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CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE

Cell-Phone–Induced Driver Distraction David L. Strayer and Frank A. Drews

University of Utah

ABSTRACT—Our research examined the effects of hands- free cell-phone conversations on simulated driving. We found that even when participants looked directly at ob- jects in the driving environment, they were less likely to create a durable memory of those objects if they were con- versing on a cell phone. This pattern was obtained for objects of both high and low relevance, suggesting thatvery little semantic analysis of the objects occurs outside the restricted focus of attention. Moreover, in-vehicle conver- sations do not interfere with driving as much as cell-phone conversations do, because drivers are better able to synchronize the processing demands of driving with in- vehicle conversations than with cell-phone conversations. Together, these data support an inattention-blindness interpretation wherein the disruptive effects of cell-phone conversations on driving are due in large part to the diver- sion of attention from driving to the phone conversation.

KEYWORDS—driver distraction; inattention blindness; atten- tion; cell phones

This article focuses on a dual-task activity that over 100 million drivers in the United States currently engage in: the concurrent use ofacell phonewhileoperatinga motorvehicle. It isnowwell established that cell-phone use significantly impairs driving performance (e.g., McEvoy et al., 2005; Redelmeier &Tibsh- irani, 1997; Strayer, Drews, & Johnston, 2003; Strayer & John- ston, 2001). For example, our earlier research found that cell- phone conversations made drivers more likely to miss traffic signals and react more slowly to the signals that they did detect (Strayer & Johnston, 2001). Moreover, equivalent deficits in driving performance were obtained for users of both hand-held and hands-free cell phones (see also Strayer, Drews, & Crouch, 2006).Bycontrast, listeningtoradiobroadcastsor booksontape did not impair driving. These findings are important because

Address correspondence to David Strayer, Department of Psycholo- gy, 380 S. 1530 E. RM 502, University of Utah, Salt Lake City, UT 84112; e-mail: [email protected].

theydemonstrate that listeningtoverbal material,byitself, isnot sufficient to produce the dual-task interference associated with using a cell phone while driving. The data indicate that when a driver becomes involved ina cell-phone conversation, attention is withdrawn from the processing of the information in the driving environment necessary for safe operation of the motor vehicle.

EVIDENCE OF INATTENTION BLINDNESS

The objective of this article is to muster evidence in support of the hypothesis that cell-phone conversations impair driving by inducing a form of inattention blindness in which drivers fail to seeobjects intheirdrivingenvironment whentheyare talkingon a cell phone. Our first study examined how cell-phone conver- sations affect drivers’ attention to objects they encounter while driving. We contrasted performance when participants were drivingbut notconversing (i.e., single-taskconditions)withthat when participants were driving and conversing on a hands-free cell phone (i.e., dual-task conditions). We used an incidental- recognition-memoryparadigmtoassesswhat informationinthe driving scene participants attended to while driving. The pro- cedure requiredparticipants to perform a simulated driving task without the foreknowledge that their memory for objects in the driving scene would be subsequently tested. Later, participants were given a surprise recognition-memory test in which they were shown objects that had been presented while they were driving and were asked to discriminate these objects from foils that had not been in the driving scene. Differences in incidental recognition memory between single- and dual-taskconditions provide an estimate of the degree to which attention to visual information in the driving environment is distracted by cell- phone conversations.

Each of the four studies we report here used a computer- ized driving simulator (made by I-SIM; shown in Fig. 1) with high-resolution displays providing a 180-degree field of view. (The dashboard instrumentation, steering wheel, gas, and brake pedal are from a Ford Crown Victoria sedan with an automatic transmission.) The simulator incorporates vehicle-dynamics,

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Fig. 1. A participant talking on a hands-free cell phone while driving in the simulator.

traffic-scenario, and road-surface software to providerealistic scenes and traffic conditions. Wemonitored the eye fixations of participants using a video-based eye-tracker (Applied Science LaboratoriesModel501)thatallowsafree range ofheadandeye movements, thereby affording naturalistic viewing conditions for participants as they negotiated the driving environment.

The dual-task conditions in our studies involved naturalistic conversations with a confederate on a cell phone. To avoid any possible interference from manual components of cell-phone use, participants used a hands-free cell phone that was posi- tioned and adjusted before driving began (see Fig. 1). Addi- tionally, the call was begun before participants began the dual- task scenarios. Thus, any dual-task interference that we observed had to be due to the cell-phone conversation itself, as there was no manual manipulation of the cell phone during the dual-task portions of the study.

Our first study focused on the conditional probability of par- ticipants recognizing objects that they had fixated on while driving. This analysis specifically tested for memory of objects presented where a given driver’s eyes had been directed. The conditional probability analysis revealed that participants were more than twice as likely to recognize roadway signs encoun- tered in the single-task condition than in the dual-task condi- tion. That is, when we focused our analysis on objects in the driving scene on which participants had fixated, we found sig- nificant differences in recognition memory between single- and dual-task conditions. Moreover, our analysis found that even when participants’ eyes were directed at objects in the driving environment for the same duration, they were less likely to re- member them if they were conversing on a cellular phone. The data are consistent with the inattention-blindness hypothesis: The cell-phone conversation disrupts performance by diverting attention from the external environment associated with the driving task to an engaging context associated with the cell- phone conversation.

Our second study examined the extent to which drivers who engage in cell-phone conversations strategically reallocate attention from the processingof less-relevant information in the driving scene to the cell-phone conversation while continuing to give highest priority to the processing of task-relevant infor- mation in the driving scene. If such a reallocation policywere observed,itwouldsuggestthatdriversmightbeabletolearnhow to safely use cell phones while driving. The procedure was simi- lar to that of the first study except that we used a two-alternative forced-choice recognition-memory paradigm to determine what information in the driving scene participants attended to while driving. We placed 30 objects varying in relevance to safe driving (e.g., pedestrians, cars, trucks, signs, billboards, etc.) along the roadway in the driving scene; another 30 objects were not presented in the driving scene and served as foils in the recognition-memory task. There were different driving scenarios fordifferent participantsandtargetobjects for someparticipants were foil objects for others. Objects in the driving scene were positionedso that theywereclearlyinviewasparticipantsdrove past them, and the target and foils were counterbalanced across participants. Here again, participants were not informed about the memory test until after they had completed the driving portions of the study.

As in the first study, we computed the conditional probability of recognizing an object given that participants fixated on it while driving. Like the first study, this analysis specifically

testedformemoryofobjectsthatwerelocatedwherethedriver’s eyes had been directed. We found that participants were more likelyto recognize objectsencountered in the single-taskcondi- tion than in the dual-task condition and that this difference was not affected by how long they had fixated on the objects. Thus, when we ensured that participants looked at an object for the same amount of time, we found significant differences in recognition memory between single- and dual-taskconditions.

After each forced-choice judgment, participants were also asked to rate the objects in terms of their relevance to safe

driving, using a 10-point scale (participants were initiallygiven an example in which a child playing near the road might receive a rating of 9 or 10, whereas a sign documenting that a volunteer group cleans a particular section of the highway might receive a ratingof1).Participants’safety-relevanceratingsrangedfrom 1.5 to 8, with an average of 4.1. A series of regression analyses revealed that there was no association between recognition memory and traffic relevance. In fact, traffic relevance had absolutely no effect on the difference in recognition memory between single- and dual-task conditions, suggesting that the contribution of an object’s perceived relevance to recognition- memory performance is negligible. This analysis is important because it indicates that drivers do not strategically reallocate attention from the processingof less-relevant information in the driving scene to the cell-phone conversation while continuing to give highest priority to the processing of task-relevant infor- mation in the driving scene.

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Cell-Phone–Induced Driver Distraction

The studiesdiscussedthusfarhave relied onexplicit-memory measures taken after the driving session to test the hypothesis that cell-phone conversations interfere with the initial encoding of information in the driving scene. However, an alternative possibility is that there are no differences in the initial encoding but rather differences in the retrieval of the information during subsequent memory tests. This distinction is more than aca- demic, because the former has direct implications for traffic safety whereas the latter does not (i.e., failing to recognize an item at a later point in time does not necessarily imply an impairment in encoding and reaction to an object in the driving environment).

Our thirdstudy tested the inattention-blindness hypothesisby recording on-line measures of brain activity elicited by events in the driving environment. Prior research has found that the amplitude of the P300 component of the event-related brain potential (ERP) is sensitive to the attention allocated to a task (e.g., Sirevaag, Kramer, Coles, & Donchin, 1989; Wickens, Kramer, Vanasse, & Donchin, 1983) and, further, that memory performance is superior for objects eliciting larger-amplitude P300s during encoding (e.g., Fabiani, Karis, & Donchin, 1986; Otton & Donchin, 2000). Moreover, ERPs recorded in flight simulation revealed that the P300 component discriminates betweendifferent levelsof task difficulty, decreasing as the task demands increased (e.g., Kramer, Sirevaag, & Braun, 1987; Sirevaag et al., 1993).

In this study, we used a car-following paradigm in which participants drove on a simulated multilane freeway. Partici- pants followed a pace car that would brake at random intervals and ERPs were time-locked to the onset of the pace-car brake lights in both single- and dual-task conditions. If the impair- ments in memory performance are due to differences in the initial encoding of objects in the driving scene, then P300 am- plitude should be smaller in dual-task conditions than in single- taskconditions.Bycontrast,ifthememorydifferencesaredueto impaired retrieval of information at the time of the recognition- memory test but not at the time of encoding, then we would not expect to find differences in P300 amplitude between single- and dual-task conditions.

The average ERPs are presented in Figure 2. Visualinspec- tion reveals a large positive potential between 250 and 750 milliseconds (the P300 component of the ERP). Our analysis indicated that the amplitude of the P300component of the ERPs was reduced by 50% when the drivers were talking on the cell phone. Thus, drivers using a cell phone fail to see information in the driving scene because theydo not encode it as well as they do when they are not distracted by the cell-phone conversation. These data suggest that drivers using a cell phone will be less able to react with alacrity in situations that demand it because of thediversionofattentionfromdrivingtothephoneconversation (see alsoStrayer, Drews, &Johnston, 2003).

Our fourth study contrasted two modes of conversation com- monlyengagedinwhiledriving:Conversationwithafriendviaa

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Single Task Dual Task

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Fig. 2. Event-related potentials (ERPs) elicited by the onset of a pace car’s brake light when talking on a cell phone (dual-task condition) and when not talking on a cell phone (single-task condition). The amplitude of the P300 component of the ERP (which is a manifestation of higher cog- nitive processing such as memory encoding) was reduced by 50% when participants were conversing on a hands-free cell phone.

hands-free cell phone versus conversation with a friend seated in the passenger seat located next to the driver in the vehicle. We hypothesized that these twoconversations woulddifferbecause passengers tend to adjust their conversation based on driving difficulty; often helping the driver to navigate and identify hazards on the roadway and pausing the conversations during difficult sections of the drive. By contrast, this real-time adjustment basedupon traffic demands isnot possible with cell- phone conversations.

Participants were instructed to drive on a multilane freeway and exitata rest stop approximately8miles downthe road.Wefound that the majority of drivers (88%) who were conversing with a passenger successfully completed the task of navigating to the rest area, whereas 50% of the drivers talking on a cell phone failed to navigate to the rest area. Analysis of the video recordings indicated that a primary difference between these two modes of communication was that the passenger helped the driver in the navigation task by reminding them to exit at the rest stop. More- over,ouranalysisofthecontentoftheconversationindicatedthat references to traffic conditions were more likely with passenger conversations than they were with cell-phone conversations.

THEORETICAL CONSIDERATIONS AND FUTURE DIRECTIONS

Whatare theimplicationsofthesefindingsforthearchitectureof cognition? Multiple-resource models of dual-task performance (e.g., Wickens, 1984) have been interpreted as suggesting that an auditory/verbal/vocal cell-phone conversation may be per- formed concurrently with little or no cost to a visual/spatial/ manual driving task. That is, given the apparent lack of overlap

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in processing resources, the cell phone and driving dual-task combination should be performed with little degradation in performance. However, given the clear and consistent interfer- ence between cell-phone and driving tasks, it would appear that multiple-resource models do not apply well to this dual-task combination.

One alternative possibility that we are currently exploring is that the dual-task interference stems from a central-processing bottleneck, wherein attending to the cell-phone conversation temporarily blocks or impedes the processing of information in the driving environment (cf. Levy, Pashler, & Boer, 2006). We hypothesize that the central-processing bottleneck forces serial processing of these two sources of information (i.e., the infor- mationnecessaryforthesafeoperationofamotorvehicleandthe cell-phone conversation) and that the cell-phone conversation may not lend itself to parsing in ways that are compatible with driving (cf. Strayer & Johnston, 2001). That is, a conversation on the phone cannot be successfully broken into arbitrary units, but instead is composed of ‘‘turns’’ that engage the central- processing bottleneck for prolonged periods of time (e.g., pausing in mid-sentence/thought impedes the flow of the con- versation); moreover, this turn-taking is often asynchronous with the processing demands of driving. Supporting this idea is the observation that in-vehicle conversations do not interfere with driving as muchas cell-phone conversations do because there is a greater ability to synchronize an in-vehicle conversation with theprocessingdemandsofdrivingthanthereiswithacell-phone conversation.

The findings reported here highlight the need for sharpening our theoretical understanding of multitasking in complex natu- ralistic environments. The usefulness of such theory increases with the ever-increasingprevalence of new technologiesallow- ing people to engage in concurrent activities. Theory develop- ment will improve our ability to determine why some tasks are successfully performed in combination whereas others are in- compatible.

In sum, the data indicate that cell-phone conversations place demands upon the driver that differ qualitatively from those of other auditory/verbal/vocal tasks commonly performed while operating a motor vehicle. Even when cell-phone drivers direct theirgazeatobjects in the drivingenvironment, theyoften fail to ‘‘see’’ them because attention has been diverted to the cell- phone conversation.

Recommended Reading Redelmeier, D.A., &Tibshirani, R.J. (1997). (See References) Strayer,D.L., &Johnston,W.A.(2001). (See References)

REFERENCES

Fabiani, M., Karis, D., & Donchin, E. (1986). P300 and recall in an incidental memory paradigm. Psychophysiology, 23, 298–308.

Kramer, A.F., Sirevaag, E.J., & Braun, R. (1987). A psychophysiological assessmentofoperatorworkloadduringsimulatedflightmissions. Human Factors, 29, 145–160.

Levy, J., Pashler, H., &Boer, E. (2006). Central interference in driving: Is there any stopping the psychological refractory period? Psy- chological Sciences, 17, 228–235.

McEvoy, S.P., Stevenson, M.R., McCartt, A.T., Woodward, M., Haworth, C., Palamara, P., & Cercarelli, R. (2005). Role of mobile phones in motor vehicle crashes resulting in hospital attendance: A case- crossover study. BritishMedical Journal, 331, 428–433.

Otton, L.J., & Donchin, E. (2000). Relationship between P300 ampli- tude and subsequent recall for distinctive events: Dependence on typeofdistinctiveness attribute. Psychophysiology, 37, 644–661.

Redelmeier, D.A., & Tibshirani, R.J. (1997). Association between cellular-telephone calls and motor vehicle collisions. The New England Journal of Medicine, 336, 453–458.

Sirevaag, E.J., Kramer, A.F., Coles, M.G.H., & Donchin, E. (1989). Recourse reciprocity: An event-related brain potential analysis. Acta Psychologia, 70, 77–97.

Sirevaag, E.J., Kramer, A.F., Wickens, C.D., Reisweber, M., Strayer, D.L., & Grenell, J.H. (1993). Assessment of pilot performance and mental workload in rotary wing aircraft. Ergonomics, 9, 1121–1140.

Strayer,D.L., Drews, F.A., &Crouch, D.J. (2006). Comparingthecell- phone driver and the drunk driver. Human Factors, 48, 381–391.

Strayer, D.L., Drews, F.A., & Johnston, W.A. (2003). Cell phone induced failures of visual attention during simulated driving. Journal of Experimental Psychology: Applied, 9, 23–52.

Strayer,D.L.,&Johnston,W.A.(2001).Driventodistraction:Dual-task studies of simulated driving and conversing on a cellular phone. Psychological Science, 12, 462–466.

Wickens, C.D. (1984). Processing resources in attention. In R. Para- suraman & R. Davies (Eds.), Varieties of Attention. (pp. 63–101). New York:AcademicPress.

Wickens, C.D., Kramer, A.F., Vanasse, L., & Donchin, E. (1983). Per- formanceofconcurrenttasks:Apsycho-physiologicalassessment of the reciprocity of information-processing resources. Science, 221, 1080–1082.

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