Integration of low-noise pavements with other noise abatement measures

Abstract

This report is produced as Deliverable 15 - "The integration of low noise pavements with other noise abatement measures" - of the EU research project SILVIA. The objectives are: . To describe noise reduction solutions taking into account the combination of pavement, tyre and vehicle design. . To address noise reduction possibilities by assessing other vehicle noise sources (e.g. reducing power unit noise). . To discuss the acoustical optimisation of local conditions (urban, semi-urban and rural roads, crossings, roundabouts, etc). . To consider traffic management measures for noise control and the effect of these on mobility. . To estimate the noise reduction of low-noise pavements when combined with noise barriers and earth mounds, with façade insulation and when used on bridges. The goal of this document is to examine the parameters of tyres, vehicles and driving conditions in order to demonstrate the potential to further the noise reductions achieved by low-noise road surfaces through a systematic optimisation of the vehicle, tyre and road surface. This report gives an overview of the general effects of various tyre and vehicle parameters, a brief review of the SILVIA database and analyses the influences of various tyre and vehicle parameters on traffic noise. These analyses are based on data present in the SILVIA database, supplemented with results from relevant literature. Preliminary examinations have demonstrated that the behaviour of the pavement referring to tyre noise is different for passenger car tyres and the tyres of trucks. A low noise road surface special designed for truck tyres might not be the most silent solution for passenger car tyres. The influence of typical and extreme operating conditions on noise will be investigated. The research contains cruise-by and accelerated pass-by measurements, uneven driving patterns as well as driving in a gradient or curve. The presence of road gradients can lead to an increased vehicle noise emission. For a given speed steep gradients can lead to noise level increases of 3 dB(A) or higher. Porous asphalt concrete was found to have similar benefits in areas where a gradient was present as on a level surface. Studies have shown that cornering can lead to increases in noise and annoyance due to stick-slip and tyre squeal, particularly at sections where speed-reducing measures are not used. Additionally the influence of slip and traction force on traffic noise will be shown. When tunnels, partial covers, opposite building façades and cuttings are present, the benefits of porous asphalt concrete versus a standard dense road surface were shown to increase. This was considered to be the result of additional multiple reflections on the road surface which can lead to a higher level of sound absorption by porous asphalt concrete. In general, the greater the degree of enclosure the larger the benefit predicted. Where opposite façades are high and closely spaced the effect will be greater than where buildings are low and wide apart. Tunnels with reflective linings are expected to benefit to the greatest extent due to the highly reverberant field caused by multiple reflections. Where absorptive claddings have previously been applied to façades and walls the benefits of laying porous asphalt concrete will be less. Porous asphalt concrete is therefore especially recommended in confined spaces where absorptive materials have not previously been applied. Road surface changes, resulting from normal maintenance or from energy or water supply work, can lead to increased annoyance to residents due to the rapid changes in the noise level as vehicles travel over the patched surface. These road works can also lead to a step-up or step-down in the road surface level, causing increased peak levels of vehicle noise due to body rattles of over 10 dB(A). For this reason care should be taken to ensure that no step-ups or step-downs exist when such road works are carried out. However, despite the increases in peak levels the problem may not be detected by measuring only average levels (LA,eq) since these may not be significantly effected. Examples of these measures might be the use of porous asphalt concrete in confined spaces, the maintenance of smooth even surfaces to reduce body rattle noise and the use of appropriate junctions to ensure smooth traffic flows and high friction surfaces to reduce tyre squeal. The influence of local conditions and traffic management measures on the noise emissions, which are other important options to offer noise protection for the population, will be studied. The term "traffic management" can be described as an application of different strategies and measures to change the flow of traffic on roads either to reduce the speed of vehicles passing by and/or to reduce the traffic volume itself. This will all have an effect on the environmental noise caused by vehicles. That means specific measures on different road and highway types and in rural as well as urban areas, including optimisation for crossings and roundabouts. Traffic management measures such as environmentally adopted "through" roads, 30 km/h zones, road humps, roundabouts, restrictions on traffic in special periods, speed control etc. are used on many urban roads in Europe. These measures are usually applied to improve traffic safety, typically by reducing the speed, and to "calm" residential areas from the environmental impact caused by the traffic in order to make the areas more pleasant to live in for the residents and more agreeable to shop and walk in for shoppers and other people. Some increases in noise at intersections can be observed due to vehicle acceleration/braking and cornering (especially at light controlled intersections). However levels may be reduced where the traffic flow is smoother e.g. at roundabouts. Reducing the speed limits on the approaches and ensuring smoother traffic flow by introducing such methods as `green waves´ would reduce noise at intersections. It is obvious that it can be a good idea to combine traffic management measures and the use of noise reducing pavements in noise abatement schemes. Generally there does not seem to be any technical arguments for not combining these measures of noise abatement. However, it must be noted that porous pavements can be damaged on bends, junctions and roundabouts sites where forces at the tyre/road interface are relatively high. This must be taken into consideration when applying porous pavements on roads specially constructed to reduce speed. Speed reducers which displace the vehicles to the left or right may be problematic for the durability of porous pavements, because this will make the vehicles drive in curves for short distances. But other types of noise reducing pavements can be used in such cases. In other parts of the SILVIA project (WP4) the noise reducing effect of different pavement types are documented. On urban roads with speeds in the range from 40 to 60 km/h noise reductions of 1 to 4 dB(A) can be achieved by using for example noise reducing thin layers or porous pavements. At higher speeds the noise reducing potential for these pavements may be up to 6 dB(A) or even more. This noise reduction is of the same magnitude as or higher than the reduction which can normally be achieved by traffic management measures. Noise reducing pavements and traffic management measures may influence the frequency distribution of road traffic noise in different ways, and this can have an influence on the total noise reduction. For simplification it can anyway be recommended to add (on a dB(A) basis) the effect of the two types of noise reduction. It is therefore generally on urban roads possible to obtain noise reductions of 3 to 8 dB(A) by combining the use of noise reducing pavements and traffic management measures. On highways with high speeds the potential for noise reduction may be up to 10 dB(A) or even more. Generally nowadays constructed noise reducing pavements have a better reduction effect on noise from light vehicles than on noise from heavy vehicles. This means that if a traffic management measure such as an environmentally adopted street or a 30 km/h zone has an effect on reducing the percentage of heavy vehicles the beneficial effects of the noise reducing pavements will be increased. Where noise problems are most severe, a combination of noise reducing measures will be required and it will not always be possible to simply add the acoustic benefits of each to obtain the combined effect. This is the case where a low-noise road surface is used with roadside noise barriers and earth mounds. The combined effect can be less than the added benefits and the discrepancy tends to increase with barrier height. There is strong evidence that noise barriers and low noise road surfaces used together can provide an optimised solution for noise abatement. The later contributes to reduce noise emitted at the source, whereas the previous acts on the sound propagation. The combination of both techniques can bring the ultimate decibels that are missing with a specific barrier. In general, due to the frequency dependence of sound wave propagation, diffraction and absorption, the global efficiency of the combination is lower than the addition of respective efficiencies. This is confirmed by theoretical predictions as well as on site measurements. In principle, when combining a low noise surface and a sound barrier, a great attention should be paid on the spectral efficiency of each in order to optimise the combination. Thus, the advantage of a porous road surface is smaller in the presence of a noise barrier than in the case of unobstructed propagation. Because they reduce noise emission at the source, low noise road surfaces bring sound reduction in the zones where noise barriers are inefficient (non shaded zones, opposite side of the road...). The use of low noise road surfaces can, in principle, reduce the barrier cost by reducing its height or length for equal performance. If pass-by measurements are performed near bridge joints, an important increase in the sound pressure level can be observed, up to 10 or 15 dB(A), depending on the quality of the joint. Results are often expressed in terms of noise increase when passing on the joint. But this indication is relative to the tyre-road noise level, i.e. to the road surface performance. In the case of a noisy joint, the emergence of the peak will be bigger on a quiet road surface than on a noisy one, thus probably making it more annoying. In the case of a silent joint, the road surface performance will dominate. Very silent joints have been developed recently, not causing any peak when vehicles are passing, even when a low noise road surface is used. If long-term noise measurements are performed in the vicinity of the joint, the effect of noise increase will be attenuated. But then the evaluation of the joint effect is more difficult because many things are mixed for example the effect of traffic, road surface and surrounding noise. It is often suggested that physical measurements may be supplemented by an evaluation of noise annoyance. There is a serious lack of knowledge on the effect of low noise surface on indoor sound pressure levels. This should be a subject for future research. Low noise road surfaces can be used in combination with improvement of façade sound insulation. The benefit for the inhabitant will be optimum. However, it is expected that due to the variation of façade sound insulation with frequency, the benefit of a low noise road surface is lower indoor than outdoor. In addition to this report a CD-ROM with some practical audio/video examples will be attached to demonstrate the effect of noise abatement measures on the human ear.