Trip Length Distributions in Commodity-Based and Trip-Based Freight Demand Modeling: Investigation of Relationships

Commodity-based and vehicle-trip-based freight demand modeling is discussed. The characteristics of the trip length distributions (TLDs) are examined, defined in terms of tons, as required in commodity-based modeling, and in vehicle trips, as required in trip-based modeling. With data used from a major transportation study in Guatemala, the TLDs are estimated for both tons and vehicle trips. The analysis revealed that ( a) the shape of the TLDs depends upon the type of movements being considered; ( b) TLDs defined in terms of tonnage differ significantly from those defined in terms of vehicle trips; ( c) TLDs for different types of vehicles, transporting similar commodities, reflect the range of use of each type of vehicle; ( d) though tons TLDs and vehicle TLDs are different, the relationship between them seems to follow a systematic pattern that, if successfully identified, would enable transportation planners to estimate one type of TLD given the other; and ( e) major freight generators affect the shape of the TLDs, so complementary models may be needed to provide meaningful depictions of freight movements.