Relationship Between Sight Distance and Operating Speed on Horizontal Curves

All guidelines give more or less interesting and considerations about stopping and passing sight distances. Sight distance is the portion of the roadway available for view at any given time. In the Italian design guidelines sight distance is considered an important factor for road safety. It is the base for the calculation of design elements such as curve radii and vertical curvatures. The amount of available sight distance is considered of the utmost importance in the safe and efficient operation of vehicles on a highway. The aim of the analysis is to investigate the relationship between the operating speed and the geometric features, into the studies of several countries, in order to create an operating speed model, where the V85 is expressed as a function of Curvature Change Rate of the single curve (CCRs) and available sight distance. In the beginning of the article, the most important national and international models, which enable to calculate V85 in the Italian highways, are summed up. Then speed surveys, varying the available sight distance, are carried out. Driving speed in the midpoint curves is evaluated. During surveys just passenger vehicles in free flow are included (headway of at least 5 seconds). Besides different constraints have been imposed to select curve sites: rural area, marked and paved roadways with constant lane width, no intersections near to curve, longitudinal grades ≤ 6%. The research project, which is carried out by the Department of Land Engineering of the University of Cagliari, aims to develop an analytical model that permits to calculate V85 as a function of CCRs and available sight distance