Factors that Influence Drivers’ Responses to Slower-Moving or Stopped Lead Vehicles 2021-01-0890

Rear-end crashes account for more than one in five fatal crashes in the U.S. The rear-end crash scenario most commonly associated with fatal crashes involves a following vehicle traveling 40 to 70 mph closing on a lead vehicle at a rate greater than 30 mph. The current research compiled an analysis of the literature to identify the kinematic factors, environmental factors, traffic-related factors and individual differences that are likely to influence drivers’ responses when closing on a slower-moving or stopped lead vehicle [LV].

In Part 1, several primarily kinematic-based methods for modeling drivers’ responses to a LV were compared for high-speed closing events. Methods utilizing looming (angular growth rate) equations were shown to predict drivers’ responses and time-to-contact methods (Inverse Tau) were conditionally accurate when applied to specific crash scenarios. However, the ratio or nominal response time methods did not predict drivers’ responses in most crash scenarios. Overall, with few exceptions, drivers responded in ways that could best be described when using the looming equation.

In Part 2, the influence of the environment, traffic in the area, and individual differences were evaluated. Drivers responded much earlier than the looming equation suggested when in congested traffic, when following closely behind the lead vehicle, when the LV was parked on the shoulder, and when the lead vehicle was sideways across the road. In reduced lighting conditions, drivers responded significantly later when the lead vehicle had narrowed taillights, when the distances between the taillights was narrowed from the normal width, and when the lights on the lead vehicle were dimmer than normal. Interchanges, work zones, and inclines (or elevated roadways) were identified as locations where rear-end crash rate increased significantly. Drivers commonly responded 0.2 to 0.35 second faster when the lead vehicle displayed emergency flashing lights. Lastly, many drivers, whether a crash occurred or not, were found to make mirror glances in the moments before reaching the looming threshold. Hence, unbeknownst to a following driver, he/she may be looking away from the forward roadway at a critical moment while closing at a dangerous speed.