Unfairness of System Optimal Traffic Assignment Strategy for Signal Controlled Network

For System Optimal (SO) criteria, small fraction of the traffic is sacrificed to the higher cost length routes because this reduces the congestion experienced by the overwhelming majority of network users. Roughgarden studied how much worse off drivers are in SO assignment than in User Equilibrium (UE). For non-signal controlled network, he quantified this worse off with the unfairness concept. Unfairness of SO criteria for signal controlled network has not received much attention. For signal controlled network, unfairness is difficult to be derived from analytical formulation because there are complex interactions between signal control, traffic flow dynamics and traffic assignment. Travel time does not always increase with traffic counts. This is different from non-signal controlled network, in which, travel time monotonously increases with flow rate. Signal control policy can divert unfairness from one route to another route. The paper extends the unfairness concept to signal controlled network and adopts simulation-based method to compute unfairness.