Contact boundaries between the head of vulnerable road users and vehicles in simulated collisions: The potential references for passive and active protection

Head injuries represent a significant risk to Vulnerable Road Users (VRUs) involved in vehicular collisions, and there is still a lack of comprehensive understanding of VRU head-vehicle contact boundary conditions to provide basic reference for vehicle safety design and assessment. Consequently, the objective of this study is to examine the head WAD (Wrap Around Distance), contact time, linear contact velocity, and angular velocity at contact of pedestrians, bicycle cyclists, and electric two-wheeler (ETW) riders through multi-body simulations, which are defined based on the distribution of accident scenarios. The findings reveal that the VRU head WAD ranges from 1.4 to 2.5 m, the VRU head contact time varies from 50 to 300 ms, a head linear contact velocity of 40 km/h is capable of encompassing approximately 80% of pedestrian and cyclist incidents and over 95% of ETW rider cases, and a head angular velocity at contact can cover nearly 80% of pedestrian cases and almost 90% of cyclist and ETW rider incidents. Furthermore, VRU head WAD increases with both the vehicle impact speed and VRU height, VRU head contact time decreases as the vehicle impact speed increases but generally increases with the height of the VRU, head linear and angular velocity at contact demonstrate a positive correlation with vehicle impact speed. A comparative analysis reveals several key findings: cyclists exhibit a significantly greater head WAD compared to pedestrians and ETW riders; VRUs generally experience a longer head WAD and later head contact time in sedan crashes than in SUV and MPV impacts; pedestrians display a notably higher head angular velocity at contact than both cyclists and ETW riders; ETW riders demonstrate significantly lower head liner and angular velocities at contact in sedan incidents than in SUV and MPV collisions.