Determinants of speed variability on the horizontal curves of two-lane undivided rural highways passing through mountainous terrain.

Drivers traversing the horizontal curves are expected to complete the deceleration manoeuvre on the tangent and transition curve and maintain a constant speed upon reaching the curve. However, this may not be true for the horizontal curves constituting a two-lane undivided rural highway passing through mountainous terrain. The objective of this study is to investigate the speed variability on a two-lane rural highway passing through mountainous terrain and to identify its determinants. The continuous speed profiles of vehicles traversing the curves were extracted using the video image processing technique. Individual speed profiles, as well as the operating speed profiles obtained through quantile regression, indicate a significant speed variability on the horizontal curve. Speed variability on the curve was modelled in terms of the 85^th percentile of maximum speed difference (MaxΔ₈₅V) using the Robust Weighted Least Square (RWLS) Method. The findings indicate that the curvature change rate, length of the curve and the speed at the point of curvature affect the maximum speed difference on a curve. The findings also suggest that the operating speed estimated based on the spot speed data collected at the curve centre might lead to erroneous estimation of design and operating speed consistencies.