Evaluation of Methods for Estimation of Intrazonal Travel Impedances

The need to account for intrazonal trips in macroscopic travel demand models has long been recognized, but the representation of all trip ends within a zone by a single centroid limits the determination of intrazonal interactions. There are no paths over which intrazonal impedances can be measured, so approximate methods have been used for the estimation of intrazonal impedances. This study proposes and tests the suitability of a new method that uses network nodes as trip ends and calculates the average trip impedance within a zone. Dachau, Germany, is used as a case study, and two approaches are explored to test the suitability of the methods. The first approach compares the estimated intrazonal distances to the reference distances computed between the building blocks within each zone; the second approach compares the estimated number of intrazonal trips to the surveyed number of intrazonal trips for different demand groups. The results show that the proposed method provides better estimates of intrazonal distances than the existing methods. However, a look at the estimated number of intrazonal trips indicates that better estimates of intrazonal impedances do not guarantee better estimates of intrazonal trips. Efforts to improve the estimation of intrazonal impedances should therefore be diverted toward refining zone systems to alleviate the need to deal with intrazonal trips.