According to the World Health Organization (WHO), noise is the second environmental risk factor in Europe in terms of morbidity, behind air pollution. It represents a major problem for public health, including global socio-economic repercussions, estimated at 147 billion euros per year, in 2021, are too often overlooked.
The noise of transport, in particular road noise, is in France the main cause of these nuisances. Different solutions can however be implemented to limit its impact. Changing acoustic spread by interposing a large obstacle between the source and residents (buildings, acoustic screen, natural mounds, etc.) can be a very effective way to reduce sound exposure, but is not always technically or financially possible and does not cover all possible situations.
Improving the acoustic insulation of buildings is also a very good technical solution, but it has the disadvantage of not protecting people outside and losing in efficiency as soon as doors or windows are open. The best option is therefore, above all, to reduce the sound emission to the source.
In this perspective, actions are regularly implemented, either in a regulatory framework (impact study of new infrastructure, “noirs of noise” resorption operations, etc.), or voluntarily by communities or road infrastructure managers to combat noise pollution.
Recent controversies on how to reduce road noise can nevertheless blur the message with the citizen and thus harm the credibility of these solutions, the role of which remains above all to fight this environmental scourge. What exactly is their technical efficiency and relevance?
Why is road traffic noisy?
Before exploring the different solutions allowing the reduction of road noise to the source, it is important to understand the causes of this noise and the parameters on which we can act to reduce it. The noise of behavior, linked to an unconventional or unsuitable driving mode, against which control measures are being evaluated in several cities, will not be considered here.
The sound issued by a road vehicle comes from two main sources:
- propulsion noise, due to the mechanical system (engine, transmission, exhaust),
- And the rolling noise, due to the contact between moving tires and the roadway.
Under normal conditions, propulsion noise predominates below 30 to 40 km/h for light vehicles (VL)-or 40 to 50 km/h for heavy goods vehicles (PL)-while bearing noise prevails beyond and increases rapidly with speed.
For current vehicles in standard operation, other potential noise sources (air flow on the vehicle, etc.) are negligible compared to propulsion noise and rolling noise.
For a constant noise, it is commonly accepted that human hearing only perceives a variation from a difference of 2 decibels A (DBA) minimum. The environmental acoustician also considers that a change below this threshold is “not significant”, because insufficiently perceptible. He uses, at first glance, this threshold as a benchmark to judge the potential efficiency of a noise reduction solution.
Act on motorization
Acting on motorization reduces sound emissions at low speed. Electric engines are generally less noisy than thermal engines, with a difference which tends to decrease for current new thermal vehicles, less sound than in the past.
The regulations also require electric vehicles to add synthetic sound below 20 km/h to improve pedestrian safety (AVAS device). An automatic gearbox also helps to reduce the sound emission, as it ensures a heat engine speed always adapted to the appearance and thus avoids excess noise due to over -dials.
Outside the access restrictions imposed on certain city cores, acting on vehicle engine is generally not the responsibility of a road infrastructure manager. The change of a large part of the rolling stock is also necessary to produce a significant acoustic impact, which is only effective in the long term.
Optimize traffic flow
For short -term effects, a second option is to optimize traffic flow by modifying vehicle flow or speed. The sound gains expected from a reduction in flow follow a logarithmic law, where a division per 2 of the total number of vehicles leads to a decrease of 3 DBA. The decrease in speed makes it possible to reduce sound emissions by acting on the noise of rolling.
In this case, beyond 40 km/h, the expected earnings are around 1 to 1.5 DBA per reduction range of 10 km/h. However, the sound emissions from light vehicles and heavy goods vehicles not being equivalent, the overall sound emission of road traffic will depend on the proportions of these vehicles which circulate there and the pavement coating. It is therefore difficult to give here a synthetic estimate of potential gains in all situations.
The Motor online application developed by our team allows everyone to test, at first glance, traffic scenarios by modifying the different parameters of noise influence in order to judge their potential efficiency on the reduction of the sound emission of a road road.
Modify the coating
A third solution is to modify the type of pavement coating in order to act on rolling noise.
We can compare the acoustic performance of many roadways, for example using our BDECHO online application, which takes advantage of the national acoustic performance base of French road surfaces.
This thus shows that the least noisy coverings are those whose surface layer is with low granulometry (coated with small aggregates) and which have a certain porosity. Conversely, that of the noisiest has a stronger grain size and is not porous.
If each solution can contribute to the reduction in road noise, each also includes drawbacks.
What advantages for these solutions?
Changing a noisy coating by another less noisy can cause a noise reduction of 2 DBA to 10 DBA to the maximum, from 25 km/h for light vehicles and from 40 km/h for heavy goods vehicles.
Expensive, this solution requires heavy implementation and a substantial intervention on the roads. The acoustic performances of the coatings also have the disadvantage of evolving over time, with a determination of faster performance for the least noisy coatings. A significant dispersion of performance (of the order of several DBA) is also observed within each category of coating, which leads to greater uncertainty about the expected performance forecast.
The speed reduction, on the other hand, is not always possible. Or, its effectiveness can be limited, for example if the speeds of the existing infrastructure are already moderate or the speed reduction cannot be applied to heavy goods vehicles, more noisy, and that the latter are already in significant proportion in traffic. The expected gains can also be less than hoped if the speeds practiced are lower than the regulatory speeds of the infrastructure, for example in chronic situation of slowdowns or congestion.
This is what was for example noted during recent acoustic measures on the Paris device, where the theoretical gains were reached only at night, when the traffic is fluid. Note that this type of solution can, however, provide interesting co-supports in terms of air pollution or road accidents, without necessarily significantly harming the perceived journey time.
Combine the different solutions
Reduction of speeds, not very noisy coatings, traffic restrictions, motorization changes … What is ultimately the best method for dropping the road sound emissions?
Reducing overall traffic will always cause a drop in emissions, audible subject to a drop in traffic by at least 30 to 40 %, if the solution is applied alone.
For other options, in reduced speed zone (less than 40km/h), playing on the engine is the most effective solution. This can go through the incitement to the adoption of electric vehicles or other less noisy engines, the methods acting on the noise of bearing being in this case less effective.
In roads where the average speed of vehicles is higher, the roads are not very noisy or the decrease in speed limits can provide interesting reductions. But their relevance must be studied on a case -by -case basis, depending on the initial properties of the infrastructure considered (speed, traffic structure, existing roadway covering, etc.).
The three solutions being compatible with each other, the ideal of course consists in associating them in order to advantageously combine the gains brought by each of them.
