Operating speed model on tangents of two-lane rural highways

The prediction of the operating speeds is a very remarkable aspect for the adjustment planning of the existing road network. The problem can be solved more easily on curve than on tangent, because the drivers’ behaviour depends on few factors as curvature, slope and adherence. Instead, on tangent, the elements that influence the adopted speeds are more numerous. They are related to different features, like length, radius of preceding and following curves, transversal section, vertical alignment, etc. Numerous studies have been conducted, in different Countries and on various typologies of roads, to determine, on the basis of the statistic analysis of instrumental direct measurements, the links among operating speeds and some characteristic parameters of the roads. The objective of this experimental study is to get relationships of general validity for the prediction, in phase of project, of the speeds that will be indeed practiced on the infrastructure. The experimental investigation, object of the present paper, has been conducted employing traffic counters, able to record, for every vehicular passage in both senses: length, instant speed and direction of the vehicle. The plan of survey has been elaborated to satisfy different objectives of search (speed in free flow conditions, in entry and in gone out of the intersections, in rural to urban transition, etc.) and it has been applied to some roads belonging to the network of the Province of Salerno. The measures have been performed holding every section under observation for 2 or 3 hours. Then the data have been used for starting the formalization of some analytical relationships to predict the operating speeds. This paper introduces a prediction model of operating speed and the other parameters of free flow conditions on more than 36 tangents. In meaningful way, the surveyed speeds are correlated to the entrance one that depends on geometric parameters of the preceding curve