I had a bike partially fall off of a rear trunk rack on the interstate. I had no idea, but another car pulled up beside me (at 70mph) and pointed and honked. I figured it out.

I put my bikes inside my car when I'm driving alone. When I travel with my BF, I use the roof rack, but with two of us we're less likely to forget that they are there.

It helps that I don't have a garage and don't do drive-through dining!