Measuring head motion across deceleration levels in automated driving: Quantifying comfort

Passenger-centred comfort measures are needed to enable adaptive driving styles in automated vehicles, particularly where comfort begins to decrease. This study examined passengers' head motion during automated braking at a crosswalk. Data from thirty-four participants were analysed, who experienced three deceleration levels (-1.5, -2.5, -3.5 m/s²) and two pedestrian visibility conditions, reflecting how quickly the AV braked after the pedestrian appeared, rating comfort respectively. As expected, head pitch and translational acceleration increased with deceleration intensity, while pedestrian visibility had no effect on head motion, even though faster AV responses were previously found more comfortable, suggesting this reflects passengers' judgement of the AV's response rather than head stability. Head-motion measures did not add predictive value beyond vehicle kinematics, likely because comfort ratings were similar across conditions. In this scenario, head-motion mainly reflected physical response to braking rather than subjective evaluation. Future research should test head-motion measures in more complex driving conditions.