Happy Trails, Marty!Doc, as Marty travels back to 1885
He [Marty] closed the [car] door again as Doc stepped out of his path. Marty put the DeLorean in gear and turned the car across the asphalt and gravel, toward the drive-in screen. / "Happy Trails, Marty!" Doc called after him. "Vaya con Dios!" / Marty waved and shifted the DeLorean up through the gears. He aimed the car for center screen and floored the accelerator. From Back to the Future Part III by Craig Shaw Gardner (quote, page 37)
"Happy Trails" was a film quote from Westerns, used by cowboys to wish each other a safe journey.