Analytical derivation and behavioral interpretation of a model combining trip generation and distribution

The disjointedness of the planning sequence of trip generation and trip distribution is the main subject of this paper. We approach this disjointedness problem by analyzing the central properties of the independently discovered balancing methods of trip-distribution models in relation to two critical issues. First, in the current planning sequence, it is usual to start by forecasting how many trips will begin (production) and end (attraction) in each zone. This forecasting is done by estimating the production of trips independently from the attraction of trips and vice versa, and then forcing some mechanical balance of total trips being generated in the urban system. Second, in the same planning sequence, the output of this trip-generation process is the input to the next one (trip-distribution process): forecasting the matrix that describes the number of trips between each pair of zones. This forecasting is done in general by updating an equivalent obsolete matrix that was obtained from an origin-and-destination survey. The updating is generally accomplished by adjusting the outdated matrix with the so-called balancing factors. It is the purpose of this paper to support the balancing-factors approach in forecasting trip-distribution matrices with a methodological interpretation and to explain behaviorally the balancing factors; and in the process, to show the spatial interaction between trip production and attraction and the emerging need for simultaneous specification and estimation of the whole trip-generation process.