Time-dependent Effects on Parameters of Freight Demand Models: an Empirical Investigation
J Holguin-Veras, I Sanchez-Diaz, C Gonzalez-Calderon, Rensselaer Polytechnic Institute, E Thorson, The City College of New York, US; I Sarmiento, Universidad Nacional de Colombia, CO
This research conducts an empirical investigation on the temporal stability of parameters of freight demand models in the short-medium term using seven national freight origin-destination samples collected in Colombia during the years 1999 to 2005.
This research conducts an empirical investigation on the temporal stability of parameters of freight demand models in the short-medium term using seven national freight origin-destination samples collected in Colombia during the years 1999 to 2005. The paper considers freight generation, freight distribution, and empty trip models. To identify time-dependent effects, models were estimated using a panel formulation with time-dependent parameters and fixed time effects, and compared with the corresponding cross-sectional models. The results indicate the presence of statistically significant time-dependent effects on all freight generation models (production and attraction), freight distribution models (both based on loaded vehicle trips and commodity flows), and empty trips models. The literature review conducted indicates that this article is, if not the first, one of the few publications studying temporal stability of parameters.
The results indicate that the amount of cargo produced by a unit of Gross Domestic Product (GDP), and the amount cargo attracted by a unit of population have increased overtime. This indicates a lowering of the unit value of the cargo transported, and an increase in the amount of goods consumed by the citizenry. The analyses of the freight distribution models indicate that the models estimated with loaded trips exhibited larger time-dependent effects, than the ones estimated using tonnage. This reflects the fact that it is easier to change loaded trip patterns?which are the outcome of logistic decisions?than to change the patterns of the tonnage distribution matrix that reflects the geographic aspects of production and consumption. The empty trip models also exhibited time-dependent effects, though much smaller than in the other models.
The results show a remarkably consistent pattern in that the components of freight demand that could change faster (i.e., freight production and attraction) are the ones that exhibit the largest rates of parameter change. The rates of change for these models are 18.29% and 26.37% respectively. In contrast, the freight distribution models of loaded trips were found to change less rapidly (10.50% and 1.94% depending on the impedance function); while the tonnage distribution model only exhibited fixed time effects. The model that changes less rapidly is the empty trip model, with a rate of change of 0.83%. In conclusion, the estimation of the aforementioned models indicates that they exhibit time-dependent effects.
Association for European Transport