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When measuring the viability of transit and automobile travel options, travelers can be classified into two groups: choice or captive users. Choice users select transit or automobile service when they view one option as superior, while captive users have only one travel option. Surprisingly, little is known about captivity effects on mode split models. This paper examines how transit service factors such as accessibility and connectivity relate to mode captivity and mode choice.

The data for this investigation comes from the Portland, Oregon 1994 Household Activity and Travel Diary Survey, the Regional Land Information System for the Portland Area, the US EPA Fuel Economy Database, and the US Dept. of Energy. Individual trip data were segmented into transit captive, auto captive and choice users based on information about private vehicle availability, transit connectivity and distance from a transit stop. Comparing traditional transit mode split models to captive conditions, the traditional models underestimate variation in mode choice behaviors for captive users, while overestimating the attractiveness of transit for choice users. These results indicate that better transit forecasts can result if accessibility and connectivity are used to help identify captive users. Additionally, among choice transit users, differences in travel times between automobile and transit modes does little to influence mode selection; the more important factor appears to be access to transit and out of vehicle time.