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, same as Scenario Editor