More than 80% of national roads in the Netherlands are surfaced with ZOAB (the Dutch acronym for dual-layer porous asphalt). Their condition has to be monitored to ensure safety and to determine if maintenance work is needed so that they comply with statutory requirements for safety, noise and ride quality. Measurement of surface damage and skid resistance are two important techniques used by TNO in collaboration with Rijkswaterstaat (RWS).
The skid resistance of a road depends on many parameters: the roughness (texture) of the surface, the type of crushed stone used, soiling of the road surface and whether the surface is wet or dry. These properties of the road surface – together with the vehicle's braking system and make of tyre – are the main factors that determine how safe it is for the motorist. We therefore assess road-surface effects on a regular basis by performing skid-resistance measurements. However, these measurements are not always made under the same conditions. TNO therefore conducted research into how variations in conditions (such as air temperature, tyre temperature and tyre pressure) affect the results of skid-resistance measurement. This resulted in recommendations to improve the measurement methods, not only for skid resistance but also for road conditions, and in guidelines on how to adjust for this, enabling skid resistance to be determined more accurately.
Ravelling is the most common form of damage that occurs in ZOAB road surfaces. It is the process by which the top stones are forced out through the road surface. When the damage becomes excessive the road has to be resurfaced. A road with serious ravelling problems reduces ride quality, increases noise and increases the risk of windscreen damage. It is apparent that automated systems can make a useful contribution to fast, safe road inspections and the timely detection of ravelling damage.
TNO is therefore conducting research for Rijkswaterstaat into the automated ravelling detection for ZOAB roads. In recent years this has led to the development and realisation of a measuring system for the ARAN-2, one of the automatic road analyser vehicles used by Rijkswaterstaat. This system produces precise longitudinal profiles of the road surface at speeds up to 120 km/h.
TNO has developed image processing algorithms and classification models. On the basis of the longitudinal profiles produced by the ARAN vehicles, these algorithms and models calculate stone loss, and therefore the maintenance work required. The measuring system thus provides safe and objective information on the condition of roads. This is essential input for Rijkswaterstaat's long-term planning of road-surfacing recommendations (MJPV) that is the basis for scheduling maintenance work on national roads from year to year.