From the Houston Chronicle:

"Man killed after falling from pickup on freeway
Copyright 2003 Houston Chronicle

A man was killed Friday night when he fell out of a pickup on the Southwest Freeway and was run over by it and other vehicles, police said.

The man, identified as William Thayer, 23, was riding in a passenger seat in a Ford pickup when he opened the door to urinate on the freeway while the vehicle was moving at freeway speed, police said."

Now, this is just an educated guess, but I bet there was liquor involved.