I do almost burst out laughing. Usually the person means well, so I don't tend to take offense. I usually reply "I bet I'm a more attentive driver, I can't use a cell, I'm ALWAYS checking my mirrors, I'm always aware of drivers around me, etc". It usually takes about 2 seconds for the person to reflect on his/her inattentive driving to realize they would be safer in a car with me any day.
