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This is a summary of the research that the department has undertaken from past to present on the modelling and appraisal of travel time variability (reliability). This is mostly for highway modelling, but has some elements of rail reliability also. It provides clarity on the position we’re at regarding research. This is a complex area for appraisal, particularly given the importance of reliability, that we need to better understand in order to potentially integrate into our modelling tools and appraisal system.