A Filed Study of External HMI for Autonomous Vehicles When Interacting with Pedestrians

Interacting with pedestrians is an inevitable situation when vehicles are driven on the road. Drivers normally explain their driving intention with speed, headlights and vehicle horns. Especially, when the distance between vehicle and pedestrian gets close enough, to express themselves, driver's nonverbal actions, such as gestures, facial expressions, and eye contact are indispensable. Without the driver's role, the existing signals could not provide sufficient and accurate information to support vehicle interacting with the environment. Thus, external HMI becomes a new solution for vehicle-pedestrian interactions. In this paper, scenarios of which pedestrians need extra instructions including crosswalk, same direction on-road walk, and inverse direction on-road walk were defined. From these scenarios, pedestrian crosswalk was chosen as the case for the research, because of its importance and high frequency. In this situation, the interaction intentions of pedestrians and vehicles were decomposed respectively. Designs with command text, status text, command graphics, status graphics were raised during this analysis on the prototype. Then, an autonomous vehicle was modified by adding an external screen to display these solutions. Twenty-four participants were involved in the field study, in next step, to figure out the preference from pedestrians by real-situation experiment. In the summary part, we discussed advantages and disadvantages of these four designs. The results from the experiment show that an external screen displaying the autonomous vehicle's command is more able to help pedestrians to improve their efficiency of crossing road than displaying vehicle's status. Besides, text form is easier for pedestrians to understand its meaning than graphic form. In addition, the external HMI design solutions in pedestrian crosswalk scene were also provided.