Road texture properties and road safety audit

The topic of the Road Safety Audit is of sure interest for both engineers and road administrators. Everybody knows that road safety depends on the interaction of three factors: road, driver, and environment. Generally, the analyses of functionality all along the road network are aimed at measuring the safety performance by means of absolute and analytical quantities, e.g. Mean Texture Depth (MTD), British Portable Number (BPN) or International Roughness Index (IRI). The integration of these quantities is useful to estimate the state of maintenance and, with this, the state of functionality of road pavements, in comparison with the traditional Specifications, Regulations and Standards. As a consequence, descriptive indexes like the International Friction Index (IFI) can easily be determined, which provide information on the state of the pavement by means of a sole number. However, this approach to safety analysis could be wrong because it does not consider the “human factor”, measurable through driver behaviour. Several studies in recent years have demonstrated that drivers are inclined to profit by the pavement adhesion properties more than we might think (the vehicle “operating speed” is very often higher than the “design speed” supposed for that road element). The importance of this aspect consists of the risk of under-estimating the safety level related to the real operative conditions of the road. This paper discusses the relationships between theoretical design adhesion, operative required adhesion and instrumental measured adhesion so as to define a procedure to evaluate the operative safety coefficient of a road element that considers not only the absolute pavement characteristics but also the way in which the driver approaches them and trusts them.