Modeling Speed and Comfort Threshold on Horizontal Curves of Rural Two-Lane Highways Using Naturalistic Driving Data

Modeling efforts for driver behavior parameters on horizontal curves have mostly focused only on the 85th percentile value. However, predicting whole distributions would help improve alignment design by allowing reliability-based design and design consistency evaluation. This paper used naturalistic driving study data to model distributions of speed and comfort threshold on horizontal curves of two-lane rural highways. Several variables along the approach tangent and curve were extracted and examined. This analysis helped determine the driver behavior parameters needed to evaluate driver behavior on horizontal curves and the headway threshold for free-flow conditions. Driver level models (DLM) and panel models (PM) were developed to predict distributions of curve speed and comfort threshold in addition to the traditional 85th percentile models. The models developed can be used in evaluating vehicle stability, driver comfort, and design consistency. Thus, the models can act as the basis for reliability analysis of horizontal curves, for which analysis methods are already established but realistic data are relatively scarce.