Communication between drivers in a road bottleneck scenario
Authors: Schuler Katharina , Quante Laura , Schießl Caroline , Beggiato Matthias , Jahn Georg 
Published: June 2022

Background. With the introduction of automated vehicles, communication between automated and human road users is becoming increasingly important. In the future, automated vehicles must be able to recognize the behavioral intentions of conventional road users to react in accordance with expectations. In urban traffic, many everyday situations are not clearly regulated by traffic rules, requiring road users to negotiate with each other as to who has the right of way. An example of such a situation is a two-sided road bottleneck, in which the right of way is not explicitly defined, so that road users must negotiate with each other who will pass the bottleneck first. Since this scenario has been little studied so far, this work’s objective is a detailed description of the communication behavior of drivers when passing a two-sided bottleneck in order to gain insights for the development of driving strategies for autonomous vehicles. Method. Video material of 100 encounters between drivers at a two-sided road bottleneck in Braunschweig, Germany, was analyzed with respect to the type, order and duration of communication signals by manual annotation. Interrater reliability showed a moderate to almost perfect agreement for all collected variables, which included both explicit (e.g. hand gestures, turn signal, headlight flasher) and implicit (vehicle dynamics) communication signals. The identified communication signals were further combined into communication sequences to map the temporal sequence of communication signals used by the drivers. Additionally, drivers were studied in more detail with respect to their arrival and passing order.Results. The results indicate that explicit communication takes a minor role in the bottleneck scenario and that drivers primarily communicate by means of implicit signals. Furthermore, no informal rule could be found with respect to arrival and passing order. The results rather indicate that the passing order of the two drivers is related to the type and timing of the communication signals shown. In the observed encounters, a braking maneuver was the most frequently shown first behavioral change, which in most cases was shown by drivers who passed the bottleneck second. The data obtained also indicate that differences between drivers who pass first vs. second can be found especially with respect to offensive and defensive communication signals. Drivers who passed the bottleneck first, showed both more frequent and longer acceleration maneuvers as well as lateral movements toward the center of the road. Drivers who passed the bottleneck second, on the other hand, communicated both more frequently and longer using defensive signals, i.e. braking, coming to a stop, or initiating a lateral movement to the side of the road. Furthermore, characteristic communication sequences were found for both driver groups, namely drivers who pass first vs. second. A communication sequence, in which the narrow passage was passed at a constant speed, in some cases accompanied by a braking maneuver followed by an acceleration, was identified for drivers passing the narrow passage first. On the other hand, communication sequences in which the driver braked and drove to the side of the road, were associated with drivers who passed the bottleneck second. Conclusion. The communication sequences identified in this work provide initial guidance on what behavioral strategies might be used to signal a drivers’ intention to pass a two-sided bottleneck first or second. The results can thus be considered as a first basis for the realization of an expectation-compliant communication behavior of automated vehicles in a bottleneck scenario. One goal of future research should be to derive concrete and generalizable action recommendations for automated vehicles, so that the automation is able to cope with the scenario in an expectation-compliant manner by means of cooperative behavior strategies.