When we talk about road safety we think of traffic lights, signs, electronic gantries, potholes, traffic, or the condition of our tyres. The last of these is directly related to the grip our vehicle has on the road surface. Tyres and the characteristics of the surface of the road on which we are traveling play an important role in our safety as drivers.
Three basic characteristics define the quality of the road surface on which we are driving. The first has to do with the macro-texture of the surface. We all know the feeling of driving on a rainy day, when water accumulates on the road surface and it is hard to see the lane markings, especially when we get behind a lorry, and we notice unexpected movements of the steering wheel when we traverse areas with more surface water.
In the photograph below of the Calatayud-Alfajarín section of the A2 motorway you can appreciate the macrotexture of the surface, the principal function of which is to evacuate water and help bind the tyre to the road.
The granulometric macrotexture should be taken into consideration when designing the surface mixture and special attention should be paid to it during on-site installation of the asphalt in order that the surface characteristics be maintained over time and ensure maximum grip.
Two other key parameters indicate the condition of the road surface, the IRI and the TFC. The International Roughness Index gauges the roughness of the surface, and, in layman’s terms, measures comfort while driving. In other words, it measures the degree of vibration and brusque movement that the drives experiences while traversing a particular road surface. While a smooth, easy-to-drive surface will show a correct IRI value, if we notice the car moving and vibrating as though we were driving along tiny potholes and deformities, the IRI will be poor. This parameter is closely related to the quality of execution of the paving and compaction of the asphalt.
The TFC is the transverse friction coefficient, and refers to what we mentioned before, the level of grip between the tyre and the road. In the previous photo you can see that the surface granulometry is composed of alternating aggregate and holes, somewhat akin to a bicycle tyre, with tiny studs protruding in order to grip the road both longitudinally and transversally. A key moment in driver safety is when it rains, and this is where the road surface plays its most important role. If the surface is not macro-textured and/or perforated, it will not drain correctly and will throw up screens of water that prevent drivers from seeing properly, particularly at night-time. In this situation, a smooth, unperforated surface acts like a mirror and it is difficult for the driver to see where he or she is going, as they can barely make out the lines on the road surface. In the photo below you can see a comparison between a properly drained surface (on the left) and one with little perforation (on the right). Obviously the level of safety on the former is far higher than on the latter.
In conclusion, beyond what is visible to the naked eye, those of us who work with paving strive to achieve maximum grip of the tyre to the road surface, independently of the prevailing weather conditions. The working formulas of the different asphalt mixtures, the materials used and the on-site execution of the paving are fundamental aspects to be taken into consideration if we are to achieve our objective.