An Investigation of the Transferability of Trip Generation Models

The cost of collecting data for travel demand modeling is high and increasing each year. Data collection costs could easily exceed the annual budget of a metropolitan planning organization (MPO) in a small or medium-sized area. Many of these agencies borrow or transfer data and/or models from other areas since they cannot afford the cost of collecting local data. A study with the objective of testing the appropriateness of transferring commonly used trip generation models from one urban area to another under specific circumstances was recently completed. The issue of the transferability of trip generation models is real and is faced by many urban areas especially those of small and medium size. A review of literature found that at the present time the appropriateness of transferring trip generation relationships from one area to another is still in question since previous studies have had mixed findings. The quantitative analyses of the study included data from four separate travel surveys, with data from 11 study areas in two states, and four trip purpose models in each area, resulting in the development of a total of 44 cross-classification models. The analysis was based on a comparison of cross-classification tables developed using the number of vehicles available and household size or number of household workers. These models are typical of those used by many small and medium-sized metropolitan planning organizations around the country. Comparisons between pairs of study areas were then made for each of the four trip purpose models. All study areas within each size category, irrespective of their state were compared to one another, and each study area within a state was compared to the other study areas in the same state. Forty area-to-area comparisons were made with four trip categories in each case for a total of 160 comparisons. This paper will present the findings of this research and offer some insights into the issue of the transferability of trip generation rates that will be helpful to planners at small and medium-sized MPOs. Though this research did not definitively answer the question of “is the transfer of trip generation models from one area to another statistically valid?” it will provide a valuable addition to the body of literature of the subject that both researchers and practitioners will be able to draw from in the future.