diff --git a/src/roadveh.h b/src/roadveh.h
--- a/src/roadveh.h
+++ b/src/roadveh.h
@@ -120,6 +120,7 @@ struct RoadVehicle : public GroundVehicl
 	bool IsBus() const;
 
 	int GetCurrentMaxSpeed() const;
+	int UpdateSpeed();
 
 protected: // These functions should not be called outside acceleration code.