Fix crash when downgrading road bridge when realistic braking enabled

src/tunnelbridge_cmd.cpp

@@ -483,7 +483,7 @@ CommandCost CmdBuildBridge(TileIndex end_tile, DoCommandFlag flags, uint32 p1, u
 			return_cmd_error(STR_ERROR_AREA_IS_OWNED_BY_ANOTHER);
 		}
 
-		if (_settings_game.vehicle.train_braking_model == TBM_REALISTIC && GetBridgeSpec(bridge_type)->speed < GetBridgeSpec(GetBridgeType(tile_start))->speed) {
+		if (transport_type == TRANSPORT_RAIL && _settings_game.vehicle.train_braking_model == TBM_REALISTIC && GetBridgeSpec(bridge_type)->speed < GetBridgeSpec(GetBridgeType(tile_start))->speed) {
 			CommandCost ret = CheckTrainInTunnelBridgePreventsTrackModification(tile_start, tile_end);
 			if (ret.Failed()) return ret;
 			for (TileIndex t : { tile_start, tile_end }) {