While traveling for a long period of time, and a car catches up to you, but likes your driving and speed enough not to attempt to pass. After miles you look through your rear view mirror to check up on your new road companion.
A typical aspect of road companion is day dreaming conversations with your road companion about other crappy drivers.
Person: How was the driver here?
Traveler: It was pretty long, but I had a pretty cool Mysterious Road Companion to keep me company.