The Fundamental Variables
The fundamental traffic characteristics are the flow (vehicles/time), speed (distance/time) and density (vehicles/distance). In general, traffic streams are not uniform, but vary over both space and time. Because of that, measurement of the variables of interest for traffic flow theory is in fact the sampling of a random variable. In fact, the traffic characteristics are only the mean values of statistical distributions, not absolute numbers. Analysis of traffic can be carried out either at the macroscopic level where the average behavior of groups of vehicles is studied, or at the microscopic level, in which case vehicles are considered individual entities. In the microscopic level the corresponding variables are time headways, individual vehicle speeds and distance headways.