The design of vertical alignments of highways is controlled by several factors, such as terrain, design speed and traffic characteristics. Two issues usually govern the design process: (1) the maximum grade allowed and (2) the maximum desirable length of grade. In current practice, the first parameter is determined arbitrarily and the second is then calculated by speed reduction criteria, depending on the design speed and the slope. The paper discusses some of the deficiencies of current practice and reviews some of the acceptable design values. In the proposed procedure, grade and length are treated as a single design element. Any practical grade can be associated with any design speed value. It is suggested that for low design speeds, any moderate grade and length combinations are adequate; for steeper grades, high design speeds require shorter lengths of grades. Furthermore, any feasible slope can be allowed for any design speed; however, the length must be checked for certain design limitations. The appropriate length of grade is derived from an operational criterion based on the speed difference between trucks and passenger cars. Additionally, the length must be checked for design uniformity to prevent, for example, very short sections with steep grades on an otherwise moderate and continuous vertical alignment. The proposed approach for determining slopes along a vertical alignment also deals with the approach grade, prior to a specific grade that needs to be designed. This allows a determination of the initial speed of cars and trucks on the grade and more accurate calculation of their relative speed difference, toward the end of the grade. If this speed difference is kept low, an integrated, more consistent and safer design is achieved.
Samenvatting