Here is some observations I made with Tropico 3. I don't know how much of this applies to Tropico 4 or 5, since I don't have those games. I imagine the game mechanic is similar if not identical (confirmation appreciated).
1) There are generally two types of buildings: Ones that provide vehicles, and others that remove vehicles, when Tropicans interact with those buildings. For example, a builder can walk into the construction office by foot and leave with a truck. But a teacher can't use the construction office to get a car. Interaction is required.
2) A garage is a special type of building since all Tropicans can just go there and get a vehicle for whatever it is they are doing.
3) Tropicans will pick the fastest way to move where they need to go.
I think the buildings that have a road-access are generally the ones that provide vehicles, regardless whether people arrived on foot or with a vehicle. Some of them are:
- Teamster office
- Construction office
- Furniture Factory
Some buildings that will cause Tropicans to "abandon" their vehicles are:
- Logging Camps
- Apartments and houses
So what does this mean in practice? Some examples will help:
A teamster can arrive at the teamster office on foot, he will then leave in a truck and go to the logging camp. Upon arriving there, he'll abandon the truck, get to the camp, and get the logs. If there is a garage nearby, he'll manually move the logs there and load them into a truck. If there is no garage, he will deliver the logs manually all the way.
A teamster who delivers logs to the lumber mill either by foot or by truck, will abandon the truck there. He'll either walk to his next destination or to a nearby garage to get another car.
A teamster can deliver lumber from the lumber mill to the furniture factory either by foot or by truck, but he will always leave in a truck and go wherever he'll go next (be it home, to the church, back to the office, etc.)
A teamster who delivers anything to the port will leave in a vehicle (I think, needs confirmation). I'm certain that when he delivers in a truck, he'll also leave in a truck.
Some other basic assumptions:
Understanding those principles lets us plot a building strategy to improve transportation routes:
- Houses need garages nearby so teamsters get to the office quickly
- Teamster offices do not need garages nearby, they already provide vehicles
- Some production needs garages nearby so the goods are transported from there on wheels, not on foot
- Public buildings (church etc.) need garages so people can get back to work/home quicker.
- The port doesn't need a garage nearby
- When building on a new part of the island, build a garage first as it allows builders to get back from the build-site and go home much faster.
TL;DR: Build a lot of garages everywhere :)