EVALUATION OF HORIZONTAL ALIGNMENT DESIGN CONSISTENCY FOR TWO – LANE RURAL HIGHWAY: A CASE STUDY ALONG SHASHEMENE - WOLAYTA SODO AND SHASHEMENE - ASELA ROAD SEGMENTS

Abstract

Design consistency is the conformance of highway’s geometric and operational features with driver’s expectancy. Techniques to evaluate consistency of a design are change in operating speed and change of vehicle stability along the horizontal alignment. To use operating speed and vehicle stability as the consistency tools requires the ability to accurately predict speed as a function of the horizontal roadway geometry. The current Ethiopian road Authority geometric design manual is based on design speed. However, due to the constraints resulting from roadway elements, right of way, and environmental features, inconsistency in operating speed based and vehicle stability based cannot be guaranteed. In order to make informed decisions to ensure highway geometric design consistency, the consistency evaluation of horizontal geometric design of operating speed and vehicle stability on simple curve and tangent section was studied. Forty five sections for both of horizontal curve and tangent section were selected from Shashemene to Wolayta Sodo and Shashemene to Asela. Roadway geometric design variables were obtained from design documents and through field measurements. The speeds of passenger cars, buses, and trucks were measured on each curve and its approaching tangent, under dry day time condition. This study focused on two major issues, in evaluating the horizontal geometric design consistency of selected alignments and developing operating speed predicting models for selected section. Therefore, the results obtained from consistency evaluation of change operating speed between simple curve and tangent section shows that twenty sections were in poor conditions, between design speed and operating speed shows that nine section in simple curves and thirty one sections in tangents sections were in poor conditions. Whereas, consistency evaluation between changes of side friction assumed and side friction demanded shows that twenty seven section in horizontal curves were in poor condition. Finally, both two models were found for horizontal curves and tangents. Model found for horizontal curve and tangents were governed by the combination of grade, super elevation and tangent length. This study would provide information about the improvement design manual for the existing level of design inconsistency and to develop guidelines that designers can use to improve the geometric design consistency of roadway features on high-speed of two-lane rural highway road.