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 (d) major freight generators affect the shape of the TLDs, so complementary models may be needed to provide meaningful depictions of freight movements.