Trying to find a story

I have spent hours searching this site trying to find a story I read several years ago. It is told from a college-age girl's POV. In the story, they take off for the beach (or maybe campground?) in the back of a pickup truck, wearing bikinis as I recall. At one point, the girl's best friend fucks a guy standing up, holding onto the roll bar, with traffic around them. I didn't think enough of the story at the time to favorite it, but I have to confess that the mental image of that scene has been stuck in my head ever since I read it.

I realize it's a longshot, but does anybody recall this and know the title or author?