Benchmarking Driver Responses in Head-On Conflicts: Expanding the Framework for TTC, Affordances, and Uncertainty

Head-on emergency events present unique challenges for evaluating both human and automated-vehicle (AV) performance because they do not conform to a direct stimulus–response sequence. Instead, driver behavior in these scenarios follows a stimulus–wait–response pattern governed by time-to-conflict (TTC), uncertainty, and environmental affordances. Prior research has often failed to distinguish between conflict types, resulting in generalized reaction-time assumptions that do not account for contextual uncertainty. This study integrates simulator and naturalistic driving data from a four-part research program to establish objective benchmarks for driver responses in head-on encounters. When an encroaching vehicle crossed the centerline 2.5 s before impact, drivers initiated braking with a weighted average of approximately 1.0 s before impact. When the encroaching vehicle crossed or was first observed at approximately 3.5 s before impact, braking typically began with a weighted average of 1.3 s before impact, consistent with a deliberate waiting period rather than an immediate reaction. Across conditions, response variability increased with TTC, with the standard deviation scaling at 0.50 times the mean. In events where the encroaching driver corrected back to the proper lane, delayed responses were associated with successful avoidance. Steering behavior was influenced by roadside affordances, drivers steered right when no right-side obstacle was present but rarely steered right when any obstacle existed and drivers were likely to steer left when right-side obstacles were present. These findings reinforce the wait-and-see principle and provide empirically grounded benchmarks for evaluating human and AV responses in head-on emergency scenarios.