Comprehensive Modeling Framework to Integrate External Trips in a Travel Demand Model

Abstract

Travel demand modeling studies often require external trips data for the model validation. This paper presents a framework to incorporate nonresident external trips within a travel demand model (TDM), whether it is a four-step model or an activity-based travel framework. This paper first describes the framework components for defining an external region and a series of interlinked models that are joined in a developed simulation platform to estimate external trips. Then, generation and distribution models for external-external (EE), external-internal (EI) and also nonresident non-home-based (NR-NHB) trips are described. The models also take into account the time-of-day (TOD) component for three essential motives: (1) to achieve more consistency in the origin-destination (OD) matrix of the external trips with the OD matrix of the internal trips for route assignment; (2) omitting the need to fit the TOD curves to disaggregate trips from 24-h trip tables into respective periods; and (3) providing an opportunity to better estimate NR-NHB trips which depend on the TOD because these trips are performed far from the home location and the number of such trips may decrease as the time progresses in a day, especially after midday. Furthermore, to minimize the need for any extra data collection, the models were developed using household travel survey data and open-source land-use data. The results suggest the suitability of the data sources to model external trips. Furthermore, the TOD-related variables, including its interaction effect with other explanatory variables, were found to be significant in many models, which signifies the importance of including a TOD component in modeling external trips. Model validation also confirmed the soundness of the proposed models. The paper, therefore, is beneficial for the practitioner community to estimate external trips and incorporate them in the travel demand model estimation.