederate (or listen to a radio broadcast of their choosing or a book on tape) while concurrently performing the tracking task. The confederate's task was to facilitate the conversation and also to ensure that the subject listened and spoke in approximately equal proportions during the dual-task portions of the experiment.

A preliminary analysis of detection rates (DR) and reaction times (RT) to traffic signals indicated that there were no differences between hands-free and hand-held cell phone groups. Neither were therre differences between radio control and book-on-tape control groups. Therefore, the data were aggragated to form a 2 (Group: Cell Phone vs. Control) x 2 (Task: Single vs. Dual) factorial design. The tables present the probability of missing simulated traffic signals. Overall, miss rates were low; however, the probability of a miss significantly increased when subjects were engaged in conversations on the cell phone, F(1,31)=8.8, p<0.01. By contrast, the difference between single and dual-task conditions was not reliable for the control group, F(1,31)=0.9, p>0.36. Analysis of the RT data revealed that subjects in the cell phone group responded slower to simulated traffic signals while engaged in conversation on the cell phone, F(1,31)=29.8, p<0.01. There again was no indication of a dual-task decrement for the control group.

(*This article was originally published in the August/September 2001 issue of the Injury Insights newsletter.)

Return to top.