1. Subtypes available to specific company at current date
-> Company::avail_roadtype



2. Subtypes available to any company at current date
FOR_ALL_ENGINES_OF_TYPE(e, VEH_ROAD) {
       /* Check if the subtype can be used in the current climate */
       if (!HasBit(e->info.climates,  _settings_game.game_creation.landscape)) continue;

       /* Check whether available for all potential companies */
       if (e->company_available != (CompanyMask)-1) continue;

       RoadTypeIdentifier rtid = e->GetRoadType();
       if (rtid.basetype != rt) continue;

       known_roadsubtypes |= GetRoadTypeInfo(rtid)->introduces_roadtypes;
}

/* Get the date introduced roadtypes as well. */
known_roadsubtypes = AddDateIntroducedRoadTypes(rt, known_roadsubtypes, _current_date);



3. Subtypes available to any company at any date
FOR_ALL_ENGINES_OF_TYPE(e, VEH_ROAD) {
       /* Check if the subtype can be used in the current climate */
       if (!HasBit(e->info.climates,  _settings_game.game_creation.landscape)) continue;

       RoadTypeIdentifier rtid = e->GetRoadType();
       if (rtid.basetype != rt) continue;

       known_roadsubtypes |= GetRoadTypeInfo(rtid)->introduces_roadtypes;
}

/* Get the date introduced roadtypes as well. */
known_roadsubtypes = AddDateIntroducedRoadTypes(rt, known_roadsubtypes, MAX_DAY);