Speed Profile Method for Evaluating the Effect of Inconsistent Road Width on Safety of Rural Highways

Design inconsistency refers to highway geometry of non-conformance with driver expectancy due to a geometric feature or combination of features that has such a high driver workload requirement that driver’s may drive in an unsafe manner. One symptom of a geometric feature that violates driver expectancy is inconsistent operating speeds in the vicinity of the geometric feature. Highway geometric design standards of India are presently based on the design speed concept. However, due to constraints resulting from physical, right-of-way, and environmental features, uniform operating speed cannot be guaranteed. In the present paper, a continuous section of 56 km of State Highway-17 connecting Bangalore and Mysore in Karnataka State, India, was chosen. Speed profile measurements were done using a ROMDAS (ROad Measurement Data Acquisition System). An attempt was made to study the design inconsistency of the selected highway using the total usable width of the roadway’s the main parameter in the analysis. The relationship between the variation of speed between consecutive sections (speed change coefficient) and change in the usable roadway width (width change coefficient) was investigated. The effect of speed change coefficient on total number of accidents and number of fatal accidents was also studied at these stretches under heterogeneous traffic flow conditions. Based on the study, it was observed that greater the change in the width of usable roadway between successive road stretches, greater was the speed change coefficient. It was also observed that both, the total number of accidents and the number of fatal accidents observed increased as the speed change coefficient deviated from unity.