EVENNESS OF ROADS AND THE SPEED OF CARS - IS THE EXPERIENCE OF A ROAD'S EVENNESS DEPENDENT ON THE SPEED OF THE CAR?

The evenness of the Danish state roads is generally good, so the drivers experience the roads as comfortable and safe to drive on at the permissible speeds. The interactions between evenness, the vehicle and speed are important factors in the analysis of the road users' comfort and safety on a road network and must become a part of the analytical tools which are used in connection with measurements of the evenness of roads. It is especially important that these parameters are included in an analysis of the road users' comfort, if the permitted speeds are increased for certain vehicle classes or certain road stretches. Since the evenness of roads changes with time, it is important to check whether the set of rules which form the basis of the demands and specifications for the evenness of roads, to a sufficient degree take these changes into account. It is also of great importance that the equipment used to measure the surface of the roads makes it possible to carry out the necessary analyses. During the last 10 to 15 years, equipment for measurement of the surface of road pavements has gone through an enormous development. The use of optical measurement equipment has replaced mechanical equipment used previously and thus given new possibilities to analyse the condition of a road surface better than before. In the Danish Road Standards it has been stated for several decades that measurements from a Viagraph and a Bumpmeter should decide the evenness of roads. Even in the most recent revision of Tender & Construction Precepts for hot-mix asphalt the viagraph number is used, whereas in the revision of the Road Standards for construction and maintenance of roads and paths the International Roughness Index (IRI) is introduced as an aim for evenness. Since these measuring methods which are used in Denmark to determine the evenness of road pavements have been used for many years, it is natural to ask whether they are really representative for what the driver experiences. This External Note 18 describes the present methods of analysing the evenness of road pavements and the factors which affect the comfort of the driver in relation to the type and speed of the vehicle.