# Stabilization Time of Running Equivalent Level \(\mathbf{L_{Aeq}}\) for Urban Road Traffic Noise
## 4 Conclusions  
The present study concerns the stabilization time \(ST\), defined as the minimum time after which the difference between the corresponding continuous equivalent sound pressure level \(L_{Aeq,ST}\) and the continuous equivalent sound pressure level \(L_{Aeq,T}\) referred a longer time \(T>ST\), including \(ST\), is never greater than a preset accuracy interval \(\varepsilon\). This parameter is very important for obtaining samples of urban road traffic noise representative of those over the longer time \(T\) taken as a reference, where \(T=1\) h normally. The observed non-normal distribution of the \(ST\) values for all the combinations of road type, hours and uncertainty \(\varepsilon\) suggests summarizing the \(ST\) values in each of these combinations by means of the 75 th percentile instead of either the mean or the median. This is a precautionary approach, being 75% of the \(ST\) values in each combination shorter than the considered \(ST_{75}\). The results obtained by the analysis on the large dataset collected on the roads in Milan clearly show that a fixed measurement time \(t\) of 10-15 min, frequently used in common practice regardless of the amount of road traffic flow, is very often not suitable for obtaining an estimate of the hourly \(L_{Aeq,Th}\) level within a preset accuracy \(\varepsilon\). On the contrary, the stabilization time \(ST\), which is affected by the SPL time variability and the presence of sound events, is an efficient tool to choose the appropriate measurement time \(t\). Furthermore, the intermittency ratio \(IR\) is an efficient metric to detect sound events, to determine their contribution to the continuous equivalent level \(L_{Aeq}\) and their influence on the stabilization time. An important parameter is the value chosen for the uncertainty \(\varepsilon\) of the \(\mathrm{L_{Aeq,Th}}\) estimate. As shown by the results, this value largely affects the duration of \(ST\): the wider \(\varepsilon\), the shorter \(ST\). The SPL time variability of road traffic in urban environments is rather high, and setting an uncertainty \(\varepsilon=\pm 0.5\) dB(A) leads to very long \(ST\) durations. At the same time, one must remember that the uncertainty \(\varepsilon\) in the \(L_{Aeq,Th}\) estimate must be combined with the other sources of uncertainty, such as that from instrumentation. As already reported in [21], setting \(\varepsilon=\pm 1.0\) dB(A) seems to be a reasonable compromise, enabling good discrimination of different urban scenarios as well as SPL fluctuations throughout the measurement time \(t\). Moreover, the stratification of the dataset by road type was effective, as it showed that E and F roads, due to the low road traffic flow and the presence of noticeable sound events clearly exceeding the low background noise, are the most critical for noise measurement time \(t\) because the stabilization time \(ST\) has a long duration. In addition, these road types are the most numerous in urban areas and cannot be neglected, considering their potential noise impact on the health of the exposed population and the large number of people living close to these roads. As a general conclusion, the stabilization time \(ST\) is much more suitable and efficient than using a preset fixed time sampling \(t\), chosen regardless of the amount of road traffic flow. Thus, \(ST\) could be a very useful tool to guide in the selection of a more adequate time sampling \(t\), also taking into account the cumulative probability to estimate the medium- and long-term \(L_{Aeq,T}\) levels within a preset accuracy interval \(\varepsilon\). A measurement time \(t\) shorter than \(ST\) would likely not be adequate to obtain the \(L_{Aeq,Th}\) estimate within the preset accuracy \(\varepsilon\). Even though the results refer to data collected in Milan, the structure of this large city is similar to those of other cities, at least in Europe, and the analysis carried out could be interesting if its methodological approach was to be replicated in other urban contexts.