Proposal of a criterion for defining the levels of service in a quantitative way: the service index

In this study a criterion able to define the traffic levels of service, not only qualitatively but also quantitatively, is proposed. Since 1950 (year of the Highway Capacity Manual first published edition) the traffic conditions have been synthetized in six levels of service that, when only considering some of the traffic influencing factors, turn out fundamental for the representation of a much more complex phenomenon. The necessity of a quantitative representation of LOS remains undoubted in many models, where a numerical index would be more applicable; on the other hand, a numerical continuity of the same index would be more representative of reality, that changes punctually and not in discrete way. The search of an index that represents the road traffic levels and that varies from 0 (worst condition) to 1 (best condition) can be lead in several levels of complexities, described in the paper and improved and tested in future research. The objective of the study is therefore the search of a function that allows to quantify the levels of service and to give them a continuous representation. It is chosen to be named "service index" the index variable from 0 to 1 representative of this function.