RLS-90, the guidelines for noise protection at roads (Richtlinien für den Lärmschutz an Straßen), was introduced on the national level by the General Circular Road Construction No. 8/1990 (Allgemeines Rundschreiben Straßenbau) by the Federal Transport Minister in accordance with the highest road construction authorities of the Länder, in Baden-Württemberg by a decree of the Transport Ministry from 11 April 1991.

The guidelines deal with noise protection measures and methods of calculation for a quantitative representation of the noise pollution. The results can also be seen in diagrams.

The purpose of these guidelines is to establish a uniform procedure. They shall help the user to consider and weigh noise protection issues in the context of road planning, to prove the necessity of noise protection measures, to develop economic and efficient solutions for noise abatement and to gear and optimize noise protection measures. The Traffic Noise Ordinance (16th Federal Immission Control Ordinance) refers to Chapter 4 of RLS-90 (calculation of the rating level). This chapter is considered as legal provision in the context of noise precaution.

The rating level at roads is determined in the guidelines of RLS-90 on the basis of traffic volume, percentage of trucks, speed limit, type of road surface and gradient. Gradients of more than 5 % get a noise addition.

The sound source is assumed at an altitude of 0.5 m above the road. Generally the noise level has to be calculated and not measured as measurements represent snapshots and lead to random results depending on the current traffic volume.

Figure 3/4 shows the dependency of the average sound level on the traffic volume and the truck percentage in a distance of 25 m from the road for a speed limit of 62 mph (100 km/h) during undisturbed sound propagation. The average distance between basic line and connection line of sound source and place of immission is 2.5 m.

Figure 3/5 gives the level corrections required for different speed limits depending on the percentage of trucks.

What is also taken into account is the distance between the place of immission and the sound source, the weakening of sound through the soil and meteorological conditions as well as reflections and barriers.

In order to determine unwanted sound reflections from noise protection walls and other reflecting surfaces, a sound level absorption of the reflected sound of only 1 dB(A) is assumed for plain building facades and of 8 dB(A) for highly absorbing noise protection walls. 

Figure 3/6 illustrates the dependency of the noise level (sound level variations) on the distance between place of immission and sound source.

Up to 3 dB(A) are added at junctions and crossings with traffic lights due to the additional noises from starting and braking.

Besides methods of calculation, RLS-90 contains extensive diagrams for the shielding effect of walls and barriers. Figure 3/7 gives an example of the barrier attenuation (in dB(A)) of a noise protection wall at a road depending on the altitude of the place of immission above the road and on the height of the wall. 

Pursuant to the guidelines, the calculations are separated into daytime (6 am to 10 pm) and nighttime (10 pm to 6 am). The sound levels apply to light wind coming from the road and they consider temperature inversions, both leading to an increase of the sound levels.

Significantly lower sound levels can occur during other conditions. This complicates a comparison between calculated and measured levels.

In case no project-related traffic figures are available, the guidelines provide information on the traffic volume for different types of roads.

Just like DIN 18005, RLS-90 contains methods of calculation for parking area noise depending on the area, the number of parking spaces and the amount of movements per parking space and hour.