Analyzing Freeway Traffic under Congestion: Traffic Dynamics Approach

This paper presents an analysis of freeway traffic flows under congestion based on the principle of traffic dynamics, using the example of recurring congestion. Queuing and discharging mechanisms are analyzed using the fundamental concept of conservation of vehicles. These analysis results are applied for reviewing the two conventional macroscopic analysis tools, deterministic queuing analysis and shock-wave analysis. Comparative study results have demonstrated that (1) the two methods are fundamentally different from the traffic modeling point of view; (2) deterministic queuing analysis always underestimates the overall magnitude of delays compared to shock-wave analysis; and (3) the area between the demand and the capacity curves in a queuing diagram is analytically equivalent to total vehicle-hours of travel in congestion as opposed to the widely accepted total vehicle-hours of delay. The study results imply that the guidelines of the 1994 Highway Capacity Manual may result in the underestimation of congestion-related statistics. For the purposes of illustration and verification, a numerical example problem is provided.