While the primary function of a highway is mobility (measured by various performance measures such as vehicle speeds), safety must also play a prominent role. Many of the safety-related aspects of highway design. Starting with the current chapter, emphasis will now turn to understanding elements of highway traffic flow and queuing, and ultimately on to other measures of highway performance and mobility.
The analysis of vehicle traffic (including traffic flow and queuing theory) provides the basis for measuring the operating performance of highways. In undertaking such an analysis, the various dimensions of traffic, such as number of vehicles per unit time (flow), vehicle types, vehicle speeds, and the variation in traffic flow over time, must be addressed because they all influence highway design (the selection of the number of lanes, pavement types, and geometric design) and highway operations (selection of traffic control devices, including signs, markings, and traffic signals), both of which impact the performance of the highway. In light of this, it is important for the analysis of traffic to begin with theoretically consistent quantitative techniques that can be used to model traffic flow, speed, and temporal fluctuations. The intent of this chapter is to focus on models of traffic flow and queuing, thus providing the groundwork for quantifying measures of performance