Car Following under Reduced Visibility

This article reports on a study of car following under conditions of reduced visibility, such as fog. The authors’ goal is to add to the understanding about safe headways in various visibility conditions. An introductory section outlines some of the approaches that have been used thus far, including automated warning systems (AWS) and variable message signs (VMS). This study analyzed real-world data on driving in fog, including values for time of arrival and departure, speed, headway, and vehicle lengths, noting how each factor affected driver’s maintenance of headway with different speeds. The study results showed no variation across sites but clear variation of headway with respect to speed across lanes. Graphs with cumulative distribution of headways are plotted to find 85th percentile headways across various sites and lanes for a particular visibility level. Drivers’ car-following behavior in the fast lane and the middle lane is not affected by drops in visibility because half of the drivers in those lanes chose headways as low as 1 second (The authors note that a minimum headway of 4 seconds would be appropriate for the visibility level studied). The authors also computed Time to Collision (TTC) and Potential Collision Speed (PCS) to measure the propensity and severity of collision; these were used as surrogate safety measures. The authors conclude by providing recommendations for safe headways for specific visibility conditions.