Hmmm, the versions I heard as a kid were a lot scarier... generally the hitch-hiker was male... and by the end of the story the driver was dead. Driven off the road in the same place where the hitch-hiker died, distracted in the rain, or just plain gobbled by the evil presence... Though some variations had the hitch-hiker just disappearing. It was never nice.

Of course, such stories were told by adults to kids to a) discourage hitch-hiking and b) discourage picking up hitch-hikers. Like it did a _bit_ of good. I spent most my late teen years (15-18) getting around on my thumb.