First of all, Sal is terrified of cars, planes, buses – anything that moves, really. Over time, we realize that her fear might stem from the fact that her mother was killed in a bus accident.
But there's a bigger issue in play here, and that's what cars allow us to do. We use cars and other forms of transportation to go on journeys. A car takes Sal all the way across the country just so she can say a proper goodbye to her mother. And even though a bus accident has killed Sal's mother, Sal is able to conquer her fears by driving a car all by herself to the site of her mother's death. As scary as cars are, where would Sal be without them?
While Sal and her mom go on their own journeys by moving from place to place, it's also worth noting that some people, like Phoebe, go on a journey having never left home. So while cars aren't totally necessary to have an awesome journey, they certainly help.
Can you think of any other ways in which cars or other modes of transportation are related to people's journeys in the story?