A Department of Transport funded research project has investigated theoretical and practical approaches to convergence monitoring in the context of highway scheme appraisals. The aim of the project was to provide practitioners with guidance on which convergence measures to use to examine convergence, which stop criteria to aim for, and how to deal with cases on non-convergence or with any remaining uncertainty in the flow and associated delay forecasts. The research included a survey amongst practitioners and model developers, and multiple model applications using actual scheme appraisals and three commercial UK packages (CONTRAM, SATURN and TRIPS). The paper reports on the main research findings and proposes findings and proposes an appropriate approach to monitor convergence in practical applications. A two-stage procedure is recommended: (1) the first step seeks a satisfactory level of convergence of each individual assignment model, by meeting pre-specified criteria (this step applies to all applications of assignment); (2) the second step handles the remaining model uncertainty in terms of overall model outcomes (and is particularly applicable in highway scheme appraisal). (A)
Abstract