New methods exploring urban traffic congestion using Lorenz and concentration curves

Congestion is a major concern affecting growing cities worldwide. Congestion mitigation depends largely on our factual understanding of congestion, its nature, distribution and variability. However, traditional measures of congestion such as the Volume Capacity Ratio (VCR) have been criticised for overly being focusing on localised (not network-wide) performance, for omitting measures of actual user travel time and for failing to consider the volume of traffic carried. This research paper presents new methods for exploring congestion, travel time performance and traffic volume impacts on traffic links distributed within an urban traffic network. The approach uses Lorenz curves, and the associated Gini Coefficient parameter, which has been developed in welfare economics as a powerful tool to study income inequality. This study uses the mathematical principle of Lorenz and Concentration curves using traffic related variables: speed and traffic volume. The proposed approach directly addresses limitations noted about VCR based approaches in that it more readily illustrates relative volume of travel across networks as well as relative travel time/speed performance as it impacts individual users. In addition the relative performance of these factors can be more readily compared on a network wide basis and even mapped to identify and prioritise problem link locations. The paper undertakes two analyses; a cumulative link VCR by cumulative link volume analysis and an analysis of cumulative link speed by cumulative volume to explore distributions of link performance across an urban network using a citywide transport network model. Both analysis (by Lorenz or Concentration curve based measure and VCR measure) of urban network are compared and evaluated. Areas for further development of the method are suggested.