PLANNING INDICATIONS
7.1.5 Driving speed and traffic calming

Noise emissions from road traffic are influenced substantially by the factors driving speed and traffic flow (see section 3.1.2.4).

Reducing the driving speed from 50 km/h (about 31 mph) to 30 km/h (about 19 mph) means a reduction of the average sound level of about 2.5 dB(A). A reduction from 80 km/h (about 50 mph) to 60 km/h (about 37 mph) leads to a noise reduction of about 2 dB(A), from 130 km/h (about 81 mph) to 100 km/h (about 62 mph) between 1 and 3 dB(A) (depending on the truck percentage).

Besides the reduction of noise, a lower driving speed also means an increase of road safety and driving quality. Another frequent consequence is the reduction of pollutant emissions, but the effects on the air quality must be considered on an individual basis (Climate Booklet for Urban Development, 2012).

The traffic flow and a person's driving habits have an enormous influence on noise emissions. At an average speed of 45 km/h (about 28 mph), a car's noise emission level is by about 4 dB(A) higher when it repeatedly brakes and accelerates than at a constant flow. At an average speed of 30 km/h (about 19 mph), the difference even amounts to about 5 dB(A). A speed reduction from 50 km/h (31 mph) to 30 km/h (about 19 mph) can reduce the maximum level by 7 dB(A) if accelerations are avoided (ALD, 2010).

Lower speeds also help to achieve the following aims:

• Reducing the attractiveness for through traffic, which can so be shifted to other, less sensitive roads.
• Promoting bike traffic, which is then less exposed to dangers from fast-driving cars (and separate bike lanes are not required in 20 mph speed limit zones).
• Improving the driving quality and reducing the divisive effect (which is also important for commercial centres).

The positive effects are opposed to the following disadvantages:

• Lower speeds on major roads could neutralize the time saving as compared with shorter roads leading through 20 mph speed limit zones. This means that drivers could possibly revert to residential areas in these cases. In order to prevent traffic on alternative routes, measures must be taken at crucial hubs, like traffic priority rules at major roads or appropriate traffic light sequences.
• Lower speeds slow down public buses as well, which makes the public transport system less attractive. If public transport shall compete with motorized individual traffic, however, it must be able to offer fast connections. A longer travel time of buses could also mean that an additional bus must be used in order to maintain the shuttle frequency (e.g. every 15 minutes), which would lead to increased costs for the operator. To avoid these undesirable effects, it could be useful to limit the speed reduction to a short section of a major road where public buses shuttle.

If the resident population shall be protected from a high noise exposure at least during the night and be granted nighttime peace, a speed limit restricted to the nighttime hours could be an adequate measure if speed limits during the day are not possible.

A slow and uniform traffic flow without disturbing braking or accelerating sounds at high engine speeds can be realized through an appropriate road design (see also section 7.1.4). This can be applied to residential roads (no confusing obstacle arrangements) and to major roads as well, which should not be oversized. The driving speed within towns or cities can be reduced through narrow roads. If the cross-section of a road is reduced, the additional space can be used for bicycle or parking lanes, green strips or constructional design elements. A uniform driving speed can also be realized through phased traffic lights or the installation of a roundabout at an existing crossing.

A reduction of the road cross-section could also lead to an increased distance between road and buildings, which in turn reduces the sound level and the subjective perception of noise as well. If the sight of the passing vehicles is prevented by vegetation for example, residents often gain the subjective impression of a noise reduction even if the actual noise impact remains unchanged.