Published evidence for the passive dispersal of land snails, especially of alien species, on cars was the subject of this post. The process is simple: if a car is parked at a spot where there are active snails, some of them may climb on the car and later go wherever the car happens to go.
Here is a piece of evidence that this indeed happens. These pictures of a snail was taken by my friend Selim Dorak on his car in the city of Antalya in southern Turkey. The car was in the parking lot of a store when Selim first saw the snail. He then drove with the snail on the car to his house and thereby demonstrated that dispersal of snails by car, at least over short distances, is possible.
The snail seems to have been a species in the family Hygromiidae. Another possibility is that it was a juvenile Helix, sp. but they usually don't have such pointed spires.