A Short-Term Naturalistic Driving Study on Predicting Comfort Thresholds for Horizontal Curves on Two-Lane Rural Highways

The comfort threshold is one of the basic controls in designing horizontal alignment. Some studies have implied that the recommended comfort thresholds are conservative because they were determined using the traditional approach for vehicle design and corresponding driver behavior during the 1930s and 1940s. Drivers exceed the recommended comfort thresholds (because they are conservative) when traversing horizontal curves, which results in overturning and lateral skidding of the vehicle, increasing the chances of crashes on the horizontal curves. Therefore, the design guidelines need to consider comfort thresholds determined using recent approaches in data collection for modern vehicle design and corresponding driver behavior. Limited studies have determined the comfort thresholds for horizontal curves. Specifically, no studies determined the comfort thresholds for the horizontal alignment design in India. Thus, this study attempted to estimate the comfort thresholds for horizontal curves using an advanced Global Positioning System (GPS) device instrumented in passenger cars for data collection on two-lane rural highways in India. An all-subset regression approach was used to model comfort thresholds, and statistics such as Akaike information criterion (AIC), coefficient of determination, and cross-validation were used in the model selection. The study results showed no significant difference in the comfort thresholds on the right-turn and left-turn curves. Among the various geometric design features of curves, the curve radius significantly influenced the variation in the comfort threshold. The estimated comfort thresholds were higher than the side-friction demand recommended in an Indian design guideline. This study highlights the importance of determining the comfort thresholds using an advanced data collection tool for modern vehicle design and corresponding driver behavior to avoid crashes on the horizontal curves.