# Stabilization Time of Running Equivalent Level \(\mathbf{L_{Aeq}}\) for Urban Road Traffic Noise
## 1 Introduction  
In 1982, [PERSON] published a critical review of the existing practice of temporal sampling techniques for the measurement of environmental noise [1]. He concluded that whilst sampling techniques provide savings in human resources and equipment, the techniques were inadequate to determine accuracy except in a number of specific situations. He also pointed out that continuous monitoring was impractical due to the resource requirement. Meanwhile, the increases in urbanization and mobility have led road traffic to become a major noise source in urban areas, with a large impact on the health of the exposed population (e.g., annoyance [2], psychological distress [3] and psychotropic medication use [4]). This noise shows large and random sound pressure level (SPL) fluctuations over time and, therefore, it is a challenge to track its time variability by sampling it. This subject has attracted the attention of many researchers, and several studies and surveys are reported in the literature (e.g., [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16]). Until relatively recently, local authorities and researchers have largely used short-duration localized noise measurements and computer-generated noise maps to comply with the Directive 2002/49/EC requirements [17]. The predictions of noise by numerical models of outdoor sound propagation and \"on the spot\" short-duration measurements areuseful tools, even though a network of permanent noise monitoring stations measuring continuously may be a preferable solution. The high cost of installing and maintaining such networks is often the main reason for not implementing them on a large scale and with adequate spatial resolution. These limitations are even more evident in urban areas, where the complexity of outdoor noise propagation, due to the spatial configuration of the built environment and the noise source distribution, leads to high variability of sound pressure levels in space and time [18; 19] and, therefore, requires spatially \"dense\" noise monitoring units. Despite the facts that the technology has improved significantly and the costs have been reduced, short-time noise monitoring is still an appealing measurement technique, also because it is usually performed by an operator, unlike the unattended permanent noise monitoring units. Among the advantages of attended noise measurement, nonrelevant sounds can be excluded or tagged in order to not be considered in post-processing and adverse weather conditions unsuitable for measurements can be avoided. Furthermore, the operator can stop the measurement when the running equivalent level \(L_{Aeq}\) shows time fluctuations within a preset range. This allows for saving time and moving to another location and, therefore, increases the spatial resolution of noise monitoring and saves resources. On the contrary, unattended continuous noise monitoring requires validation of the collected data that could be difficult, cumbersome and time consuming in the necessary post-processing phase. In any noise time sampling, the value of the continuous equivalent sound pressure level \(L_{Aeq,T}\) referred to medium- and long-time \(T\) is estimated by the \(L_{Aeq,t}\) level measured for a shorter time \(t\), usually much shorter than \(T\) (\(t<<T\)). For instance, the Italian legislation on road traffic noise requires that \(T=1\) h, and it is common practice that the hourly \(L_{Aeq,Th}\) value is estimated to be equal to the measured value \(L_{Aeq,t}\) over time interval \(t\) within the selected hour. The duration of \(t\) and, therefore, the time sampling ratio \(m=t/T\), which is always \(m\leq 1\) being t << T, is variable and in common practice is very often fixed at 10-15 min in urban areas and road traffic noise, regardless of the amount of road traffic flow.