Modeling Relationship between Geometric Design Consistency and Road Safety for Two-Lane Rural Highways in the West Bank

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Mohammed Ghassan Dwikat
The objectives of this study are to investigate and quantify the relationship between design consistency and road safety for two-lane highways in the West Bank. This study produced speed prediction method using real time traffic speed data obtained from Google Earth maps, which were used to estimate the 85th percentile speed along an alignment that includes both horizontal curve sections and tangent sections. A comprehensive crash and geometric design database of two-lane rural highways has been used to investigate the effect of several design consistency measures on road safety. Previous studies showed that the most promising consistency measures identified in previous research fall into four main categories, namely: operating speed, vehicle stability, alignment indices, and driver workload. Five crash prediction models, which relate design consistency to road safety, have been examined. The generalized linear regression approach has been used for model development. All models adopted in this study showed acceptable levels of goodness of fit and over-dispersion. The developed models verified that the main design consistency measures have an important impact on safety. The consistency measures used in model development are: variation between the design speed and the operating speed, absolute difference of the 85th percentile speeds between successive design elements, difference between side friction supplied and demanded, average radius of curvature, average tangent length, maximum radius of curvature to minimum radius of curvature, curvature change rate, ratio of individual curve radius to average radius of the section, and visual demand of familiar drivers of the section. Validation step was performed; the goal was not only to compare the accuracy of different models developed, but also to evaluate the overall accuracy of Crash Prediction Models for use on rural two-lane highways in the West Bank. Validation requirement was to demonstrate that a model is appropriate, meaningful, and useful for the purpose for which it is intended. The models can be used as a quantitative tool to evaluate the impact of design consistency on road safety. An application is presented where the effectiveness of crash prediction models, which incorporate design consistency measures, is compared with those, which rely on geometric design characteristics. The study concluded that models, which explicitly consider design consistency, can identify the inconsistencies more effectively and reflect the resulting impacts on safety more accurately than those which do not. Finally, a systematic approach to identify geometrically inconsistent locations using the safety consistency factor has been proposed.