Kapitel 6.2.1.5

Planning indications

6.2.1.5 Driving speed and traffic calming

Figure 3/5 in section 3.1.2.2 shows that a lower driving speed in road traffic results in a lower noise pollution. A reduction of the driving speed (fig. 6/6) from 44 mph to 25 mph (70 km/h to 40 km/h) or from 31 mph to 19 mph (50 km/h to 30 km/h) reduces the noise level by 3 dB. This roughly corresponds to the halving of the traffic volume and the sound energy. A reduction of the driving speed from 81 mph to 62 mph (130 km/h to 100 km/h) only leads to a sound level reduction of about 1.5 dB with a truck percentage of 20 %.

What also plays a major role in this context is engine speed. The higher the engine speed, the louder the driving sounds, especially in a lower gear. Driving quietly means driving at lowest possible engine speed. Figure 6/7 illustrates the dependency of the driving sounds on the gear selection.

About 70 dB(A) are produced in second gear at 25 mph (40 km/h), only about 63 dB(A) in fourth gear. Figure 6/7 refers to a single passing car, while the noise pollution level at a road is the sum of many passing cars and is therefore represented by the average sound level. The average sound level, however, is reduced only half as much with increasing distance from the road as the maximum sound level from a single passing car given in Fig. 6/7.

A slow and uniform traffic flow without disturbing braking or accelerating sounds at high engine speeds can be realized through appropriate road design. This can be applied to residential roads (no confusing obstacle arrangements) and to major roads and bypasses, which should not be oversized. The driving speed within towns or cities can be reduced by narrow roads. Uniform driving speed can also be realized through phased traffic lights.

Figure 6/8 compares the noise reduction through a speed limit with that of a noise protection wall. You can see that the effect on the third floor of a house at 40 m from the road is the same in the case of a reduction of the speed as shown in the figure and of a 4.5 m noise protection wall. Another consequence of a speed reduction is a significant drop in the exhaust gas emissions (BAUMÜLLER and REUTER, 1990; CLIMATE BOOKLET FOR URBAN DEVELOPMENT, 1998,) (available in English and German).

The road traffic authorities in accordance with the municipality can stipulate traffic restrictions within built-up areas on the basis of § 45 of the Road Traffic Regulation for the protection of the population from noise and exhaust gases or for supporting structured urban development measures. This means especially the establishment of 20 mph speed limit zones in residential areas with heavy pedestrian and bicycle traffic. These regulations do not apply to supra-local traffic roads and priority roads.

No crossings controlled by traffic lights and no junctions must be installed within a 20 mph speed limit zone, except for pedestrian lights, and the general rule of traffic is "right before left".

In addition to that zone speed limits of less than 19 mph (30 km/h) can be ruled in central urban areas (e.g. traffic-calmed shopping areas).

Surveys on a 20 mph speed limit (carried out by the Federal Ministry of Transport, Building and Urban Affairs in 1989) show a big driver acceptance, less so in the case of speed limits on motorways (as shown in figure 6/9).

The effect of a 20 mph speed limit in residential quarters is more and more judged in a positive way, the same applies to housing satisfaction. Most critical statements on a 20 mph speed limit refer to building measures. This is why there are high requirements for planning.

Structural measures like the rebuilding of roads are necessary especially in the case of isolated 20 mph speed limit zones. They are not required as for areal 20 mph speed limits. Rebuilding measures can become necessary at selected places like at junctions with 20 mph speed limit zones or at places with a high potential risk of accidents near schools, children's nurseries or stops of local public transport. The local situation has to be taken into account, however, in the process of restructuring circulation areas.

Bicycle traffic will also increase, especially in traffic-calmed zones, as 50 % of all car drives take place within a distance of 5 km. Many examples of the management of bicycle traffic are given in the booklets "Empfehlungen für Radverkehrsanlagen" ("Recommendations for handling bicycle traffic" published by the neighbourhood association Stuttgart, 1991) and "Mit dem Fahrrad in der Stadt" ("Cycling in the city" published by the Ministry of the Interior of the Land of Baden-Württemberg, 1989).

Many towns and cities have decided on the designation of 20 mph speed limit zones on the whole territory. Speeds of more than 20 mph will then only be permitted on a road network designed for supra-local and transit traffic. 240 of these zones are designated in Stuttgart. This means that a 20 mph speed limit is realized on about 65 % (1,000 km) of Stuttgart's roads.

The inner development of residential areas mentioned above is also important for planning in this context. A detailed development without through roads obligatorily leads to slower driving (see section 6.2.1.4, figure 6/3).

Speed reductions in rail traffic are also an effective noise abatement measure. According to Schall 03 (see section 3.1.2.3), a halving of the speed of trains corresponds to a noise level reduction of about 6 dB(A). But there are practically no realization possibilities for city planning in this field. We must not forget, however, that trains are an environment-friendly means of transport and depend on adequate travel speeds in order to fulfil their mission.

	Fig. 6/6: Speed limit (B 10 Stuttgart- Esslingen)


	Fig. 6/7: Maximum sound level of a passing car in a distance of 7.5 m depending on the driving speed and gear selection, source: General German Automobile Association (ADAC)


	Fig. 6/8: Sound level variations through speed limits or noise barriers (motorway, distance from the development = 40 m, M = 2,400 Fz/h, 15 & trucks), source: Krell


	Fig. 6/9: Drivers' opinions on speed limits, source: EMNID


	Fig. 6/10: Speed limit?

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