A visual-looming-based connected collision warning trigger for rainy driving: Effect on brake reaction time, deceleration behavior, and surrogate safety outcomes

Rear-end crash prevention depends strongly on warning timing, yet most connected warning studies still rely on fixed trigger rules that are only loosely aligned with drivers’ perceptual urgency and are rarely evaluated across different car-following conditions under degraded visibility. This study evaluated a vehicle-to-vehicle-enabled connected collision warning (CCW) strategy triggered by the visual-looming-based τ−1 metric, which provides a perception-consistent indicator of time-critical risk. A driving simulator experiment was conducted under rainy conditions, in which 37 licensed drivers were exposed to 12 lead vehicle deceleration events each. Behavioral responses and surrogate safety outcomes were analyzed using linear mixed-effects and mediation models. Relative to the no-warning condition, the τ−1-triggered CCW reduced collision occurrence and shortened brake reaction time by 10.1% at far headway and 7.5% at near headway, while producing limited changes in brake transition time, maximum deceleration, and the proportion of uncomfortable deceleration. The safety effects of the warning were jointly moderated by headway and lead vehicle deceleration, with stronger benefits observed under moderate urgency and attenuated effects under extreme urgency. Mediation analysis further indicated that 39% of the reduction in collision severity was transmitted through brake reaction time, whereas 61% was attributable to direct warning effects on post-braking control. These findings support perception-consistent and scenario-adaptive trigger calibration for connected warning systems, in which warning thresholds should be adjusted according to headway condition, lead vehicle deceleration intensity, and degraded-visibility risk rather than relying on a single fixed time-to-collision (TTC) or distance threshold.