Silent Roads for Cost Effective Noise Reduction

By Wim van Keulen
March 2004

The Author is senior consultant at Vankeulen Advies in Vlijmen, the Netherlands. His work includes engineering of low noise road systems for various conditions of traffic load, speed; testing of the acoustical quality of road surfaces; engineering of noise barriers and facade isolation; noise mapping, studies on noise exposure and acoustical quality of cities and countries; technical aspects of noise legislation; effectiveness of existing and new noise policies. Furthermore, he is specialised in the field of psychoacoustics. He holds a MSc title in Applied Physics.

Noise pollution due to traffic is a major problem. The most effective way to reduce traffic noise is application of silent road types. Especially in countries with less experience with silent roads they also have to be durable in order to limit expensive maintenance. In many cases silent roads are very cost efficient and reduce the application of costly noise barriers. However, application of silent roads needs proper legislation for measurements en monitoring.

Keywords: Noise reduction, environment, silent roads, costs

1. Noise polution

Environmental noise has become an important issue in western European countries. From numerous questionnaires it was concluded that road traffic noise is the major stress factor of inhabitants in cities. About 60 % of the urban European population is exposed to traffic noise levels over 55 dB(A), a level that is considered to seriously affect ones well being. About 25% is exposed to levels over 65 dB(A), a level where health effects start to occur.
Not only does traffic noise seriously jeopardise the quality of urban live, also the non-urban areas are more and more covered with a blanket of noise. A background level of road traffic noise exceeding 40 dB(A) affects nature quality. Noise levels exceeding 47 dB(A) can disturb nature life, especially the breeding of birds.
Silence is becoming a rare species. Therefore, extended national and international legislation has been implemented to limit the noise immission levels from road systems.
The last 25 years in the EU the police to reduce traffic noise concentrated on noise reduction of vehicles. However, in practice traffic noise did not diminish. This very disappointing fact follows from ignoring the contribution of rolling noise. The rolling noise of tires did not drop during the last 50 years. In almost all situations rolling noise is the dominant source (see figure 1).

Figure 1

Figure 1 - respective contributions of engine noise and rolling noise for light vehicles (continuous line) and heavy vehicles (dashed line) on a dense surface.

2. Some Initial Measures

In order to lower the annoyance caused by traffic noise a number of simple measures can be taken. Some of them are:

  1. checking and fining motorists with illegal exhausts;
  2. stimulating free flowing traffic by applying controlled traffic lights.

As example with respect to the first point, in Athens, Greece joint Ministry of the Environment and Athens Police Force started motorcycle noise spot measurement controls in April 1996. Apart from the fines the drivers had to prove that they had dealt with the noise emission problem of their vehicle in a re-examination process usually after a 2-week period (see Figure 2).

Figure 2

Figure 2 - temporal variation of the Athens Noise Control Roadside Check results (percentage of motorcycles with non-permissible noise emissions in the total sample)

The trend shows that initially (for a 9 month period) over 50% of the noise emissions were found to exceed the permissible limits. After another 9 month period the percentage had dropped to a quite constant 9%. From calculations based on the above assumption it is estimated that a reduction of the number of illegal noise emissions by only 40% will lead to a overall noise reduction of the adjacent buildings of about 5 dB(A). The peak noise levels will of course reduce even more (>10 dB(A)).
With respect to the second point, it is known that traffic which is slowing down and accelerating causes higher noise levels than free-flowing traffic because of more engine noise. This is especially the case near crossings that are controlled by traffic lights. From indicative measurements on trucks it follows that this effect can be 5 dB(A) or more in the vicinity of traffic lights. In order to minimise the number of vehicles that accelerate the traffic lights should be in phase. With this the traffic will flow more freely causing significantly less noise. Additionally, other (gas) emissions will drop and considerably fuel savings can be achieved.

3. Silent Roads

The severity of traffic noise requires the large-scale introduction of low noise technology. Of the several technologies available to road authorities, the application of low noise surfaces is not only the most cost-effective but also can be implemented on short notice. These advantages have led to the development and application of several low noise surfaces. Silent road surfaces are considered to be among the most effective means of reducing traffic noise and, as a result of continuous study and optimisation, large improvements in both the total reduction and the application range of the reducing effect are to be expected in the next years. Noise reductions presented in this paper are relative to Dutch dense asphalt in good condition. This type of asphalt is about 2 to 3 dB(A) more silent than comparable references in other countries.

Porous Asphalt

The noise reduction of porous asphalt (PA) is based on acoustical absorption. Rolling noise as well as engine noise is absorbed. The PA 6/16 (in the Netherlands the standard road type on motorways) has besides absorption a very coarse texture. At high speeds the noise reduction is about 4 dB(A), however, at lows speeds there is no significant noise reduction. For low speed situations the double layered 4/8-6/16 variant seems to be more suitable. With this road surface noise reduction of more than 4 dB(A) at 50 km/h can be achieved. However, PA needs intensive maintenance. Without proper maintenance the acoustical effect drops rapidly. Furthermore, the durability of PA is low especially under urban circumstances. These facts make the suitability of PA limited in countries with less experience with PA.

Stone Mastic Asphalt (SMA)

The limited suitability of PA has lead to the development of SMA. The noise reduction is based on an optimal texture of the surface and is at maximum 2 dB(A) at 50 km/h. The optimal stone size for noise reduction is 0/6. Finer or coarser stone sizes lead to less noise reduction.

Optimised Cement Concrete

Exposing the aggregate by brushing or washing out the fresh surface can silence the surface of newly put cement concrete surfaces. From extensive research in the Netherlands [1] it followed that a high speed situations (motorways) a noise reduction of 2 dB(A) can be achieved. A great advantage of these road surfaces is its very large durability. Furthermore, these road types hardly need maintenance.


A rather new development is so-called microlayers. These thin surfaces (thickness 25 mm) combine the positive effects of PA and SMA. The durability is comparable to that of SMA and the noise reduction compares to that of 2-layered PA. These types of road surfaces need only standard maintenance. For countries with less experience with silent roads microlayers seem to be optimal. Noise reductions can be expected from 2 to 4 dB(A) at low speeds and up to 7 dB(A) at high speeds.

Silent block pavements

Traditionally block pavements are very noisy. Therefore, mainly in the Netherlands, silent block pavements have been developed. Relative to the dense asphalt noise reductions of 1 dB(A) can be expected. This means that relative to classic block pavements noise reductions up to 10 dB(A) can be expected.

Third generation pavements

For the Dutch ministry of Roads and Waterworks a new type of road has been developed by VANKEULEN advies [2]. It was specially designed to reduce noise from trucks. The optimal result was obtained by applying various layer thicknesses. From measurements followed a noise reduction of 8 dB(A).
Besides that noise reduction is optimal in the initial phase also the reduction has to be durable in time. The washing out of pollution to the side of the road has been improved. De grading has been chosen such that less clogging will occur and absorption will remain longer intact. Also cleaning is less needed.


In general, silent roads are more expensive than standard road surfaces like dense asphalt. Costs are determined by construction costs, maintenance en durability. Also it holds that the higher the noise reduction of a road surface the higher the costs. However, a silent road surface not only carries traffic, it is also a noise reduction measure like noise barriers. Application of silent roads will lead to lower noise barriers or no barriers at all. This leads to a significant cut in overall expenses. In many cases are silent roads the most cost efficient solutions. The costs are less than half compared to the classic solution with high barriers.

Silent roads are not luxury products but they contribute significantly to the welfare of everybody.


  1. "The Acoustical Properties Of Optimised Exposed Aggregate Concrete In The Netherlands", W. van Keulen and A. van Leest, 9th Symposium on Concrete Pavements, 4-7 April 2004, Istanbul, Turkey;
  2. "SilentTransport - a road with various sounds!", J.H. Dijkink and W. van Keulen, Internoise 2004, Prague.


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