DESIGN SPEED, ACTUAL SPEED AND ROAD DESIGNING

The paper presents the authors' current research results and analyses referring to the relation between design speeds and actual speeds of vehicles on roads with various geometric characteristics (situational layout, longitudinal profile, cross section). The research, that has been carried out over a period of more than ten years, was intended to create two categories of design criteria - one criterion relating to a single section of road geometry and the other to the spatial arrangement of these elements. The authors present a general philosophy of road design from the point of view of design speed. There is a proposal to introduce into the Polish normative for road design with regard to design speed the idea of a primary and a computing speed. The first one would allow classifying roads from the aspect of the economic policy of the country and particular regions; the second one would have the function of an actual design parameter. Both types of speed are related to each other and at the same time co-related with the actual traffic conditions. This means that a designer is able to select geometric elements of the road being entirely aware of the consequences on the expected level of traffic conditions. Additionally, selection out of a full range of geometric parameters is made possible due to this suggested method of assessing vehicles' speeds. This makes it possible to determine a distribution of speed or a selected parameter of this distribution for a chosen road section. This approach enables the designer to make effective attempts to reach optimal safety conditions, comfort and driving flow.