Operating-Speed Models for Horizontal Curves on Two-Lane Rural Highways: Case Study in Nigeria

Geometric elements of highways play a significant role in accident occurrences on curves. Hence, curved sites and the corresponding transition sections represent the most critical locations when considering safety measures on rural highways. Excessive speeds may cause inconsistency in a horizontal alignment. Hence, attaining geometric consistency is vital in designing and redesigning two-lane rural highways. Operating speed models were developed to evaluate operating speed consistency on a two-lane rural highway in Nigeria using speed data for 111 horizontal curves. The test of significance on the vehicle speed in the different lanes and the consistency of speed measurements show no statistical difference between the speeds in both directions. Also, at the 5% significance level, there is no statistical significance between the speed data obtained using a stopwatch and an automatic counter. However, the automatic counter allows for more observations. Based on the different categories of curve radius (CR) considered in the model development, Model 1 with CR < 1200 m has a statistically significant and influential relationship. Thus, this model is highly recommended for operating speed prediction on horizontal curves in Nigeria.