SILENCE E-LEARNING FOR ENGINEERS - TRAFFIC MANAGEMENT
Traffic management: Basic traffic noise relations
Most of the traffic flow measures presented here have not been subject to surveys aimed at assessing their effects on noise emissions. Therefore, the possible impacts of these measures are based on the effects of traffic volume, traffic composition, speed and driving pattern on noise emissions. This chapter gives an overview of these effects.
The traffic-noise relations can also be used to assess the effects of various traffic management measures for which the impact on traffic flow and therefore on noise is so dependent on local conditions that it cannot be generally presented here. One example of this, could be improvements in public transportation or conditions for cyclists, which may lead to a shift in people’s choice of transport modes and thereby to a decrease in car traffic.
Changing the traffic volume affects the noise levels. Given that the traffic composition, speed and driving patterns are unchanged, the logarithmic nature of the dB scale means that a 50% reduction of the traffic volume results in a 3 dB reduction in noise levels, regardless of the absolute number of vehicles.
A reduction in the traffic volumes on a road will often lead to increases in speed because the remaining vehicles can drive more freely, unless measures are taken to keep the speed down.
Increased speed will work against the reductions in noise caused by the reduced traffic level. If traffic flows more freely, this is also a change in driving pattern. Decreases in the number of accelerations and decelerations are likely to result in lower noise levels. However, more room for driving may also lead to harder accelerations, which will increase the noise emissions.
The reduction of traffic volumes is a measure which is mainly applicable on minor roads or for certain (smaller) areas, where a variety of measures may be used to move the traffic onto major roads.
On major roads, it is hardly ever realistic to reduce traffic to an extent that it will significantly reduce noise levels. Some reduction may be achieved through long-term town and traffic planning which aims at moving people from cars to other modes of transport.
The composition of the traffic in terms of vehicle categories is important for the noise levels. The figure below shows German results of noise emission as LAmax from various categories of road vehicles in free flowing traffic. There are clear differences in noise levels depending on the size of the vehicles. At 60 km/h for instance, the LAmax level from a truck with more than three axles is 83 dB, from a truck with up to three axles it is 80 dB, for a public transport bus it is 79 dB, for vans it is 75 dB, for motorcycles 74 dB and for passenger cars it is 73 dB. This means that a public transport bus at 60 km/h makes as much noise as 4 passenger cars, a truck with up to three axles as much as 5 cars and a truck with more than three axles as much as 10 passenger cars.
Source: Steven 2005 (LDV is “light duty vehicle” (a van), HDV is “heavy duty vehicle” (a truck))
In reality, the effect of heavy vehicles is usually not as important as these figures suggest. On most urban roads, heavy vehicles only account for a small percentage of the total traffic. In combination with the often higher speed of the light vehicles, the effect is that the light vehicles usually dominate the noise emissions. On most high-speed roads, especially on motorways, the speed of the light vehicles is considerably higher, and they therefore also dominate the noise emission in these situations even though the percentages of heavy vehicles often are fairly high. Only in cases with very high percentages of heavy vehicles and/or small or no differences in the speed of light and heavy vehicles, the heavy vehicles will dominate the LAeq levels of a road.
Although heavy vehicles usually do not dominate the noise emissions given as LAeq, they represent peaks in the emitted noise, which may annoy and disturb those living, working or walking along the road. At night, the peak levels caused by heavy vehicles represent noise events that may wake up or cause alterations in sleep depth to people living along the road.
A number of traffic flow measures may lead to changes in the traffic composition.
For example: traffic calming schemes may cause heavy vehicles to choose other routes; night-time bans on heavy vehicles – and perhaps on two-wheelers – will reduce the number at night but may lead to increased numbers during the day; and city logistics may reduce the overall number of trucks entering central city areas as well as the distance travelled by trucks within the areas. The effect of these types of initiatives will have to be assessed individually for each location in which they are implemented, as this depends on local road and traffic conditions.
The propulsion noise (Lprop) increases with increasing engine revolutions. That is, it increases with increasing speed within the same gear, but drops when the driver shifts to a higher gear. There is, however, also an overall tendency towards increasing noise levels at higher gears, and thereby at higher speed.
Source: Steven 2005
The dotted lines are the noise from the accelerating car. The fully drawn lines show the noise level as it would be at the given speed without acceleration. The line labeled ‘v, ave’ shows the vehicle speed.
The effect of changes in speed is derived by Andersen (2003) from measurements of more than 4,000 vehicles carried out in 1999 and 2000. The effect of speed changes (driving with constant speed) is given in 10 km/h intervals in the table.
The effect of the driving pattern on noise is important to take into account when evaluating the noise effect of various traffic management measures. Installing road humps, changing the layout of road sections and intersections, setting up signs of speed reduction, etc. are all initiatives which may change the way people drive. This may cause an increase or decrease in the number and intensity of accelerations and decelerations.
There is a correlation between acceleration and noise for passenger cars. For low speeds around 30 km/h, the average noise increase due to acceleration is 2 dB. For normal urban speeds around 50 to 60 km/h, the increase due to acceleration is 1 to 1.5 dB. At high speeds, the increase is marginal. In each individual case the noise increase depends on the level of acceleration.
Source: Steven 2005 (The measurements have been done on actual traffic.)
Similar relations are drawn by Steven (2005) for light goods vehicles (LGV), heavy goods vehicles (HGV) with power ratings below 75 kW, between 75 and 150 kW, between 150 and 250 kW and higher than 250 kW. The same tendency to an increase in noise during acceleration is seen for all these vehicle categories. The differences at 30 and 50 km/h are shown in the table.
The effect of individual traffic management measures on noise levels is often small, and the main effect is usually due to changes in speed.
However, even when noise is reduced due to reduced speed, annoyance to those living along the road may increase because the measures also result in uneven driving patterns with accelerations and decelerations.
These do not necessarily change the LAeq level very much, but where the noise without the measures may have been a steady sound, the accelerations and decelerations cause the sound pattern to shift, thus making it more noticeable. This is important to consider when implementing traffic management measures.