SPEEDS, FRICTION FACTORS AND ALIGNMENT DESIGN STANDARDS

Abstract

The basis of, and values for, the low speed alignment design standards in the national association of Australian state road authorities' interim guide to the geometric design of rural roads are considered against international practice and research and development in alignment design. The operating speed approach adopted for the interim guide is found to be consistent with developments in European countries, whereas North American practice has persevered with the traditional design speed approach. As operating speeds on low speed alignments are generally greater than the traditional design speed, it is not valid to use the speed assumed for design as a basis for comparing alignments when different design speed concepts are involved. The conservatism introduced by assuming conservative values for design criteria in the traditional design speed approach is largely offset by operating speeds being in excess of the design speed. When standards are compared on the basis of the balance between standards for different elements, the interim guide standards are shown to be consistent with those adopted in other countries. Criticisms of the interim guide are considered, particularly the specification of longitudinal friction factors which are high relative to the friction which can be expected on wet surfaces. An alternative means of presenting stopping sight distances is proposed (a). The isbn of the microfiche version is 0-86910-350-4.