New prediction method of operating speed and its application to highway alignment evaluation

To predict operating speed in highway design phase and ensure the coordination of alignment elements, models of lateral tolerated acceleration, longitudinal acceleration and deceleration, brake attenuation, environment speed concerned highway alignment were developed to determine the desired speed along the highway. A roadway-driver-vehicle-environment system (RDVES) was established to simulate vehicle models driving on 3D road-surface. With the RDVES, the operating speed of test roads was obtained and analyzed. The results show that (1) similar circle radii and tangent length along roadways was the best solution to avoid speed fluctuation, and shorter tangent should be adopted; (2) the acceleration distance of a vehicle when exiting curves was usually larger than the brake distance when impending curves, and both were much larger than the spiral length; (3) interchange of revolution and translational energy of driving vehicle will cause speed change when driving on mountainous roads containing sharp curves; (4) coordination of highway alignment by the operating speed method would be not applicable for evaluation of fourth-class grade highways; (5) the effect of corner cutting increases as deflection angle decreases, and hence the operating speed on the curves would be higher.