Safety Effects of Geometric Design Consistency on Two-lane Rural Highways: The Case of Ethiopia

Different studies conducted, regarding the development of statistical models used to predict crash occurrences along two-lane rural highways, around the globe and particularly in Ethiopia, rely on geometric characteristics than explicitly considering measures of design consistencies. Recently, the issue of geometric design consistency in highway design is emerging as an important criterion. This study is therefore carried out to evaluate the design consistency on horizontal alignments using measures of design consistency and develop safety functions that exclusively include design consistency measures. Elements of all road sections considered in the study were rated as good, fair or poor design using design evaluation criteria. Poisson regression and Negative Binomial regression modeling approaches were used to assess the relationship between measures of design consistency and road safety. Using information criteria goodness-of-fit, Poisson regression model was found better fit to the data than NB regression models. Three different crash prediction models which explicitly incorporate design consistency measures were successfully developed. The models can be used to predict crashes of two-lane rural highway having similar characteristics with the road considered under this study. Finally, based on the results of this research, further detailed study on effect of highway design consistency on road safety and current design guides related to design speed selection to account for design consistency revision was recommended.