Calculating the accessibility (and mode shifts) for levels of transit quality deemed affordable to a given region is obviously a complex exercise that requires an accurate regional model. However, it is possible to estimate the scale of the transit investment that is needed to capture a sizable fraction of auto users.
Downs (1994) did this by calculating the number of TODs needed to accommodate the average population and employment growth during the 1980’s of metropolitan areas with a 1990 population of one million or more. He concluded that TODs could handle the growth if their numbers were large, but that this would require a regional transit system that a city might not find financially feasible.
This leads to the inference that considerable numbers of travelers who live outside of TODs need to be attracted to transit, both for work and non-work trips. Again, this will entail investments in new transit, possibly in the form of neighborhood circulators, e.g., small vans, buses, or personal rapid transit vehicles that deliver passengers to TOD station areas and, with well-coordinated intermodal transfers, to a regional network of TODs.
A number of metro regions have modeLED the transportation performance of various TOD scenarios based on assumptions concerning the portion of new growth that will be accommodated by existing and new activity centers. There is not space here to review these
