The Effect of Dynamic Speed Information and Timing of Displaying EHMI on Automated Vehicle and Pedestrian Interactions

Pedestrian-AV (automated vehicle) interaction attracts a significant amount of attention in recent years. However, there are controversial opinions about whether an external human-machine interface (eHMI) that communicates AV’s intent is needed. The authors propose that vehicle built-in cues (e.g., dynamic speed information) and the timing of displaying eHMI when communicating an AV’s yielding intent are both important. An online study was conducted to understand the impact on a 2-way street with a marked but not signalized crosswalk. As predicted, pedestrians were more likely to cross, with higher trust in AV and perceived safety, when the eHMI was displayed earlier. Information of the AV’s decreasing speed did not have a significant effect on crossing decisions, however, it increased trust and perceived safety. In addition, eHMI did have a positive add-on effect on pedestrian’s trust in AV, especially when it was displayed earlier, in the scenarios that dynamic speed of an AV was visible. Moreover, both the timing of eHMI and dynamic speed information played a role in enhancing a pedestrian’s judgment consistency in the short period of the interaction with AV, i.e., viewing the approaching AV from 20 meters away until it stopped near the pedestrian. The study indicated that both information about the vehicle’s physical behavior (i.e., speed) and the eHMI are beneficial in pedestrian-AV communication. More in-depth virtual reality or field studies are needed to further validate the findings.