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Thin Asphalt Layers as a Traffic Noise Intervention
Low-noise thin asphalt layers (TALs) are a feasible solution to mitigate road traffic noise in urban environments. Nevertheless, the impacts of this type of noise intervention are reported mostly regarding noise levels, while non-acoustic aspects influencing the population perception are still little-known. This study investigates the implementation of TALs in two streets of Antwerp, Belgium. The effectiveness of the intervention was measured via noise modelling and acoustic measurements of road traffic noise. A reduction of 2.8 dB in noise exposure was observed in Lden and Lnight, while SPB measurements showed decreases up to 5.2 dB on the roadside. The subjective impacts of the TALs were evaluated via self-administered surveys and compared to results from control streets. The annoyance indicators were positively impacted by the TALs implementation, resulting in annoyance levels similar or lower than in the control streets. The TALs did not impact the reported physical complaints, sleep quality, and comfort level to perform activities.
Long-term exposure to road traffic noise has been associated with non-auditory health outcomes such as cardiovascular diseases, cognitive dysfunction, sleep disorder, among others . The build-up of somatic disease arises from physiological responses triggered by exposure to high levels of (road traffic) noise. These somewhat unconscious behavioral reactions to noise exposure can be subjectively measured by the “noise annoyance”. This indicator is assessed at the population level via social surveys and is more easily reported by the exposed population to describe (road traffic) noise exposure than the manifestation of somatic disease; see  for an elaborate review on this topic. Thus, annoyance could be considered an early warning signal for health risks, playing a key role in setting noise exposure limits and creating action plans for noise exposure mitigation .
Although many studies have focused on the objective noise reduction of road traffic noise interventions, only a limited number of studies have examined the effectiveness of road traffic noise interventions on human health. In this sense, this research reports on the possible impacts of TALs as a road traffic noise intervention for an urban environment. The main goal of our study is to establish whether TALs are a valid option for noise reduction in such environments. The research questions are, therefore, two-fold: firstly, what is the objective effectiveness of the implemented TALs to reduce noise levels and noise exposure, meaning its effect on Lden and Lnight? Secondly, do objectives changes in noise levels affect the perception and well-being of the exposed population?
The key to this study is the road traffic noise intervention by means of TALs implemented in two urban streets (TAL A and TAL B).
In order to evaluate the effectiveness of this intervention, social and socioacoustic aspects were investigated before and after the TALs were laid. The objective impacts were assessed via acoustic measurements of road traffic noise, the Close‐ProXimity (CPX) and Statistical Pass-By (SPB) methods, and noise modelling. The subjective impacts caused by the change in noise levels were quantified via social surveys distributed to the residents of the streets in question. The main parameters retrieved from this survey were the annoyance indicators (namely Annoyance, ΔAnnoyance, and RTA) and the subjective noise indicators (physical complaints, sleep quality and comfort level to perform activities). Furthermore, a control group of (quiet) urban streets was selected to serve as a comparison for the survey’s results.
2. Effectiveness of the Noise Intervention
All five TAL sections presented LCPX lower than the DAC 10 reference surface (REF - B). These differences ranged between 2.3 and 2.8 dB(A) for the P1 tyre and 0.7 to 1.9 dB(A) for the H1 tyre. SPB measurements revealed noise reductions ranging between 3.8 and 5.2 dB(A). These decreases should be significant enough for the residents close to the road to perceive the difference after the TALs construction.
In the original condition, the residents were exposed to an average Lden of 64.3 ± 0.9 dB(A) and 53.8 ± 1.1 dB(A) for Lnight. After the TALs were laid, the average Lden dropped to 61.5 ± 0.8 dB(A), while Lnight was reduced to 51.0 ± 0.9 dB(A). Thus, the TALs placement led to a reduction of 2.8 dB(A) in both indicators.
3. Effects of the Noise Intervention
3.1. Direct Subjective Perceived Noise: Annoyance Indicators
|Pre||Post 1||Post 2|
|Annoyance *||2.23 (0.99)||2.75 (1.11)||2.48 (1.01)||2.42 (0.75)|
|ΔAnnoyance ×||0.46 (0.85)||0.87 (0.79)||0.16 (1.07)||0.00 (1.22)|
|RTA *||2.29 (1.08)||2.86 (1.10)||2.51 (0.92)||2.41 (0.99)|
|Post 1||Post 2||n.s.||n.s.||n.s.|
|χ2(3) = 12.62, p = 0.006||χ2(3) = 19.17, p = 0.000||χ2(3) = 4.17, p = 0.006|
The average Annoyance in the control streets (2.23 ± 0.99) indicates that respondents are ‘slightly annoyed’, compared to an average of 2.75 ± 1.11 (close to ‘moderately annoyed’) in pre-survey on the experimental streets. A similar condition is reported for RTA. For both indicators, the mean ranks difference is statistically significant. This contrast partially justifies the implementation of a noise intervention;
After the TAL construction, the ΔAnnoyance scores reveal that the residents experienced a lesser increase in annoyance by noise over the 1-year window prior to the post-surveys than before the pre-survey. Therefore, the residents report positively experiencing a change in Annoyance and RTA, most likely attributed to the noise intervention;
In the first post-survey, Annoyance and RTA have decreased in comparison to the pre-survey and are no longer significantly different from the control groups, where the average noise annoyance is close to the Flemish average reported in SLO-4  (Annoyance = 2.11 and RTA = 2.19; based on >5000 respondents). This effect appears to be sustained even at the time of the second post-survey;
The three noise indicators did not differ statistically between post 1 and post 2. Thus, the lower traffic intensity might not be as influential on the reported subjective indicators as we anticipated, at least not in the short term;
Similar means for Annoyance and RTA across all cases possibly indicate that either the respondents did not differentiate between the noise sources causing annoyance or road traffic noise is clearly identified as the main source of Annoyance in general. The last option is more reasonable, as RTA is distinguishably the highest among the annoyances from the different noise sources: the second higher reported mean annoyance comes from ‘priority vehicles (ambulances, fire trucks, etc.)’, ranging from 1.64 to 1.78 across the three cases.
3.2. Indirect Subjective Perceived Noise: Physical Complaints, Quality of Sleep and Comfort Level to Perform Activities
|Physical complaints (1)||Headaches|
|Fatigue||0.14 * (n = 220)||0.13 * (n = 211)|
|Dizziness||0.||0.15 * (n = 210)|
|Insomnia||0.22 ** (n = 224)||0.16 ** (n = 214)|
|Heart palpitations||0.14 * (n = 221)||0.15 * (n = 213)|
|Gastrointestinal complaints||0.13 * (n = 223)||0.12 * (n = 215)|
|Sleep quality (2)||Sleep duration (night)|
|Sleep duration (day)||−0.13 * (n = 212)|
|Time to fall asleep|
|Waking up too early||0.15 ** (n = 221)||0.14 * (n = 213)|
|Difficulty waking up|
|Feeling well-rested||−0.14 * (n = 220)|
|Comfort level to perform activities (3)||Concentration during reading||In||0.21 ** (n = 223)||0.20 ** (n = 214)|
|Out||0.56 ** (n = 222)||0.16 ** (n = 209)||0.20 ** (n = 213)|
|Concentration during working or studying||In||0.22 ** (n = 218)||0.13 * (n = 210)|
|Out||0.31 ** (n = 218)||0.13 * (n = 205)||0.18 ** (n = 208)|
|Concentration during watching TV||In||0.18 ** (n = 223)||0.14 * (n = 213)|
|Speech intelligibility during a conversation||In||0.20 ** (n = 221)||0.12 * (n = 213)|
|Out||0.30 ** (n = 223)||0.24 ** (n = 210)||0.26 ** (n = 213)|
|Speech intelligibility on the telephone||In||0.18 ** (n = 224)||0.15 * (n = 211)|
|Out||0.29 ** (n = 222)||0.21 **(n = 209)||0.22 ** (n = 212)|
|Relaxing or unwinding||In||0.23 ** (n = 223)||0.17 ** (n = 210)||0.16 ** (n = 214)|
|Out||0.38 ** (n = 222)||0.26 ** (n = 209)||0.24 ** (n = 213)|
This study aimed to quantify the effectiveness of low-noise thin surface layers (TALs) as an intervention on road traffic noise and its impacts on the residents’ perception. Objectively, a noise exposure reduction of 2.8 dB(A) for both Lden and Lnight was obtained.The noise exposure after the TALs placement still could not meet the recommended levels by the WHO  (Lden 61.5 dB and Lnight 51.0 dB compared to the limits of 53 dB and 45 dB, respectively). However, this is where the subjective impacts of the noise intervention become relevant, as the literature suggests that the impact of noise exposure on the build-up of non-auditory health disorders can be better assessed by the annoyance indicators rather than the actual noise levels.
Firstly, both the annoyance levels caused by noise in general and specifically by road traffic noise were significantly reduced after the TALs placement. The percentage of highly annoyed people (%HA) was reduced by 15.2% and 4.8%, for the cut-offs between highly annoyed and not highly annoyed at 60% and 80%, respectively. These reductions are considerably higher than based upon calculations using exposure–response relationships (ERRs) found in the literature (). Perhaps the residents’ satisfaction was increased by the implementation of a policy to enhance their well-being.
The indirect subjective noise indicators, which included physical complaints, sleep quality, and comfort level to perform activities indoors or outdoors, did not present a change after the noise intervention. However, the annoyance indicators presented weak but still significant relation with physical complaints and a strong correlation with difficulty to perform most of the activities outdoors and indoors.
The results support the policies of (road) authorities in general and in cities such as Antwerp in particular, to continue investing in projects that reduce road traffic noise annoyance, provided that sufficient residents in the region can benefit from the intervention and that it is technically feasible.
This entry is adapted from 10.3390/su132212561
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