Alternative Design Consistency Rating Methods for Two-Lane Rural Highways

Design consistency refers to the conformance of a highway's geometry with driver expectancy. Drivers make fewer errors in the vicinity of geometric features that conform with their expectations. Techniques to evaluate the consistency of a design documented within this report include alignment indices, speed distribution measures, and driver workload. Alignment indices are quantitative measures of the general character of a roadway segment's alignment. Potential indicators of geometric inconsistency include a large increase in the magnitude of the alignment indices for a successive roadway segment or feature or a high rate of change occuring over some length of road. Speed distribution measures—including variance, standard deviation, coefficient of variation, and coefficient of skewness—were investigated as potential candidates for a consistency rating method. The results indicated that speed variance is inappropriate as a design consistency measure for horizontal curvature. Driver workload is a measure of the information processing demands imposed by roadway geometry on a driver. The efforts for this study used both objective and subjective measures to model geometric features and combinations of features in terms of the difficulty that they pose to drivers. Vision occlusion, subjective difficulty ratings, a driving simulator, and an eye-mark system were used during the research.