I have a pretty good idea of how this went down. This particular commuter got into his car with his arms full, carrying a Pepsi. He probably put the Pepsi on the roof of his car while he fished out his keys, and put his briefcase, jacket and other items in the back seat. He then got in the front, started his car, and drove off. Forgetting the Pepsi on the roof, of course.
As he drove off, the Pepsi fell over, and rolled around on the roof of his car. Because of his roof rack, it prevented the Pepsi from going off the edge. Instead, the Pepsi merely went back and forth, hitting the rack, and going back in the opposite direction. Due to the low speed limit, there was not enough speed to send the can flying.
One little twist, however. This was my Pepsi. My car. And my commute.
I realized where I had left my Pepsi when I heard a thunk on the roof. I then heard a few more as the Pepsi rolled back and forth, smacking the rack and rolling back to the other side. I even made two turns, and the can continued its back and forth rolling. This was absolutely embarrassing, I couldn't believe that not only I left a soda on my car, but the damn thing wouldn't fall off. I finally made a turn onto a long, straight road which allowed me to get up to a decent speed, and I went from 35 to 50mph pretty quickly. It was at that point I didn't hear any more rolling, and instead, in my rear view mirrow, saw a can of Pepsi flying through the air.
I felt bad about littering. But I felt worse thinking other drivers might have been pointing and laughing at me. And possibly Twittering about it.