HORIZONTAL ALIGNMENT DESIGN CONSISTENCY FOR RURAL TWO-LANE HIGHWAYS. FINAL REPORT

The state of the practice in highway geometric design consistency was determined through a review of U.S. and foreign geometric design policy, practice, and research. Models, and a menu-driven microcomputer procedure for their use, were developed for operating-speed and driver-workload consistency evaluations of rural two-lane highway horizontal alignments. The operating-speed model was calibrated based upon speed and geometry data for 138 horizontal curves and 78 of their approach tangents in 5 States. The driver workload model was calibrated based upon 2 occluded vision test studies on a total of 55 subjects. The operating-speed data suggest that 85th percentile speeds generally exceed the design speed of horizontal curves whose design speed is less than drivers' desired speed (i.e., 85th percentile speed on long tangents). A preliminary evaluation comparing model-estimated operating-speed reductions versus degree of curvature as predictors of accident experience was conducted using a data base of 1,126 curve sites in 3 States. The evaluation suggests that accident experience increases as the required speed reduction from an approach tangent to a horizontal curve increases.