Take a good, long look at the big, steaming pile of movies in which a mismatched pair has to travel a long distance in pursuit of different goals, hijinks ensue, and they learn something about themselves along the way. Then take “Due Date” (2010) and toss it on that pile. Zach Galifianakis is better in smaller doses. Robert Downey Jr. can’t carry the movie by himself. It has some funny sidetrips but it hits all the cliche plotholes that make “road” movies interchangeable and unmemorable. Preposterous vehicle wrecks. Inexplicable epiphanies. Injured people who keep falling/crashing but don’t get hurt worse.