Allow changing road vehicle driving side when all road vehicles in depots

src/road_cmd.cpp

@@ -180,14 +180,26 @@ RoadType AllocateRoadType(RoadTypeLabel label, RoadTramType rtt)
 }
 
 /**
- * Verify whether a road vehicle is available.
- * @return \c true if at least one road vehicle is available, \c false if not
+ * Verify whether a road vehicle has been built.
+ * @return \c true if at least one road vehicle has been built, \c false if not
  */
 bool RoadVehiclesAreBuilt()
 {
 	return !RoadVehicle::Iterate().empty();
 }
 
+/**
+ * Verify whether a road vehicle has been built and is not in a depot.
+ * @return \c true if at least one road vehicle has been built and is not in a depot, \c false if not
+ */
+bool RoadVehiclesExistOutsideDepots()
+{
+	for (const RoadVehicle *rv : RoadVehicle::Iterate()) {
+		if (rv->IsFrontEngine() && !rv->IsChainInDepot()) return true;
+	}
+	return false;
+}
+
 static DisallowedRoadDirections GetOneWayRoadTileDisallowedRoadDirections(TileIndex tile)
 {
 	if (IsNormalRoadTile(tile)) return GetDisallowedRoadDirections(tile);

src/settings.cpp

@@ -1483,8 +1483,8 @@ static void MaxNoAIsChange(int32 new_value)
  */
 static bool CheckRoadSide(int32 &new_value)
 {
-	extern bool RoadVehiclesAreBuilt();
-	return (_game_mode == GM_MENU || !RoadVehiclesAreBuilt());
+	extern bool RoadVehiclesExistOutsideDepots();
+	return (_game_mode == GM_MENU || !RoadVehiclesExistOutsideDepots());
 }
 
 static void RoadSideChanged(int32 new_value)