Discovering the unknown can dredge up a variety of fears, and where best to meet the unknown than on a road trip? Instead of snacking on gas station food and singing along to the radio, these stories take us on a literal journey as the characters try to escape the monsters that follow them.
Books:

The Talisman by Stephen King and Peter Straub follows a young boy who needs to travel across America. He can travel faster if he goes into a realm reminiscent of the Upside Down, but it’s a whole lot scarier…

Night Road by A.M. Jenkins involves two vampires on a road trip, one meant to train the other. I thought this was a great original take on vampires.

Winnebago Graveyard by Steve Niles is a graphic novel that uses many of the common tropes to the road trip horror story, when a family’s Winnebago is stolen, leading them into a town full of Satanists.
Movies:

The Hills Have Eyes (2006) has another family in a camper, who must survive the night when it breaks down and the inbred mountain people come out to attack them.
Wrong Turn (2003) is the quintessential road trip horror movie, about a group of friends who find themselves on the highway to hell. And also discover some inbred mountain people.

The Hitcher (1986) will teach you never to pick up a hitchhiker. I mean, if you’ve seen any horror movies at all you should know this, but just in case, this movie will cure you.
There are so many good road trip horror movies out there! What’s your favorite?