(svn r20230) -Fix [FS#3961]: road vehicles could be dead locked with one way roads. This allows one wayness to be removed if there are vehicles on a tile; it does not allow you to add one wayness to roads that have vehicles on them as it makes turning vehicles jump

src/road_cmd.cpp

@@ -499,12 +499,21 @@ CommandCost CmdBuildRoad(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32
 								if (ret.Failed()) return ret;
 							}
 
-							CommandCost ret = EnsureNoVehicleOnGround(tile);
-							if (ret.Failed()) return ret;
+							DisallowedRoadDirections dis_existing = GetDisallowedRoadDirections(tile);
+							DisallowedRoadDirections dis_new      = dis_existing ^ toggle_drd;
+
+							/* We allow removing disallowed directions to break up
+							 * deadlocks, but adding them can break articulated
+							 * vehicles. As such, only when less is disallowed,
+							 * i.e. bits are removed, we skip the vehicle check. */
+							if (CountBits(dis_existing) <= CountBits(dis_new)) {
+								CommandCost ret = EnsureNoVehicleOnGround(tile);
+								if (ret.Failed()) return ret;
+							}
 
 							/* Ignore half built tiles */
 							if ((flags & DC_EXEC) && rt != ROADTYPE_TRAM && IsStraightRoad(existing)) {
-								SetDisallowedRoadDirections(tile, GetDisallowedRoadDirections(tile) ^ toggle_drd);
+								SetDisallowedRoadDirections(tile, dis_new);
 								MarkTileDirtyByTile(tile);
 							}
 							return CommandCost();