CONSISTENCY OF HORIZONTAL ALIGNMENT FOR DIFFERENT VEHICLE CLASSES

The objective of this study was to develop guidelines for evaluating the consistency of horizontal alignment of two-way two-lane highways. A number of simple and continuous horizontal curves were selected for this purpose. Roadway geometric design variables were obtained from design plans and field measurements. The speed of passenger cars, light trucks, and trucks was measured on tangent and horizontal curves. In the analysis, the speed reduction between tangent and curve or successive curves was considered the inconsistency indicator. The results of the analysis indicate that the degree of curve, length of vertical curve, gradient, and pavement condition have a significant effect on consistency of simple horizontal curves. For successive curves separated by a short common tangent, the speed reduction was found to be highly affected by the radii of curves. Also, the results reveal that a good consistent design can be achieved if the radii of successive curves are equal. Finally, the speeds on a common tangent between successive curves are investigated. For each type of vehicle, the results indicate that the degree of successive curves and the tangent length significantly influence the speed on the common tangent.