Design strategies compared: How eHMI are perceived in relation to the exterior design of automated vehicles
Authors: Gadermann Lars , Fischer Lutz , Holder Daniel , Ihle Niklas , Schlecht Julius , Maier Thomas 
Published: June 2023

Communication between road users, especially between vulnerable road users and drivers, is an important part of road traffic. Often, small nuances such as gestures, facial expressions or eye contact determine the subsequent behaviour of those involved. For example, eye contact with the driver helps pedestrians to make a decision about when to cross the road. With automated vehicles (AVs), the ability to communicate with the person in the driver's seat is limited, as they no longer necessarily control the vehicles behaviour. AVs perceive their environment via a multitude of sensors. Towards the environment, however, a communication channel has been missing until the latest research about external human-machine interfaces (eHMI). For eHMIs, there are various concepts and studies on design, modality and positioning. So far, there is a lack of experience on the integration of eHMIs into the exterior design of vehicles in terms of their perception by pedestrians and other road users. A large number of studies have already shown that vehicle exterior design has an influence on people's subjective perception and associated actions. In this paper, this preconditioning will be investigated in relation to the degree of integration of eHMI in interaction with visible or invisible sensor clusters. For this purpose, different degrees of eHMI integration are applied to a concept car and investigated in an eye-tracking study with regard to their influence on the sensations of other road users. This results in findings for the design of safety-relevant eHMIs to strengthen the trust in AVs among the general population.For this paper, we conducted a literature review on the design and limitations of designing of eHMIs. In particular, studies on the location of eHMIs, their modality (light, symbol or text) and colour stood out. In order to be able to make statements about the integration of eHMI into the vehicle exterior design, the research results were categorised according to the degree of integration into additive, integrative and integral eHMI.Since visual attention plays a significant role when viewing vehicles with eHMIs, the study was designed with eye tracking anaylsis. In this, the test persons are shown renderings in ¾ front and ¾ rear perspective of a concept vehicle on the screen, which represent different degrees of integration of eHMI. The stimulus patterns show additive eHMI in the form of attached roof bars, surrounding light text bands on the roof, as integrative eHMI as well as integral eHMI in the exterior design in the form of central, fully integrated display surfaces at the front and rear. In addition, to test interactions with the visibility of sensor clusters, each configuration was combined with both visible and invisible sensor clusters.The test persons evaluated each of the stimulus patterns by means of a questionnaire regarding their subjective impression in the context of use cases (road crossing scenario). The evaluation with regard to different factors provides statements about the system trust through eHMI, the appealing and recognisability of eHMIs as well as the interaction with the sensor clusters typical for automated vehicles.RESULTS AND CONCLUSIONThe research has shown that the type and degree of integration of eHMI into the vehicle exterior design is of relevant importance for design perception, evaluation and thus design. The degree of integration has an impact on perceptual factors such as recognisability or appealof AVs. To replace the subtle but efficient communication between driver and pedestrian in the context of a future vehicle exterior design, it is beneficial if the eHMI represents a salient stimulus and can be interpreted clearly.OUTLOOKFurthermore, it should be investigated whether vehicles with a personifying exterior design and a human-interacting eHMI (cf. Jaguar Eye-Pods) show more positive effects on communication and recognisability than conventional, machine-like eHMI. This would come close to merging classic vehicle design with robotic design, meaning the transfer of human characteristics to machines.ACKNOWLEDGEMENTSThis research was supported by Federal Ministry for Economic Affairs and Climate Action in the national research project RUMBA.