Influence of Speeds on Design Standards in Switzerland

In Switzerland, a speed diagram is used to check the consistency of the alignment of a section of highway. The sequence of alignment elements is subject to maximum permitted differences for the design speed, and the highest design speed is coupled to the general speed limit applicable to the category of highway in question. After a significant reduction in the speed limit for rural highways [100 to 80 km/h (62 to 50 mph)], the application of this model as a control instrument in practice has become problematic. Consideration is being given to an alternative procedure, such as specifying the permitted radii sequence. The analysis deals principally with the changes in speed behavior compared with previous surveys from the period when a higher speed limit was in place. The main focus of this study was the relationship between curve radius and running speed; the influence of other design elements in curves and speed behavior on curves on rural highways were also investigated. The alignment model in Swiss geometric design standards is described. On the basis of the new findings, the problems of the Swiss alignment model and the implications for existing design standards were analyzed.