One of the unique aspects of mixed-land use developments is the potential for internal trip capture, defined by ITE as trips between two distinct on-site land uses without traveling on the roadway network external to the site. Although it is acknowledged that some internal trip capture occurs in the mixed-land use environment, there is not a universally accepted approach as to how to best estimate internal trip capture for trip generation estimates used in traffic impact analyses. Some reviewing agencies have adopted methods recommended by ITE; others publish their own methodologies, while others negotiate reductions on a case-by-case basis. In this paper, three methodologies to account for internal trip capture at mixed-land use developments (a fixed-percentage reduction, the current ITE-recommended practice, and a new methodology based on forthcoming NCHRP research) are validated against observed trip generation counts at five mixed-use sites. For the five mixed-use development sites examined in this study, the proposed estimation methodology from the NCHRP research project was found to most accurately replicate the observed trip generation, with average error approximately one-fifth to one-third that of other methods currently in practice. The findings of this paper can be used by practitioners and reviewing agencies to guide the preparation of trip generation estimates for traffic impact analyses of proposed mixed-use developments.
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