Freight Generation, Freight Trip Generation, and the Perils of Using Constant Trip Rates
J Holguin-Veras, M Jaller, L Destro, X Ban, C Lawson, Rensselaer Polytechnic Institute; H Levinson, Independent Consultant, US
The paper analyzes freight trip generation practices and concludes that the accuracy of these models depend on: the consistency between the model's structure and actual patterns, and other factors not currently taken into account.
The paper has produced a number of findings, of great importance, that call into question current freight demand modeling practices. The paper's chief conclusion is that the accuracy of freight generation (FG) and freight trip generation (FTG) models depend on: the consistency between the model's structure and actual FG/FTG patterns, the degree of internal heterogeneity of the economic/land use aggregation used to estimate the model, and the appropriateness of the spatial aggregation procedure used to obtain the desired FG/FTG estimates.
Regarding model structure, the paper establishes that there are strong practical and theoretical reasons to treat FG and FTG as separate concepts because the latter is the output of logistic decisions, while the former is determined by the economics of production/consumption. As a result, the connection between business size variables, e.g., employment, and FG is relatively strong because they are economic input factors; while the one with FTG is weaker as inventory and transportation costs come into play. Moreover, while FG is expected to grow with business size, FTG may not necessarily do the same. This means that it is not generally correct to assume proportionality between FTG and business size, and that using constant FTG rates could be problematic. For instance, only 18% of the industry sectors in New York City exhibit constant FTG rates per employee.
The paper also analyzes the role played by the economic/land use aggregation used to define the industry segments included in a FG/FTG model, and the spatial aggregation for which the FG/FTG estimates are desired. In terms of economic/land use aggregation the chief conclusion is that the finer its level of detail the better, as independent variables would have better chance of explaining FG/FTG. In the case of spatial aggregation, the paper concludes that the correct aggregation procedure depends on the underlying disaggregate model. In essence, for a FG/FTG model to work well both economic/land use and spatial aggregations must be appropriate.
Association for European Transport