Reducing speed, changing road surfaces... What solutions are there to combat road noise pollution?

Acoustics researchers have compared different options for reducing noise pollution caused by road traffic. Depending on the situation, it may be more beneficial to opt for quieter electric engines, change the road surface or optimise traffic speed. However, in most cases, there is no single optimal solution.

According to the World Health Organization (WHO), noise is the second highest environmental risk factor in Europe in terms of morbidity after air pollution. It is a major public health issue, with overall socioeconomic impacts estimated at 147 billion euros per year in 2021, repercussions which are too often overlooked.

Transportation noise, particularly road traffic noise, is the main cause of this nuisance in France. However, various solutions can be implemented to limit its impact. Modifying sound propagation by placing a large obstacle between the source and residents (buildings, noise barriers, natural mounds, etc.) can be a very effective way of reducing noise exposure, but this is not always technically or financially feasible 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 its effectiveness when doors or windows are open. The best option therefore remains, above all, to reduce noise emissions at source.

With this in mind, measures are regularly implemented, either within a regulatory framework (impact studies for new infrastructure, operations to reduce ‘noise black spots’, etc.), or on a voluntary basis by local authorities or road infrastructure managers to combat noise pollution.

Recent controversies over how to reduce road noise may nevertheless confuse the message to the public and thus undermine the credibility of these solutions, whose role remains above all to combat this environmental problem. What exactly is their technical effectiveness and relevance?

Why is road traffic noisy?

Before exploring the various solutions for reducing road noise at source, it is important to understand the causes of this noise and the parameters that can be acted upon to reduce it. Behavioural noise, linked to unconventional or inappropriate driving, for which measures are currently being assessed in several cities, will not be considered here.

The sound emitted by a road vehicle comes from two main sources:

• propulsion noise, caused by the mechanical system (engine, transmission, exhaust),

• and rolling noise, caused by contact between the moving tyres and the road surface.

Under normal conditions, propulsion noise predominates below 30 to 40 km/h for light vehicles (LVs) – or 40 to 50 km/h for heavy vehicles (HVs) – while rolling noise predominates above this speed and increases rapidly with speed.

For current vehicles in standard operation, other potential sources of noise (airflow over the vehicle, etc.) are negligible compared to propulsion noise and rolling noise.

It is generally accepted that human hearing only perceives a change, in a constant noise, from a minimum difference of 2 décibels A (dBA). Environmental noise control experts therefore consider any change below this threshold to be ‘insignificant’ because they are not sufficiently perceptible. As a first approximation, they use this threshold as a criterion to judge the potential effectiveness of a noise reduction solution.

Taking action on the vehicle’s engine

Taking action on a vehicle’s engine helps reduce noise emissions at low speeds. Electric engines are generally quieter than combustion engines, although the difference is becoming less pronounced in new combustion vehicles, which are quieter than in the past.

Regulations also require electric vehicles to add a synthetic sound below 20 km/h to improve pedestrian safety (AVAS). An automatic gearbox also helps reduce noise emissions, as it ensures that the combustion engine RPM is always adapted to the speed of the vehicle, thus avoiding excessive noise due to over-revving.

Outside the access restrictions imposed on certain city centres, however, taking action on vehicle engines is not generally within the remit of a road infrastructure manager. In addition, a significant proportion of the vehicle fleet would need to be replaced in order to have a significant impact on noise levels, which would only be effective in the long term.

Optimising traffic flow

For short-term effects, a second option is to optimise traffic flow by modifying vehicle throughput or speed. The noise reduction expected from reduced traffic flow follows a logarithmic law, whereby halving the total number of vehicles results in a reduction of 3 dBA. Reducing speed also reduces noise emissions by reducing rolling noise.

In this case, above 40 km/h, the expected gains are in the order of 1 to 1.5 dBA per 10 km/h reduction. However, as noise emissions from light vehicles and heavy goods vehicles are not equivalent, the overall noise emission from road traffic will depend on the proportion of these vehicles on the road and the road surface. It is therefore difficult to give a summary estimate of the potential gains in all situations.

The Motor online application developed by our team allows anyone to test traffic scenarios by modifying various noise-influencing parameters in order to assess their potential effectiveness in reducing noise emissions from road traffic.

Modifying the road surface

A third solution is to modify the type of road surface in order to reduce rolling noise.

The acoustic performance of a wide range of road surfaces can be compared, for example using our online application BDECHO, which based on the national database of acoustic performance data for French road surfaces.

This shows that the quietest surfaces are those with a fine-graded surface layer (mix of small aggregates) and some porosity. Conversely, the noisiest surfaces have a higher volume size and are not porous.

While each solution can contribute to reducing road traffic noise, each also has its drawbacks.

What are the advantages of these solutions?

Replacing a noisy surface with a quieter one can reduce noise by between 2 dBA and 10 dBA at speeds of 25 km/h for light vehicles and 40 km/h for heavy goods vehicles.

This solution is expensive and requires major work and modification on the road. The acoustic performance of road surfaces also has the disadvantage of changing over time, with faster performance degradation for the quietest surfaces. Significant performance variation (in the order of several dBA) is also observed within each surface category, leading to greater uncertainty in predicting expected performance.

Speed reduction, on the other hand, is not always possible, and its effectiveness may be limited, for example if the speeds on the existing infrastructure are already moderate or if speed reduction cannot be applied to heavy goods vehicles, which are noisier and already account for a significant proportion of traffic. The expected gains may also be lower than hoped for if the speeds driven are lower than the regulatory speeds for the infrastructure, for example in situations of chronic slowdowns or congestion.

This was observed, for example, during recent noise assessments on the Paris ring road, where the theoretical gains were only achieved at night when traffic was flowing freely. It should be noted, however, that this type of solution can bring significant co-benefits in terms of air pollution and road accidents without necessarily significantly affecting perceived journey times.

Combining different solutions

Reducing speeds, low-noise road surfaces, traffic restrictions, changes in engine types... what is ultimately the best way to reduce road noise pollution?

Reducing overall traffic will always lead to a drop in noise emissions, which will be audible provided that traffic is reduced by at least 30 to 40% if this solution is applied on its own.

For other options, in low-speed zones (below 40 km/h), changing the engine type is the most effective solution. This could involve encouraging the adoption of electric vehicles or other quieter engines, as methods that reduce rolling noise are less effective in this case.

On roads where the average speed of vehicles is higher, low-noise road surfaces or speed limits can bring significant reductions. However, their suitability must be assessed on a case-by-case basis in advance, depending on the initial characteristics of the infrastructure in question (speed, traffic patterns, existing road surface, etc.).

As the three solutions are compatible with each other, the ideal solution is, of course, to combine them in order to benefit from the advantages of each.