Behavioral adaptation in mixed traffic: the roles of AV penetration rate and human driving style

Automated vehicles (AVs) are designed to improve traffic safety, mobility, and driver comfort; however, their benefits depend on the widespread adoption of AVs. As full market penetration remains unlikely in the near term, AVs will continue to operate alongside human-driven vehicles (HDVs), making it essential to understand their interactions and the implications for traffic safety. This study investigated how different AV penetration rates (0%, 25%, 50%, 75%) influence HDV drivers' behavioral adaptation at the tactical level and subjective evaluations, while considering drivers' individual driving styles (aggressive, moderate, and defensive). Thirty-six drivers participated in a driving simulator experiment involving two driving scenarios (left-turn and lane-change scenarios) under varying AV penetration rates. Drivers' adaptive decision-making in response to AVs' defensive driving, as the AV penetration rate changed, was measured by the frequency of left turns executed without yielding and the distance maintained from surrounding vehicles during lane changes. Subjective evaluations were assessed through perceived safety and anxiety ratings collected after each trial. Results indicated that the influence of AV penetration rate was moderated by driving style. In the lane-change scenario, increased AV penetration rate resulted in more adaptive decision-making among aggressive and moderate drivers, whereas in the left-turn scenario, this effect emerged only for aggressive drivers. In contrast, AV penetration had no significant effect on defensive drivers' behavior in either scenario. These findings suggest that higher AV penetration may compromise safety in mixed traffic by provoking more aggressive decision-making and aggressive behavior among certain driver types, highlighting the need to account for driver adaptation patterns in AV deployment strategies.