...concurrently, carrying conversations, tuning the radio, or eating.

Although these and similar activities have been found to be associated with increased accident risk (see Stevens & Minton, 2001; Stutts & Hunter, 2003), developments in in-car information and communication systems provide further possibilities for time-sharing. In addition to navigation displays, telephones, and various in-car entertainment systems, in-car computers that support all features associated with a standard Windows-driven laptop (e.g., G-Net's Auto PC) are now available, as are intelligent transport systems and wireless Internet. Such technology allows the driver access to a wide range of information sources and to perform a variety of additional tasks, effectively providing a mobile office. There are undoubted benefits associated with such systems (e.g., for those engaged in sales; Eost & Galer Flyte, 1998). However, if they are used while driving, safety may be compromised through competition for limited perceptual, cognitive, and/or response processing capacity (e.g., see Wickens, 1991, for a discussion of capacity limitations).

A related task, the use of cellular phones while driving, has been the subject of substantial recent research interest (for reviews, see Goodman, Tijerina, Bents, & Wierwille, 1999; Haigney & Westerman, 2001). Cellular phone use has been found to be associated with increased driver workload (Aim & Nilsson, 1994; Brookhuis, De Vries, & De Waard, 1991 ; Haigney, Taylor, & Westerman, 2000) and, consistent with this, degraded situational awareness (see Endsley, 1995, for a description of the concept of situational awareness, including the association with mental workload).

When using a cellular phone, drivers are less effective in their responses to events in the driving environment, such as braking in response to visual stimuli (Ahn & Nilsson, 1994), taking evasive action to avoid objects (Cooper et al., 2005), detecting leading car deceleration (Lamble, Kaurenen, Laakso, & Summala, 1999), and taking evasive action to a range of traffic scenarios (McKnight & McKnight, 1993). This may be part of a more pervasive reduction in drivers' interactivity with the driving environment and a tendency to execute either the current, or a previously determined, "schema." In support of this proposition, Haigney et al. (2000) found that the variability of gas pedal movement was reduced during cellular phone use, and Brookhuis et al. (1991) found a decrease in variability of lateral position when drivers were using a mobile phone, particularly under motorway driving conditions. Such reduced responsiveness to external events may be an involuntary result of increased competition for attentional resources when using a cellular phone. Alternatively, drivers may be aware of threats to performance when using a cellular phone, and this may be part of a process of strategic control designed to facilitate timesharing.

There is some limited evidence that drivers compensate for demands associated with cellular phone use by increasing safety margins. Haigney et al. (2000) and Alm and Nilsson (1994) found speed reductions when drivers were taking a phone call. However, Aim and Nilsson (1994) found this only in an "easy" driving condition, and this was not replicated by Aim and Nilsson (1995). Cooper et al. (2003) found that drivers were more cautious in response to changing traffic lights when engaged in a cellular phone task. Nevertheless, cellular phone use has been found to be associated with increased risk of accident involvement (Redelmeier & Tibshirani, 1997; Violanti, 1998), and legislation has been enacted in several countries to restrict the use of cellular phones by drivers. On the basis that structural interference (see Kahneman, 1973) is likely to pose the greatest threat to driving performance, some countries (e.g., Germany and the United Kingdom) have banned only handheld cellular phone use. However, it would seem that cognitive activity associated with hands-free phone operations may also have a detrimental effect on driving performance (Goodman et al., 1999; Lamble et al., 1999).

In this paper, we report a study addressing the possibility that a portion of the cognitive interference associated with hands-free cellular phone use results from demand characteristics of the conversation. In this respect, a potentially important distinction can be drawn between driver-controlled and externally controlled communication. Given that pauses in conversations have been found to influence the inferences drawn, such as those relating to the speaker's character (Tree, 2002) or the listener's interests (Gardner, 1998), it would follow that in normal conversation there is pressure to maintain a particular pace of response. This effect may be stronger for cellular phone conversations than for conversations with passengers, as the passenger will be aware of changes in driving demand (e.g., negotiating a junction; see Parkes, 1991).

It can be hypothesized that there are advantages associated with giving the driver greater control over the pace of the conversation. This can readily be achieved using an in-car messaging system. For example, an in-car E-mail system would allow the driver to regulate the time at which messages are opened, (i.e., attended to). We are aware of only one relevant study on the use of this type of in-car information system. Lee, Caven, Haake, and Brown (2001) conducted a simulator-based investigation of car-following task performance when using a driver-paced, speech-based E-mail system. They found that the response to a lead car braking was impaired and self-reported workload increased when drivers used the E-mail system, although this did not interact with a manipulation of driving demand. There was no effect on situation awareness, as assessed by means of a postdrive questionnaire. No comparison was made with a system-paced version of the task. A manipulation of E-mail interface complexity (by varying the number of menu options) did not influence performance.

Experimental Aims

This study was designed to replicate and extend the findings of Lee et al. (2001). Effects of driver interaction with an in-vehicle simulated "E-mail" message system were examined. Performance was tested in two interface conditions, one in which the time that messages were opened was controlled by the in-car system and one in which the timing was controlled by the driver. Driving demand was manipulated across four levels, using a range of driving scenarios that varied in their requirements for both environmental monitoring and driver response. Several measures of driving performance were assessed, including a measure of anticipation of unfolding traffic events. Consistent with previous research, we hypothesized that concurrent task performance would distract drivers' attention, leading to reduced ability to anticipate changes to the driving environment and poorer vehicle control, especially in more demanding driving environments. However, it was also predicted that use of the driver-controlled interface, in which the driver was notified of the arrival of an E-mail but could delay presentation, would reduce vulnerability to distraction and that...

NOTE: All illustrations and photos have been removed from this article.



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