PLANNING INDICATIONS
7.1.3 Road and railtrack surfaces

The sound level is strongly influenced by the type of road surface. Cobbled pavement is by up to 6 dB(A) louder than most other used asphalt surfaces (table 7/1). Consequently low-noise road surfaces can reduce the sound level by up to 5 dB(A). Cobbled pavement is yet acceptable in old town centres or traffic-calmed zones ("play streets"), where it is often used for design purposes as the vehicles drive slowly there but it should not be used on major roads as a means of slowing traffic down. Other aspects should also be considered in this context, like the surface’s practicability for pushchairs and wheelchairs. Cobbled pavement must generally be well laid as loose stones cause more noise.

Figure 7/3 depicts this circumstance deduced from measurements separated into cars and trucks.

New, open-pored asphalt surfaces (often called "whispering asphalt") achieve significant sound level reductions of up to 5 dB(A) (sometimes even more). But the effect is only noticeable as of a speed of 60 km/h (about 37 mph). Negative aspects are a low durability (of about six to eight years) and the gradual clogging of the pores through tire particles and other impurities, which slowly reduces the noise reduction effect. High propulsive and shearing forces (e.g. when a truck turns or brakes at a junction or a roundabout) quickly damage the surface. Since the upper asphalt surfaces are water-permeable, the road drainage system is very costly. As this surface can only be applied on a large scale, its use is restricted to streets where local diggings are excluded (because of the drainage infrastructure below the street). This is why the installation of open-pored asphalt surfaces is usually not possible within developed areas.

Within towns or cities, noise-reducing road surfaces have only been tested for some years, but this very successfully (e.g. in Düsseldorf). Sound level reductions of 2-4 dB(A) can be reached with noise-optimized asphalt surfaces (LOA 5 D) or low-noise stone mastic asphalt surfaces (SMA LA) as compared with conventional mastic asphalt.

Noise-reducing road surfaces (especially open-pored asphalt) are more expensive than traditional asphalt or concrete surfaces. But the extra costs must be seen in the context of the possible savings from otherwise required noise abatement measures, like the construction of a noise protection wall. Furthermore, low-noise road surfaces are a structural measure against road traffic noise, which is one of the most consistent with urban development.

The condition of the road surface also plays an important role in the development of noise. In addition to an increased production of noise (through damaged sewer covers for example), heavy vehicles may cause vibrations (the so-called "secondary sound") and therefore substantial disturbances in the adjacent residential areas, especially when driving on roads with an uneven surface.

These disturbances are often intensified through sound level peaks caused by the throbbing sound of vehicle components or their cargo for example. The additions or deductions given in table 7/1 refer to intact road surfaces.

Table 7/1: Sound level additions D_StrO in dB for different road surfaces depending on the driving speed pursuant to RLS-90

As for rail traffic, the traditional track structure consisting of a ballast bed and wooden or concrete sleepers is acoustically more advantageous than tracks embedded in the road. These produce a sound level addition of 5 dB. According to Schall 03, grass-covered tracks for trams reduce the noise level by 2 dB. Particularly in the context of rail vehicles, the above-mentioned vibrations can have a significant negative influence if they pass closely by or under residential development.

Further possible noise reduction measures for the wheel/rail system are:

• a regular grinding of the rail surface to remove ripples
• rail web damping, especially in the case of tram tracks, by integrating the tracks into an elastic material or by covering them with grass