Design Consistency Evaluation of Rural Highways

Abstract

Rapid growth of population and increasing economic activities are the twin factors that contribute to the tremendous increase in the vehicle population which, in turn, contributes to the growing number of road accidents. Road accidents are complex events involving a variety of factors, including highway geometry, driver behaviour, weather conditions, and speed limits. Several studies have been conducted to determine the relationship between factors associated with accidents and their effect on safety. Improving highway design consistency is considered to be an important strategy for improving traffic safety. Geometric design consistency evaluation is a widely used method of determining sections of highways which require alignment improvement. Identifying and treating any inconsistency on a highway can significantly improve its safety performance. A critical factor in highway design is maintaining a good consistency with a road geometry that meets the driver’s expectations. Considerable research has been undertaken to explain this concept, including identifying potential consistency measures and developing models to estimate them. However, considerable amount of work has not been carried out to evaluate the geometric consistency in India. Rural roads constitute about 80 per cent of Indian road network. Therefore, rural road safety accounts for a considerable share of the total road safety problem. In Dakshina Kannada District, and even in Karnataka state, intermediate lane highways make more than 50 per cent of the principal road network. The majority of these intermediate lane rural highways are historical routes and many of them do not follow a specific design code. Therefore, road safety of intermediate lane roads is a very important issue in the development of the country. This study aims to enhance the safety of these existing highways based on an understanding of actual driving behaviour by means of field data measurements, and to verify their conformance to a consistency evaluation model. Both horizontal and vertical alignments are the main focus of this study. The horizontal alignment of a road consists of a straight tangent followed by horizontal curve, whereas the vertical alignment of the road consists of a level section followedvii by a vertical curve. This study deals with developing appropriate design consistency evaluation criteria of horizontal and vertical curves using geometric, speed, and accident data of eight intermediate lane rural highways. Geometric details of a road were collected from the field and CAD (Computer Aided Design) drawings were prepared using the details of surveying. A spot speed survey was conducted for passenger cars on the approach tangent and at the beginning, middle, and end points of horizontal curves and on the approach tangent, limiting point and summit point of vertical curves. The accident details were collected for six years (from 2005 to 2010) from different police stations. The operating speed prediction models were developed for both horizontal and vertical curves. The speed change experienced by the same driver from tangent to curve i.e. speed differential was also studied for horizontal curves, and the models were developed to predict this change. Consistency evaluation criteria for horizontal curves and vertical curves were also developed to enhance the safety of the alignment. Alignment indices, are the another set of quantitative measures, were used to identify the inconsistencies that exist on intermediate lane rural highways. Finally, an attempt was made to develop the relationship between safety and alignment indices.