https://doi.org/10.1177/0361198118777091 Transportation Research Record 2018, Vol. 2672(22) 58–66 © National Academy of Sciences: Transportation Research Board 2018 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0361198118777091 journals.sagepub.com/home/trr TRR JOURNAL OF THE TRANSPORTATION RESEARCH BOARD Article Interpersonal communication between road users is an inte- gral aspect of using the transportation system. Whether driv- ing a motor vehicle, riding a bicycle, or crossing the street, safe transportation requires that everyone share the road. Communication between road users helps to coordinate behaviors, regulate road use, signal intent, and encourage drivers to behave in certain ways (1, 2). 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 acknowledgment of what is occurring, such as when a pedestrian crosses the street and a driver yields to him or her. Zaidel (3) 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 (4). In response to the critical need for interpersonal com- munication, modern automobiles are equipped with tech- nologies to facilitate communication between drivers, such as turn signals, hazard lights, and car horns. For example, drivers are required to indicate their intent to turn or change lanes through the use of a turn signal to other road users, yet research shows that turn signals are not consistently used. Sullivan, Bao, Goudy, and Konet (5) conducted a naturalis- tic 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 envi- ronment, other technologies have been developed to further extend communication options while driving, such as remote-controlled expressive car signs that utilize mount- able screens that can display short messages or express emotions to other drivers (6). The development of automated and connected vehicles comes with promise that driver-error-related crashes will be reduced. The National Motor Vehicle Crash Causation Survey (NMVCCS) found that about 94% of crashes were due to driver error (7), and advocates of automated vehicles have predicted that automated vehicles will reduce number of crashes by 80% (8). However, even with just a few auton- omous vehicles on the road at this time, an automated vehicle was already involved in a crash recently when it attempted to merge into traffic, to maneuver around an obstacle on the road, as a bus was approaching from behind (9). The autono- mous 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 777091TRR XX X 10.1177/0361198118777091Transportation Research RecordStanciu et al research-article 2018 1 University of Michigan Transportation Research Institute, Ann Arbor, MI Corresponding Author: Address correspondence to Sergiu C. Stanciu: sergiucs@umich.edu Pedestrians/Bicyclists and Autonomous Vehicles: How Will They Communicate? Sergiu C. Stanciu 1 , David W. Eby 1 , Lisa J. Molnar 1 , Renée M. St. Louis 1 , Nicole Zanier 1 , and Lidia P. Kostyniuk 1 Abstract Interpersonal roadway communication is a vital component of the transportation system. Road users communicate to coordinate movement and increase roadway safety. Future autonomous vehicle research needs to account for the role of interpersonal roadway communication. This literature review synthesizes research on interpersonal interaction between drivers, bicyclists, and pedestrians while also directing attention to implications for autonomous and connected vehicle research. Articles were collected from TRID, PsycINFO, Google Scholar, and ScienceDirect using search terms relevant to driving, communication, and vulnerable road users. The synthesis documents that interpersonal communication not only takes place but is also an important and understudied aspect of safe roadway travel. The review also found that road users employ a variety of communication methods that include gestures, facial expressions, and built-in vehicular devices. Comprehension of messages is influenced by several factors including culture, context, and experience. These results shed light on potential issues and challenges of interpersonal communication and the introduction of autonomous vehicles to the roadway.