diff --git a/src/openttd.h b/src/openttd.h
--- a/src/openttd.h
+++ b/src/openttd.h
@@ -56,9 +56,10 @@ enum TransportType {
 	 * the values for road and rail.
 	 */
 	TRANSPORT_BEGIN = 0,
-	TRANSPORT_RAIL = 0,
-	TRANSPORT_ROAD = 1,
-	TRANSPORT_WATER, // = 2
+	TRANSPORT_RAIL = TRANSPORT_BEGIN,
+	TRANSPORT_ROAD,
+	TRANSPORT_WATER,
+	TRANSPORT_AIR,
 	TRANSPORT_END,
 	INVALID_TRANSPORT = 0xff,
 };