Mitigating Initialization Bias in Transportation Modeling Applications

Abstract

Traffic simulation model is a useful tool to evaluate real world transportation solutions in a risk free environment. Traffic simulation model requires some form of initialization before their outputs can be considered meaningful. Models are typically initialized in a particular, often “empty” state and therefore must be “warmed-up” for an unknown amount of simulation time before reaching a “quasi-steady-state” representative of the systems’ performance. The portion of the output series influenced by the arbitrary initialization is referred to as the initial transient and is a widely recognized problem in other areas, but less emphasized in the transportation application. After reviewing methods of accounting for the initial transient bias, this paper selects and evaluates three techniques; two popular methods from the general simulation field, Welch’s and MSER method, and one from the current state of the practice in the transportation application, Volume Balancing. VISSIM models were created to compare the selected methods. After presenting the results of each method, advantages and criticisms of each are discussed as well as issues that arose during the implementation. It is hoped that this paper informs the current practice in transportation application as to how to account for the initial transient in order to continue facilitating meaningful and reliable results.