Drastic mobility restrictions during SARS-CoV-2 pandemic: an opportunity to learn about constraints on the way to a pollution-free city.

Road traffic is one of the main sources of pollution in modern cities. If there is a desire to move towards healthier cities, it may be necessary to modify the current model of mobility. The SARS-CoV-2 pandemic, together with the measures applied by most governments in the world to control the mobility of citizens, offered a unique opportunity to assess the changes in pollution levels after a drastic reduction in road traffic. In this study, air and noise pollution levels and road traffic flow were analyzed in the city of Cáceres, Spain, before and during the state of emergency imposed by the Spanish government. The values obtained were compared with the quality limits established by both the Spanish government and the World Health Organization (WHO). A traffic noise prediction model has been employed to evaluate the acoustic impact resulting from the reduction in traffic flow. As a result of this study, it was found that air pollution was indeed reduced due to the mobility restrictions imposed to control the pandemic, but that the WHO's recommendations for the values of the day-evening-night noise indicator (Lden) and the night-time noise indicator (Ln) for road traffic noise, which should not be exceeded, were not met. These findings highlight the need to review current urban mobility models if the WHO's recommendations are to be reached with regard to reducing the effects of exposure of the population to urban noise.

Effects of noise on pedestrians in urban environments where road traffic is the main source of sound.

Research combining the measurement of objective variables with surveys of people's perception of noise on city streets is useful in terms of understanding the impact of urban noise on the population and improving the environment. Although previous investigations have analysed the factors that may influence the noise annoyance of citizens, it is usually considered as a global aspect. This paper presents research based on in situ surveys and objective variables (urban, meteorological and noise indicators) to evaluate some specific effects of noise on pedestrians in urban environments where road traffic is the main source of sound. The results show significant relationships of the effects of noise and perceptions of how noisy urban environments are with variables such as building height, road category and temperature, with correlation coefficients ranging from 0.37 to 0.64. Significant correlations between these subjective variables and the acoustic variables were also found, with explanations of variability that reached values of up to 50 %. A multivariate analysis revealed that both urban variables (especially the category of street) and environmental variables can be an alternative or a complement to models predicting the effects and perception of environmental noise based only on acoustic variables.

A comprehensive experimental study of the influence of temperature on urban road traffic noise under real-world conditions.

The effect of road traffic noise in urban environments is an issue of social and scientific interest, due to its public health and economic impacts. Scientific literature showed a decrease in the level of tyre/road noise generated as temperature increases, but usually under standardised traffic conditions in non-urban environments. Based on a wide network for the hourly monitoring of road traffic flow, air temperature and noise levels across the city of Madrid (Spain), this work proposes and applies a new experimental methodology for studying the dependence of urban road traffic noise on temperature. This study was conducted under real-world traffic conditions involving a wide variability in urban configurations and in the type and state of preservation of vehicles, tires and pavements. From the analysis of data for a whole year, a time interval was identified (from Tuesday to Thursday and between 8 a.m. and 8 p.m.) in which the variability in road traffic flow for the whole city of Madrid was stable enough to allow for a linear regression study between temperature and noise levels from urban road traffic. The relationships found were highly significant (p ≤ 0.001) for data from all the noise monitoring stations, with values of higher than 20% and up to 42% for the explanation of the variability in the measured noise levels by temperature at most of the measurement points. The values of the slope coefficients at the noise monitoring stations ranged from -0.036 to -0.125 dB/°C, with an average value of -0.090 ± 0.011 dB/°C. These results are within the range of values reported in the scientific literature for experimental tests conducted under conditions of controlled or free-flowing traffic.

Variability of traffic noise pollution levels as a function of city size variables.

Noise levels measured in 27 cities with different areas (from 0.6 km2 to 59.27 km2) and populations (from approximately 2000 to 70,000 inhabitants) were compared with respect to five different urban characteristics (population, area, total street length, density, and linear density). Comparisons were conducted for both overall city noise levels and noise registered on five types of roads with different functionality using the Categorisation Method. The results showed that four of the five cities' characteristics presented a significant correlation with the noise levels (all except for density). The calculated correlations were better for noise levels in the different categories than the overall noise values, with higher explained variability on the streets with more traffic. Therefore, the road categorisation method can be used not only to assess the noise variations within cities, but also to better explain the effect of noise on the analysed city characteristics. The results of the calculated relationships enable the estimation of noise levels both currently and in future urban developments of noise values on different types of streets.

A proposal for producing calculated noise mapping defining the sound power levels of roads by street stratification.

The European Noise Directive proposes using strategic noise maps as tools to assess populations affected by environmental noise. It recommends using computational methods instead of in situ measurements when possible. A sound source's emission power is an important factor in the calculation of noise indicators. For traffic noise, this parameter is usually defined based on vehicle flow considering an emission spectrum that depends on the type of vehicle and its speed. This study analysed the possibility of using the categorisation method to propose an alternative method of defining a sound source's emission power to develop noise maps. This was accomplished using previously published values of the emission power per unit length. Another method is also proposed that estimates traffic flows. To verify their estimation capacity, the results of both methods were compared with the values obtained from in situ measurements. The results demonstrated similar uncertainties in both methods and were in the range of expected average uncertainties compared to the results obtained by calculating a noise map with the measured experimental values. In particular, for the differences between calculations and measurements, in absolute values, the mean uncertainties were approximately 2 dBA in estimating different long-term noise indicators. For the differences, the mean of the uncertainties obtained via the categorisation method did not present significant differences for the null value for all the analysed noise indicators. Street stratification is a rapid and low-cost approach for road traffic noise mapping without increasing uncertainties.

Evaluation of exposure to road traffic noise: Effects of microphone height and urban configuration.

Noise pollution is a major environmental problem due to its impact on human health and implications for other spheres of society. Since road traffic is the main source of noise pollution, the use of measurement methodologies to accurately determine the environmental noise levels to which the façades of buildings in cities are exposed is an important issue. This paper presents an experimental study in urban environments that uses different configurations to evaluate the influence of the position of the microphone and the parking lanes on the levels of road traffic noise to which the population is exposed. In urban settings in which sound waves propagate without obstacles between the lanes of traffic and the receivers, broadband results for the differences between noise levels measured by microphones placed at heights of 4.0 and 1.5 m showed a significant increase with an increase in the distance between the microphone and sound source of between -0.8 and 0.9 dBA over a range from 2 to 8 m. This difference between the two microphones was greater at points where a lane of parked vehicles was located between the road traffic lanes and the receivers were placed near the façades of building. At the same heights, the broadband difference in sound levels ranged from 2.7 to 4.5 dBA. This acoustic shielding effect due to the presence of parked vehicles started to be relevant in the 250 Hz band and increased progressively with frequency. Taking into account these experimental results and the recommendations in the European Noise Directive, it would be important to apply corrections to sound indicators for road traffic noise that are related to the height of the microphone. Making a distinction between urban configurations with and without lines of parked vehicles between the microphone and the road traffic lanes would be advisable.

Effect of parking lanes on assessing the impact of road traffic noise on building façades.

The use of strategic noise maps as a means for estimating population exposure to environmental noise and defining action plans to mitigate its effects on human health has become a reality since the publication of the European Noise Directive. In this context, it is known that some differences can be found between the values obtained for sound indicators through simulation and measurements due to different causes. One of these factors is the presence of elements in urban environments not currently considered in calculation methods but certainly present in validation measurements. This paper presents an assessment of the acoustic shielding effect due to parked vehicles on urban streets using computational methods. First of all, a process of validation of the software model by means of different simulation methods and in situ measurements was carried out. Then, a study was developed varying different variables related to urban planning and noise modelling, as well as considering different typologies of real streets according to a categorisation method. Broadband results show that this shielding effect can be significant in common configurations in urban environments, even to receiver heights of 4 m considered as a reference in strategic noise maps. The magnitude of this effect varied depending on the distances between the building façade, parked vehicles and sound sources, as well as the receiver height. Differences up to 4 dBA in sound levels were found in several configurations between situations without and with cars parked at 4 m, although in some specific cases it reached up to 8 dBA. Therefore, results of this study indicated that parking lane shielding effect should be considered in calculations and validation measurements for strategic noise maps in order to obtain an adequate estimation of population exposure to road traffic noise.

Perceptions and effects of the acoustic environment in quiet residential areas.

Many cities have historical areas clearly distinguished from the rest because of the architecture, urban planning, and functionality. In many cases, these aspects give one the possibility of finding a characteristic acoustic environment and also developing quiet areas. Through an examination of sound levels and surveys, the perception of residents and passers-by concerning the acoustic environment of the old town of Cáceres and its relation with the characteristics of the urban environment were analysed. In addition, the perception and the effects of noise pollution of low intensity were studied. The results indicate that absence of daytime noise is the most influential environmental characteristic on the overall perception of the urban environment studied, even surpassing the feeling of security. The absence of daytime noise was also the most valued characteristic of the urban environment according to respondents. The most annoying noise source proved to be the road traffic. However, for similar levels of sound exposure, the percentages of people who were annoyed and whose sleep was disturbed were lower than those found in previous studies. Bells and birds, both soundmarks of the soundscape of this urban environment, were among the most annoying to passers-by.

Can weekly noise levels of urban road traffic, as predominant noise source, estimate annual ones?

The effects of noise pollution on human quality of life and health were recognised by the World Health Organisation a long time ago. There is a crucial dilemma for the study of urban noise when one is looking for proven methodologies that can allow, on the one hand, an increase in the quality of predictions, and on the other hand, saving resources in the spatial and temporal sampling. The temporal structure of urban noise is studied in this work from a different point of view. This methodology, based on Fourier analysis, is applied to several measurements of urban noise, mainly from road traffic and one-week long, carried out in two cities located on different continents and with different sociological life styles (Cáceres, Spain and Talca, Chile). Its capacity to predict annual noise levels from weekly measurements is studied. The relation between this methodology and the categorisation method is also analysed.

Analysis of Sampling Methodologies for Noise Pollution Assessment and the Impact on the Population.

Today, noise pollution is an increasing environmental stressor. Noise maps are recognised as the main tool for assessing and managing environmental noise, but their accuracy largely depends on the sampling method used. The sampling methods most commonly used by different researchers (grid, legislative road types and categorisation methods) were analysed and compared using the city of Talca (Chile) as a test case. The results show that the stratification of sound values in road categories has a significantly lower prediction error and a higher capacity for discrimination and prediction than in the legislative road types used by the Ministry of Transport and Telecommunications in Chile. Also, the use of one or another method implies significant differences in the assessment of population exposure to noise pollution. Thus, the selection of a suitable method for performing noise maps through measurements is essential to achieve an accurate assessment of the impact of noise pollution on the population.

Urban noise functional stratification for estimating average annual sound level.

Road traffic noise causes many health problems and the deterioration of the quality of urban life; thus, adequate spatial noise and temporal assessment methods are required. Different methods have been proposed for the spatial evaluation of noise in cities, including the categorization method. Until now, this method has only been applied for the study of spatial variability with measurements taken over a week. In this work, continuous measurements of 1 year carried out in 21 different locations in Madrid (Spain), which has more than three million inhabitants, were analyzed. The annual average sound levels and the temporal variability were studied in the proposed categories. The results show that the three proposed categories highlight the spatial noise stratification of the studied city in each period of the day (day, evening, and night) and in the overall indicators (L(And), L(Aden), and L(A24)). Also, significant differences between the diurnal and nocturnal sound levels show functional stratification in these categories. Therefore, this functional stratification offers advantages from both spatial and temporal perspectives by reducing the sampling points and the measurement time.

The temporal structure of pollution levels in developed cities.

Currently, the need for mobility can cause significant pollution levels in cities, with important effects on health and quality of life. Any approach to the study of urban pollution and its effects requires an analysis of spatial distribution and temporal variability. It is a crucial dilemma to obtain proven methodologies that allow an increase in the quality of the prediction and the saving of resources in the spatial and temporal sampling. This work proposes a new analytical methodology in the study of temporal structure. As a result, a model for estimating annual levels of urban traffic noise was proposed. The average errors are less than one decibel in all acoustics indicators. A new working methodology of urban noise has begun. Additionally, a general application can be found for the study of the impacts of pollution associated with traffic, with implications for urban design and possibly in economic and sociological aspects.

Stabilisation patterns of hourly urban sound levels.

In this paper, the stabilisation times for all 24 h of the day are analysed for 12 measurement stations located in the Spanish town of Malaga and throughout 5 years (2007-2011) of sampling environmental noise levels. For the results to be generalised to sound level measurements made in other streets or cities where there have been no long-term measurements, this study was developed for different road types and urban shapes. This distinction was made according to the types of roads indicated in other studies in which a statistically significant relationship was found between noise levels and the road type. The final objective of the study is to determine the capacity to estimate and approximate the real equivalent hourly noise level (L Aeq,1h) from the integration of a number of consecutive minutes (short-term measurements) less than 60 (L Aeq,1h ≈ L Aeq,T being T ≤ 60 min). Clearly, this strategy would save time and resources by making measurements of reduced duration. In summary and according to this analysis, a short-time measurement of 15 min is adequate to work with 90% confidence levels and errors of ±2 dB, with 80% confidence levels and errors of ±1 dB, and 50% confidence levels and errors of ±0.5 dB. However, it is necessary to consider the measurement hour period to achieve these levels of confidence due to the high variability throughout the day.

Analyzing nocturnal noise stratification.

Pollution associated to traffic can be considered as one of the most relevant pollution sources in our cities; noise is one of the major components of traffic pollution; thus, efforts are necessary to search adequate noise assessment methods and low pollution city designs. Different methods have been proposed for the evaluation of noise in cities, including the categorization method, which is based on the functionality concept. Until now, this method has only been studied (with encouraging results) for short-term, diurnal measurements, but nocturnal noise presents a behavior clearly different on respect to the diurnal one. In this work 45 continuous measurements of approximately one week each in duration are statistically analyzed to identify differences between the proposed categories. The results show that the five proposed categories highlight the noise stratification of the studied city in each period of the day (day, evening, and night). A comparison of the continuous measurements with previous short-term measurements indicates that the latter can be a good approximation of the former in diurnal period, reducing the resource expenditure for noise evaluation. Annoyance estimated from the measured noise levels was compared with the response of population obtained from a questionnaire with good agreement. The categorization method can yield good information about the distribution of a pollutant associated to traffic in our cities in each period of the day and, therefore, is a powerful tool for town planning and the design of pollution prevention policies.

Urban streets functionality as a tool for urban pollution management.

Pollution derived from traffic can be considered one of the major problems of modern cities. Although considerable efforts have been devoted to gathering information about pollution and its control, little attention has been paid to the analysis of relationships between pollution distribution and town planning. The existence of these relationships would enable better prediction and prevention of pollution through town planning. In this work, an analysis of one pollutant derived from traffic (urban noise) in 27 cities is presented. Non-parametric tests and ROC analyses were employed, using the equivalent sound level (L(eq)) values as the dependent variable. For the characterization of the pollutant, an alternative concept to accessibility is analyzed: the concept of functionality. Results of statistical inferential analysis showed the existence of significant differences between the sound levels of the different category results, confirming that noise is stratified in the studied cities and that the five categories proposed based in the concept of functionality highlight this noise stratification. Moreover, high sensitivity and low non-specificity were obtained by using ROC analysis. Results of this analysis also showed an overall average value of prediction capacity close to 90%. Therefore, because the proposed categories highlight the noise stratification of the studied pollutant in all the towns studied, the functionality concept can be considered an interesting tool for urban planning and for designing pollution prevention policies. Finally, as traffic is a source of other urban pollutants, the concept of functionality may be a new concept for wide environmental pollution management.