I'm not sure I read this here, but this site is the most likely suspect. The main character is a man in say his 30s traveling to I think Florida. He sees a young female hitch-hiker. I think he sees her twice, once when he's getting off the highway to eat and again when he's getting back on the highway. The second time she's holding a sign that says "I will blow you for a ride to XXX," with XXX being Florida or whatever state the guy is driving to.
He's surprised and picks her up, but tells her she doesn't have to go through with it. The rest of the story is them falling for each other. When they get to Florida, he thinks he's lost her because the boyfriend who dumped her and caused her to hitch is there waiting. He leaves but she tracks him down to his house.
Ring any bells for anyone? I'd have guessed it was by Cat5, but apparently not.