in fact, passengers are generally not aware that these levels are high, because experience has led motorists to expect levels commonly exceeding 65 dBA. Surfaces Roadway surface types contribute to different noise levels. Of the common types of surfaces in modern cities, there is a 4 dB difference between the loudest and the softest: chip seal type and grooved roads being the loudest, and concrete surfaces without spacers being the quietest, and asphaltic surfaces being about average. Rubberized asphalt (which uses recycled old tires) is much quieter and is already widely used. Experimental Porous Elastic Road Surfaces (PERS) might cut road noise in half. PERS is made by adding ground up tires to asphalt paving material. Studies have shown that cutting longitudinal grooves in the pavement reduces noise. Tires Tire types can cause 10 dB(A) variations in noise, based on a 2001 sample of 100 commercially available tires. As of 2001, there was no correlation between grip and noise. Quieter tires may have slightly lower rolling resistance. Tire labeling for noise, grip, and rolling resistance has been widely introduced in Europe, with noisy tires being taxed. Geometry Roadway geometrics and surrounding terrain are interrelated, since the propagation of sound is sensitive to the overall geometry and must consider diffraction (bending of sound waves around obstacles), reflection, ground wave attenuation, spreading loss and refraction. A simple discussion indicates that sound will be diminished when the path of sound is blocked by terrain, or will be enhanced if the roadway is elevated so as to broadcast; however, the complexities of variable interaction are so great, that there are many exceptions to this simple argument. Wind Micrometeorology is significant in that sound waves can be refracted by wind gradients or thermoclines, effectively dismissing the effect of some noise barriers or terrain intervention. Obstacles Geometry of area structures is an important input, since the presence of buildings or walls can block sound under certain circumstances, but reflective properties can augment sound energy at other locations. Computer models At a macro level, ongoing research is required for national and worldwide responses to road noise pollution - issues include road surface choices, the regulation and taxing of noisy designs, and the ongoing inspecting of individual vehicles. At the micro level of managing particular roads, because of the complexity of the variables discussed above, it is necessary to create a computer model that can analyze sound levels