Like I said I don’t have a definite answer. You’ll have to ask one of the devs for that one. 🙂
I just know that every person that spawns in the game world have four preset destinations. A home building, a Workplace, a shopping destination and a place of leisure. These can be in different cities – live in one city, work in another and so on)
I’m just basing my assumption on observing my lines casually – I don’t really see people hanging back when the vehicles arrive. But then again I don’t really have a lot of lines that would allow for a detour – Only a few bus lines in my starter cities.