Process Validation of Urban Freight and Logistics Models

A number of innovative modelling approaches for the analysis of urban freight demand and its impact upon the built environment and transport infrastructure have been proposed over the past several years. These range from new and more robust synthetic models to tour-based formulations based on truck survey data to agent-based microsimulation models. As impressive as these contributions are, most have only included nominal validation efforts, typically limited to comparing the flow estimates to observed traffic counts. In many cases in both research and practice the quality and quantity of these counts are disappointing, and definitive conclusions about model validity and accuracy are difficult to draw from them. Fortunately, increasing the number of counts is far from the only option open to modellers. A far more expansive practice known as process validation can not only overcome the limitations of count data, but admit a far wider spectrum of information, data, and knowledge to the task. This paper illustrates how the process was applied to a tour-based microsimulation model of urban freight, and offers suggestions how it can be more widely applied to freight and logistics models.