# Effects of Vehicular Speed on the Assessment of
## 3 Results and Discussions  
### Road pavements Evaluation Methods Comparison  
The first step of the analysis concerns the study of the correspondence of the results provided by all the three methods previously described (IRI by ASTM E1926, PSD by ISO 8608 and \(a_{uzz}\) by ISO 2631). In order to compare the results obtained using the different pavement assessment methods, four different ride quality levels were defined, identifying specific threshold limits for each index. The IRI thresholds taken in account were the limit values proposed by [PERSON] et al. (2015) for speed equal to 80 km/h; which is also the speed at which the vertical weighted acceleration (named \(a_{uzz,80}\)) was calculated for this comparison. These IRI thresholds are very close to those proposed for the same speed by [PERSON] (2017). The choice of this speed is based on the velocity range characterizing the examined roads (i.e., 60-90 km/h) and on the fact that IRI is frequently used for the evaluation of roads having legal speed limit equal or higher than 80 km/h. In Table 3 it can be noted that, in order to compensate the absence of specific limitations between the comfort levels provided by ISO 2631, it was chosen to consider the mean value of each overlapping zone as the limit between two adjacent comfort levels (for example 0.565 is the mean of 0.63 and 0.5 m/s\({}^{2}\)). Finally, for the ISO 8608 approach the ride quality levels correspond to the road profiles classes defined in the same standard, as shown in Table 3.  
A first comparison between the abovementioned pavement evaluation methods concerns the analysis of the percentage distribution of all the analyzed pavement sections (621) among the four ride quality levels defined in Table 3. The results are shown in Figure 3, where it can be noted that except for the ride quality level _\"Very Good\"_ there is a significant variability in the assessment of the pavement sections. For example, the percentage of sections judged as _\"Poor\"_ varies from 3% to 27% depending on the evaluation method used. Similar differences can be also observed for the other classes _\"Good/Fair\"_ and _\"Mediocre\"_. \begin{table}
\begin{tabular}{c c} \hline \(a_{uzz}\) **Values** & **Comfort Level** \\ \hline less than 0.315 m/s\({}^{2}\) & not uncomfortable \\
0.315–0.63 m/s\({}^{2}\) & a little uncomfortable \\
0.5–1 m/s\({}^{2}\) & fairly uncomfortable \\
0.8–1.6 m/s\({}^{2}\) & uncomfortable \\
1.25–2.5 m/s\({}^{2}\) & very uncomfortable \\ greater than 2 m/s\({}^{2}\) & extremely uncomfortable \\ \hline \end{tabular}
\end{table} 
Table 2: Comfort levels related to \(a_{uzz}\) threshold values as proposed by ISO 2631 for public transport. In particular it can be seen that adopting IRI or ISO 8608 approaches more than 50% of the total sections belongs to _\"Very Good\"_ and _\"Good/Fair\"_ classes, while using the \(a_{uz,80}\) index as the evaluation method more than 60% of the sections are rated as _\"Mediocre\"_ and _\"Poor\"_. This great variability found in terms of profile classes distribution highlights how the choice of the evaluation criterion is very important and significantly affects the road maintenance plan. The main differences found between the three assessment methods depend on the contribution provided by each kind of discontinuity to the global comfort level perceived by users. In particular, in ISO 8608 classification, there is not an evaluation of the various wavelengths contribution to the vibration level perceived by users. IRI, instead, was developed with the aim to represent the effects of roughness related to both drivers comfort and forces transmitted from wheels to road pavements. In IRI calculation, however, mechanical parameters that do not represent real vehicles are used. On the contrary, the introduction of frequency weighting coefficients in the calculation of the \(a_{uzz}\) index reflects the perception of vibration by human beings.