Aaron asked me why I looked frightened. What was I supposed to tell him? He was my friend, and he had rebuilt his car by himself. He was so proud of it. He always talked about how the car was over 70 years old, as if that excused all the dents and the problems with the motor. As I looked over at him, I wanted to say, "The problem is that I don’t like barreling down the road in a tin can on wheels," but I wanted to keep the peace, so I said, "I’m not scared. Keep driving!" What does the author mean by the figurative phrase "barreling down the road in a tin can on wheels"?