# Effects of Vehicular Speed on the Assessment of
## 1 Introduction  
Good ride quality on road networks is required in all modern countries. In order to achieve this outcome, large amounts of synthetic indices have been proposed by researchers during last few decades, with the aim to represent roads geometric characteristics and effects on vehicles (consumption and wear), passengers (comfort and safety), and pavement (dynamic loads increment). Among these, the most popular and widely used one is the International Roughness Index (IRI), proposed by [PERSON] [1, 2], which is a measure of the effects induced by road pavement irregularities on a conventional vehicular suspension. This index is calculated considering a simulated passenger car (also known as \"golden car\") travelling at 80 km/h. Many authors, like [PERSON] and [PERSON] [3, 4], have studied mutual relationships between IRI and other similar indices (e.g., Three Band Indicators), with particular attention to unevenness index (C) and waviness (\(w\)) defined in the ISO 8608 standard [5], where a road surface profiles classification is provided. These studies are focused on the use of artificial profiles, which can be used to evaluate many specific issues, like, for example, the influence of different wavelength contribution on the various examined indices. The use of artificial profiles also allows having the full control over their characteristics (e.g.,roughness level, profile length) [6; 7; 8]. Besides, to validate the models, a huge number of samples are necessary, which generally are not always obtainable from real surveys that require, furthermore, considerable costs and time. Thus, starting from the equation provided by ISO 8608 for fitted Power Spectral Density (PSD) calculation, it is possible to generate random road profile belonging to the desired ISO class, through the implementation of various methods. Among them, two of the most commonly adopted ones are the so-called shaping filter and the sinusoidal approximation, described by [PERSON] et al. [9]. Many studies are based on the use of these theoretical road profiles in order to evaluate, for example, dynamic loads on road pavements due to the roughness [10; 11; 12] or to calculate the transfer function needed to estimate road roughness using vehicle acceleration measurements [13]. Other authors also have underlined the need to consider indices different from IRI, in particular for the assessment of roads having maximum legal speed limits lower than 80 km/h, for which the use of IRI is not fully appropriate. To overcome this problem, [PERSON] and [PERSON] [14] suggested a new index, named Vehicle Response Index (VRI), which is based on the PSD of profile slope and on the response gain of a quarter-car model to road input. In this way, it is possible to take into account both pavement surface characteristics and vehicle parameters in the ride quality evaluation; also being able to assess the effects on the vehicle running at different speeds. In order to correspond to these needs, other methods have been proposed in the literature. For example, [PERSON] et al. [15] developed a speed-related ride quality IRI thresholds, while other authors, like [PERSON] and [PERSON] [16] and [PERSON] and [PERSON] [17], focused their works on the relation between road roughness and human whole-body vibration (WBV), calculated starting on the vertical accelerations recorded in a vehicle riding on a rough pavement. The last approach gives to road agencies the opportunity to assess comfort level perceived by road users, evaluating the possibility to reduce speed limits or taking suitable actions in order to improve road features. Furthermore, this approach allows estimating the speed at which users can travel along a road section while suffering a tolerable level of discomfort. In other words, it can be possible to assess the velocity reduction needed in order to maintain the comfort and quality of travel considering actual road pavement conditions.