Why are you on the move?
It's a fair question. Why ARE the PCs moving around so much? I've run campaigns where no one ever asked the question, because the answer was, because that's the format these adventures take, but you owe it to yourself to do better. They're probably like migrant workers, looking for opportunities for coin and experience. Maybe they're like students out of college, backpacking through Europe. Nothing wrong with a wandering spirit, but in that case, are they planning on returning? Is home your true destination? The starship from the earlier example might be on a mission to explore and report back. Or are the PCs being chased by someone - the authorities, an angry father whose daughter has been jilted, a powerful villain seeking revenge, a toxic environment encroaching on once fertile homelands? Are the characters part of a race? And it doesn't have to be the same for each character, obviously. Sometimes, there's a clear destination (a specific quest), but even if you're just going to wander the game setting, a focused motivation can give the story momentum and open up interesting avenues for role-playing.
Who leaves, and who arrives?
The same characters, hopefully, but will they be the same people at both the start and the finish line? The point of most RPGs, on a player satisfaction level, is to increase in experience. The rookies leave home and come back heroic veterans. Or is that really the story you want to track? As a player, you might want to change it up, and angle your choices towards different outcomes. Let's look at specific road movies here. Thelma & Louise sees two women embark on a crime spree and reach the literal end of their characters. Where COULD those characters go once they crossed the point of no return? Nebraska and Chef are about fathers and sons connecting in a meaningful way; is that something your characters would be interested in? I've had players play three brothers, two of them estranged, with the third trying to keep the family together. The relationships were the real journey, no matter where they actually went together. What lesson does your character NEED to LEARN? That's a good starting point for imagining where he might end up.
When there's a specific destination (Mount Doom, say), it would be easy to design the journey as a straight line of challenges on the way there. But you should allow your players (and yourself) to take detours, to stay in one place for a time, or even to not have a clear destination at all. (Obviously, this depends on the reason for the journey; don't pull a Frodo and wait a year before going off to destroy that Ring; that stuff's important!) The point I'm trying to make is that the journey itself can be its own point. In two of the movies I watched last week - Tracks and Wild - a young woman sets out to find herself by undertaking an dangerous trek through a wilderness. Do the characters want to be tested? Are the players brave enough to start out as ciphers and let the journey mold them into full-fledged characters - or even PERSONS - each experience and choice building that character?
When it's not important, fast forward
Lastly, if the destination is all that's important, and in some scenarios, that's surely the case, don't be afraid to skip ahead. Sometimes, all you need is a plane jetting off, and a plane landing elsewhere. Or screw that. All you really need is a shot of Paris with a card that says "Paris, France". And hey, you don't even need that last part, do you? Go where the story is. If it's not on the road, we don't need the campfire strategies or the random encounters. "Some weeks later..." and the characters are where the action is. In other words, it's not always a road movie.
I'll end on a list of some favorite road movies which haven't been mentioned or alluded to yet. For extra credit, ask yourself why the characters of these movies are on the move, and how they're changed by the experience. Maybe you'll find the key to your own picaresque campaign...
Mad Max Fury Road - National Lampoon's Family Vacation - Run Lola Run - Smokey and the Bandit - Life of Pi - The Adventures of Priscilla, Queen of the Desert - The Darjeeling Limited - The Road - Sideways - The Love Bug - Little Miss Sunshine - Planes, Trains and Automobiles - Broken Flowers - Dogma - Bring Me the Head of Alfredo Garcia - O Brother Where Art Thou? - Easy Rider - Monsters - The Muppets - The Motorcycle Diaries - The Adventures of Baron Munchhausen - The Wizard of Oz - Duel - Bonnie & Clyde - Stand by Me - Zathura - Gravity - Interstellar - Apocalypse Now... and many, many others.