Interpersonal communication has to do with the exchange of information between people that enhances mutual understanding of a situation, indicates behavioral intent, or responds to the actions of another. Effective communication involves all parties communicating their intent in a manner that is received and understood. Alternatively, miscommunication is either a failure to communicate intent effectively (e.g. clearly visible, easily understood) or a misunderstanding of a communication attempt. Lack of communication is the failure to communicate intent or response within a meaningful timeframe. Interpersonal communication between road users is an integral aspect of using the transportation system. Whether driving, riding a bicycle, or crossing the street, safe transportation requires that everyone share the road. Communication between road users helps coordinate behaviors, regulate road use, signal intent, and encourage other drivers to behave in certain ways (Renge, 2000; Shor, 1964). These interactions can involve reminding road users of formal traffic rules when they appear to be breaking them, or using eye contact to establish mutual acknowledgement of what is occurring, such as when a pedestrian crosses the street and a driver yields to him or her. Zaidel (1992) argues that the road can be considered a social environment, where driver decision making is partly determined by the way that drivers communicate with each other. Road users consider such communication important, and when drivers fail to communicate their intent, other road users view their behavior less favorably (Ba, Zhang, Reimer, Yang, & Salvendy, 2015). In response to the critical need for interpersonal communication, modern automobiles are equipped with technologies to facilitate communication between drivers, such as turn signals, hazard lights, and car horns. For example, drivers are required to indicate to others road users their intent to turn or change lanes through the use of a turn signal, yet research shows that turn signals are not consistently used. Sullivan, Bao, Goudy, and Konet (2015) conducted a naturalistic investigation of turn signal use at intersections and found that signals were only used 71-75% of the time. When they were used, turn signals alone were not sufficient to judge a driver’s intersection behaviors because signal use was partly determined by the road type, turn direction, and state of the surrounding traffic. In recognition of the need for interpersonal communication in the driving environment, other technologies have been developed for the purpose of further extending communication options while driving, such as remote-controlled expressive car signs that utilize mountable screens that can display short messages or express emotions to other drivers (Drivemocion, 2010). Lack of communication and miscommunication on the road also represent safety concerns. The National Motor Vehicle Crash Causation Survey (NMVCCS) found that “false assumption of others’ actions” was the critical reason for 4.5% of all crashes, of which at least a part could be likely attributed to miscommunication between road users while driving (National Highway Traffic Safety Administration, NHTSA, 2008). The safety effects of miscommunication may be even greater when vulnerable road users are considered. Of pedestrians and bicyclists, 25.5% and 27.0% of fatal crashes, respectively, were attributed to “failure to yield right of way” in 2014, with a large proportion of these crashes likely attributed to lack of communication or miscommunication (Fatality Analysis Reporting System, FARS, 2017). Despite these statistics, there is not a good way to estimate the prevalence of crashes for which communication problems are primary causal factors, and, thus, a full grasp of the safety aspects of miscommunication or the lack of communication in the transportation system is lacking. The development of automated and connected vehicles comes with promise that drivererror-related crashes will be reduced. The NMVCCS found that about 94% of crashes were due to driver error, and advocates of automated vehicles have predicted that automated vehicles will reduce 80% of crashes (Iliaifar, 2012; NHTSA, 2008). Currently, automated vehicles do not have the same capacity to communicate with other road users as drivers. For example, automated vehicles do not have a means to signal intent beyond the use of a turn signal, they cannot indicate whether or not they intend to yield, and they cannot yet read and interpret gestures from vulnerable road users (Parkin, Clark, Clayton, Ricci, & Parkhurst, 2016). Considering how ubiquitous interpersonal interaction is within the context of driving, this is a potential safety concern worth addressing. Recently, an automated vehicle was involved in a crash when it attempted to merge into traffic, to maneuver around an obstacle on the road, as a bus was approaching from behind. (Ziegler, 2016). The autonomous vehicle's programming assumed that the bus driver would yield when the autonomous vehicle attempted to merge into traffic; yet, this assumption was incorrect and the conflict could not be resolved through interpersonal communication. The transportation system is not only undergoing a technological transformation but also a social-communicative transformation. As the development and deployment of automated and connected vehicles continues, there will be a mixed fleet of autonomous, semi-autonomous, and non-automated vehicles, as well as vulnerable users (e.g., pedestrians and bicyclists) that all must safely interact with each other. Thus, it is critical to better understand the nature of how, why, and when people communicate with each other on the roadway. Such information will be useful for developing future vehicles and behavioral countermeasures that will not only prevent crashes but improve the well-being of all road users. The purpose of this report is to synthesize the literature on interpersonal communication between drivers, bicyclists, and pedestrians with the intent of identifying issues and challenges that may face developers of autonomous and connected vehicles. This review highlights future directions for automated vehicle research within the context of interpersonal communication on the road. (Author/publisher)
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