Only recently have travel models advanced to tour based models that recognize that all trips are not made from the home, but that people 'run errands', and combine several trips into a single tour. This has implications not only for automobiles but for all modes.
Every trip begins and ends with a walk. It is not always a very long walk, but it always exists. Every transit trips begins with a walk to the station/stop. And ends with a walk from the station/stop to the final destination. For automobiles, the initial walk distances is typically very short, as most people park closely to where they live. But the walk after parking, across the parking lot, or out of the parking garage, may be quite long. In both cases, the portion of trip spent in-vehicle is only part of the journey.
It would be better if each portion of that journey was considered a trip on a tour. Transportation models ought to consider pedestrian trips, and pedestrian scale transportation networks will require further developments in this area. Considering only the in-vehicle time ignores the significance of the walking portion of these trips, and the significance of the walking environment.
Much of the research on roads is on the effect of pavement quality on travel speed. There should likewise be extensive research on the effects of the quality of the pedestrian environment on both the likeliness to walk and the distances walked. This research will have two strands: The first geometric, measuring network qualities such as connectedness and the directness of paths through the network; the second qualitative, measuring the quality of the travel path.
Secondly, for development considerations, it would be better to consider each portion of the trip (Walk, Vehicle, Walk) as a separate trip. Specifically for purposes of retail gravitation. If the vehicle access point (station/stop/garage) is a trip end, then centers where a greater number of trip ends concentrate should be more attractive. Where someone catches a train (or keeps their car) may vary by virtue of what else is there.