Impact of On-Curve Speed Variability on the Safety and Consistency Assessment of a Two-Lane Rural Highway in Mountainous Terrain

This study investigates how the speed variability on curves affects the geometric design consistency and safety assessment of a two-lane undivided rural highway in mountainous terrain. Previous studies have shown that speed is not constant on the curve, and the minimum speed (VMin) and the maximum speed (VMax) do not consistently occur at specific points on the curve. Naturalistic driving (ND) data can only reveal such speed fluctuations on the horizontal curves; however, getting such data from mountainous terrain poses several challenges. To address this, the study first develops correction factors for the operating speed at the curve center to estimate maximum and minimum 85th percentile speeds. Then, the geometric design consistency was evaluated in terms of Lamm’s safety criteria for major commuting vehicles (cars and loaded trucks). To consider the speed variability, this study used VMax85 in place of V85 for evaluating the design consistency. Similarly, VMax85 was used in place of V85 for estimating the demanded side friction, fRD. For evaluating the operating speed consistency, VMin85 was used in place of Vi+185. The findings corresponding to these changes were compared with the results of Lamm’s safety criteria. The effect of using VMax85 and VMin85 in the consistency evaluation was assessed through the level of agreement. The agreement level between safety criteria and the crashes corresponding to trucks showed that the application of VMax85 and VMin85 improves the identification of problematic sections. The levels of agreement between the overall safety module and endangerment level corresponding to VCC85 and, VMax85 and VMin85 are 79.3% and 84.4%, respectively. Furthermore, the crash frequency models corresponding to the three safety criteria revealed that the VMax85 and VMin85 significantly impact the safety evaluation. It was also found that the speed variables, VMax85 and VMin85 play a vital role in predicting the crash frequency compared to the geometric parameters. Thus, the study recommends considering speed variability through VMax85 and VMin85 when assessing geometric design consistency and safety of undivided highways in mountainous terrain.