Perceived level of service, driver, and traffic characteristics: Piecewise linear model

The objective of this research is the analysis of perceived highway level of service in relation to drivers’ personal characteristics and traffic conditions. A field survey was carried out for that purpose, in which drivers were asked to assess the traffic conditions experienced on a freeway segment, just after having actually driven along this segment. At the same time, traffic counts were taking place. Exploratory analysis revealed a large dispersion in drivers’ assessment of level of service for each volume-to-capacity (v/c) value, especially at moderate traffic conditions. Furthermore, the effect of driver’s age, gender, driving experience, familiarity with the road, vehicle capacity, and v/c on perceived level of service was examined by means of nonparametric statistical tests. It was found that, in this sample, only traffic conditions affected drivers’ assessment of level of service. Perceived level of service was then modeled against v/c by means of a piecewise linear regression technique for three scenarios, covering different types of drivers with respect to their tolerance of congestion. The results confirm that the relationship between perceived level of service and traffic conditions has a piecewise linear form with significant differences in slopes and breakpoints. Moreover, they suggest that drivers perceive no more than two or three levels of traffic conditions. More specifically, only low-tolerance drivers appear to distinguish level of service A from B and only high-tolerance drivers appear to distinguish level of service D from E. On the contrary, an impressive reduction of perceived level of service occurs within the v/c interval of 0.55–0.70 in all scenarios. These findings are further discussed in relation to their accordance with the highway capacity manual approach for level of service and its possible adjustments.