Discrete Mode and Discrete or Continuous Shipment Size Choice in Freight Transport in Sweden
G de Jong, ITSm University of Leeds, UK / Significance and NEA, NL; D Johnson, ITS, University of Leeds, UK
The paper will present estimation results, elasticities and values of time for different specifications of disagreggate models for the choice of mode and shipment size, based on the 2001 Swedish Commodity Flow Survey
Discrete mode and discrete or continuous shipment size choice in freight transport in Sweden
Gerard de Jong ? ITS Leeds, Significance and NEA
Daniel Johnson ? ITS Leeds
Mode choice in freight transport is usually studied in isolation (or in combination with network assignment, as multi-modal assignment). However, mode and shipment size are closely linked decisions. Large shipment sizes usually coincide with higher market shares for non-road transport, whereas there is a high correlation between road transport and small shipment sizes. Previous models that combined both choices in a single model include McFadden et al. (1985) and Abdelwahab (1998). These authors estimated joint discrete-continuous models, where mode choice is the discrete and shipment size the continuous choice. Other authors (e.g. Chiang et al. 1981, de Jong, 2007) have studied this joint decision-making problem as two simultaneous discrete choices, classifying shipment size into discrete categories. This paper will study whether it makes a difference for the main policy outcomes of the model (notably the transport time and cost elasticities per mode) whether the disaggregate model is specified as:
? An independent discrete mode choice model (which is the most common formulation)
? A joint model with discrete mode and discrete shipment size choice
? A joint model with discrete mode and continuous shipment size choice.
The Swedish 2001 Commodity Flow Survey is a unique data source in Europe. It details about almost 1 million individual shipments to or from a company in Sweden, with information on origin, destination, modes used, weight and value of the shipment, sector of the sending firm, commodity type, access to rail tracks and quays, etc.. Whilst the US Commodity Flow Survey has been analysed several times, its Swedish counterpart has barely been used for model estimation so far. Using this Swedish CFS mode and shipment size choice at the individual shipment level can be explained from characteristics of the shipper, the shipment and transport time and cost on the networks.
Earlier work (De Jong, 2007) estimated both mode and shipment size as discrete choices, but clearly shipment size is a continuous variable. The paper further develops this earlier work to simultaneously estimate shipment size (as a continuous variable) and mode choice (as a discrete variable) using full information maximum likelihood estimation methods for sample selection problems applied to the Swedish 2001 Commodity Flow Survey.
Both assuming independence between mode and shipment size choice and discretising the continuous information on shipment size may be interpreted as forms of specification error. The question is how important this will be for the results of the model. The paper will present estimation results for all three specifications distinguished above. Mode-specific transport time and costs elasticities of the mode shares and values of time from the joint discrete/continuous model will be compared against those from the independent discrete mode choice model and the joint discrete choice model. This will show whether these differences in specification lead to differences in the elasticity values and values of time - the model outputs that are typically used to evaluate transport policies.
Abdelwahab, W. (1998). Elasticities of mode choice probabilities and market elasticities of demand: evidence from a simultaneous mode choice/shipment-size freight transport model. Transportation Research E, pp. 257-266.
Chiang, Y., P.O. Roberts and M.E. Ben-Akiva (1981.) Development of a policy sensitive model for forecasting freight demand, Final report. Center for Transportation Studies Report 81-1, MIT, Cambridge, Massachusetts.
Jong, G.C. de (2007) A model of mode and shipment size choice on the Swedish commodity flow survey. Paper presented at UTSG 2007, Harrogate.
McFadden, D.L., C. Winston, and A. Boersch-Supan (1985). Joint estimation of freight transportation decisions under non-random sampling. In E.F. Daughety (Ed.) Analytical Studies in Transport Economics. Cambridge University Press, Cambridge.
Association for European Transport