Robert Downey Jr. and Zack Galifianakis bicker and bond during a long road trip in “Due Date,” a retread of “Planes, Trains & Automobiles” that doesn’t admit the paternity but definitely comes up way short by comparison. This movie from Todd Phillips (“The Hangover”) is one excruciatingly unfunny journey.

Downey plays Peter Highman, an architect finishing up a job in Atlanta and preparing to fly back to Los Angeles in time to be with his wife Sarah (Michelle Monaghan), who’s scheduled to undergo a C-section and deliver their first child. But at the airport he’s unfortunate enough to literally bump into Ethan Tremblay (Zach Galifianakis), a prissy doofus traveling to Hollywood with the hope of getting into television—especially the show he loves, “Two and a Half Men” (not coincidentally a Warners property). He’s accompanied by his dog, which he carries in a shoulder-bag, and a coffee can that—we later learn—contains the ashes of his recently-deceased father.

By foolishly saying things about bombs and terrorists, the intrusive, oblivious Ethan (who’d already managed to accidentally put his weed—medicinal, he calls it—into Peter’s luggage) gets them both tossed off the plane and put on a no-fly list. Peter, whose money, credit cards and IDs are in his checked baggage, has little choice but to accept a ride from Ethan in his rent car.

As you’d expect, their long trip does not go well. They stop to buy Ethan replacement drugs from a Birmingham dealer (a limp cameo from Juliette Lewis). They have a violent run-in with a mean-spirited Western Union clerk (Danny McBride). After a crash caused by Ethan, they get help from a friend of Peter’s in Dallas (Jamie Foxx), whom Ethan inanely suggests may be too close to Sarah, leading Peter to become jealous. And they make an accidental detour into Mexico, which becomes the occasion for a big set-piece, the obligatory car chase.

What’s amazing about all these episodes is how dark, mean and humorless they all are, especially the sequence with McBride, which is more nasty and sour than amusing; the Foxx sequence, the big joke in which is literally tasteless; and the one featuring Mexican border guards, which is based on crude stereotypes and prejudices. With Downey and Galifianakis on hand, one would expect comic sparks. But the characters they play are so obnoxious, and the performances so shrilly one-note, that they never elicit a viewer’s sympathy. They wind up as friends, of course, but that seems like nothing more than a necessity of the script, since we never feel them actually growing close. (A scene at the Grand Canyon, where they finally come together, is especially unbelievable.) And though they supposedly warm to one another, we certainly don’t warm to them. The result is that there’s nothing to root for, except for the movie to end.

Among the rest, McBride again fails to invest a brutal character with any saving quality, Foxx seems to be coasting, and Monaghan is simply bland. RZA does a cameo as a airport security man; it’s supposed to be a cool moment, but like so much in the picture is nasty instead. On the other hand, when it goes for sentiment, the movie is totally phony. And of course the dog’s on hand to provide not only the inevitable slew of reaction shots but one of the movie’s grossest moments, too.

“Due Date” doesn’t even look particularly good; the locations don’t much resemble the different places along the route they’re supposed to, and Lawrence Sher’s cinematography gives everything a dank, depressing appearance, especially in the night scenes.

All Phillips’ effort accomplishes, in fact, is to remind us how much better John Hughes, Steve Martin and John Candy did this nearly a quarter-century ago. The best advice is just to watch “Planes, Trains & Automobiles” again and keep this stink-bomb off your itinerary.