Iván Sarmiento

Time-Dependent Effects on Parameters of Freight Demand Models: Empirical Investigation

Seven national freight origin-destination samples collected in Colombia from 1999 to 2005 were used to conduct an empirical investigation of the temporal stability of parameters of freight demand models in the short to medium term. Freight generation, freight distribution, and empty trip models were considered. To identify time-dependent effects, models were estimated with a panel formulation with time-dependent parameters and fixed time effects and then 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 (based on both loaded vehicle trips and commodity flows), and empty trip models. A literature review indicates that few studies are available on the temporal stability of parameters. 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 those 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%, respectively, depending on the impedance function), while the tonnage distribution model exhibited only fixed time effects. The model that changes least rapidly is the empty trip model, which has a rate of change of 0.83%.

Network Flow Methodology for Estimating Empty Trips in Freight Transportation Models

Commodity-based freight transportation models, for example, the Freight Analysis Framework, transform volumes of commodities into traffic flow by the application of vehicle use factors. However, these models cannot accurately capture the number of empty trips that result as a consequence of the movement of these commodities. Previous research has proposed a number of methodologies to incorporate empty trips as a function of loaded trips and some assumptions about the trip chains. Parameters in these models are calibrated with extensive origin–destination surveys. However, freight transportation data are usually scarce or not available in proper form for use in models. This paper presents a novel methodology to estimate the number of empty trips on the basis of a time-expanded network flow approach that captures the behavioral and operational characteristics of the carriers. The model was formulated as a linear optimization problem that minimized the system costs associated with truck trips. The model was validated with a commercial vehicle survey from Colombia. The results showed that under some operational assumptions, the model appropriately represented the amount of empty vehicle hours in a freight transportation network.

Important Aspects to Consider for Household Travel Surveys in Developing Countries

Characteristics and aspects to be considered in conducting household travel surveys (HTS) in developing countries are discussed. The main differences with the HTS conventional approach (interviewing people by telephone or mail) are highlighted. These topics are covered in the context of a case study that presents the HTS methodology used in the metropolitan area of Medellín, Colombia, in 2011 and 2012 with a sample of 20,000 face-to-face interviews. Results of the HTS are presented and analyzed for the case study. A low number of trips per inhabitant, 1.7, was found. A modal split is relevant in a city that restricts car use on the basis of license plate numbers during all weekday peak periods and that has a large number of taxis, nearly 8,000 per million inhabitants. Also shown are the difficulties that the survey interviewers experienced: planning process, accessibility to households owing to different social aspects (e.g., fear, social events, and international sporting events), and security concerns, among others. Finally, challenges for future modeling are presented.