A company can build a RoadSubType when:
- A vehicle for it is available for purchase to the company
- The company owns a vehicle for that RoadSubType but the vehicles are expired (like RailTypes)
- The RoadSubType doesn't have vehicles bound to it but is compatible with other RoadSubTypes, like OFFR/DIRT, HWAY

Possible other cases:
- Scenario Editor, it should be possible to build every RoadSubType at any time (as it works with industries, bug?)
- Cities, should be able to build which RoadSubType they want (still missing support for it, only build the first one) but based on introduction date?
- GameScripts, they should be able to ask what kind of availability they need