On my ride this past Sunday I pulled up behind a mid-size sedan waiting at a light red. I was positioned mid-lane so he could clearly see me in his rear view mirror. I noticed the driver was trying to find something on the passenger side seat or floor. When the light changed, he was still looking under seats, didn't see the green light and didn't move. So I did what any other vehicle would do. I waited a few seconds and then gave him a couple of loud dings of my bell. His head pops up suddenly, startled, he looks at me in his mirror, looks at the green light, and gives me a little "I'm sorry" wave before finding his gas pedal and going through the intersection so we could all be on our way.
I figure if I hadn't done it, one of the cars behind me probably would have.