Most modern devices for measuring texture in the macro and megatexture ranges are based on the principle of “laser triangulation”. A laser beam is projected perpendicularly onto the surface to be investigated and a special camera “observes” the patch of light on the road surface. The camera is capable of determining the height of the patch. By moving the measurement system (most often, but not necessarily, parallel to the road centre line) and measuring and recording the height of the patch at fixed intervals, it is possible to obtain a two-dimensional profile of the road surface.
The TRIMM project (acronym of Tomorrow’s Road Infrastructure Monitoring & Management) was completed late in 2014. It was funded under the European Union’s Seventh Framework Programme for research (FP7, 2007-2013). Its objective was to strengthen the role of condition monitoring in road management and to highlight its beneficial effects. The condition of a road network is, in fact, decisive for its level of service to road users. Maintenance is required to preserve or improve the condition of bridges and roads.
The Belgian Road Research Centre has a footway profilometer to measure the longitudinal evenness of cycle tracks and other surfaces. A scooter with a trailer travels at a constant speed of 20 km/h on the surface to be surveyed. Using a laser and an accelerometer, the distance between the trailer and the road surface is recorded every 3 cm. The GPS coordinates and the distance travelled are recorded as well. From the measured longitudinal profile is possible to calculate the coefficient of evenness (for wavelengths of 0.5 m and 2.5 m) or other indicators of evenness.
The Belgian Road Research Centre has a longitudinal profile analyser for continuous measurements of the road profile. The measured data can be used to quantify the longitudinal evenness of a road and to calculate inticators such as the coefficient of evenness (CP, which stands for coefficient de planéité in French) and the international roughness index (IRI). Longitudinal evenness is an important factor for the safety and comfort of road users. The high accuracy of measurement makes the APL also suitable for research purposes. The measuring wheel bumps up and down with the relief of the road. This movement causes a change in the angle formed by the wheel-bearing arm in its hinge point. An inert pendulum in the bearing arm transforms the road profile into an electric signal. The angle between the bearing arm and the pendulum is recorded every 5 cm. This allows a graphic representation of the “pseudo-profile” of the road surface. A constant survey speed of 21.6 km/h, 54 km/h or 72 km/h is required.
We do not instantly think of it when travelling on a road, but it is worth having a closer look at the road surface. Quite a few interesting characteristic properties can be derived from a road surface condition survey. The results of non-destructive measurement methods are converted into «indicators» that are easy to understand – usually a figure on a scale.