Proper planning will get you far, but sometimes life happens and you'll need a plan B. Think about some of the variables in your life that could affect your attendance at work. Sick children, traffic, car breakdowns... the list goes on. Most likely one of these issues has affected you before. How did you handle it? It may not be possible for everything to work out so smoothly, getting your shift covered or still making it to work on time. Your interviewer is looking to see that you are proactive and that you can handle the stress of unexpected situations that can arise.
"I stay calm and composed when issues arise. I got rear ended one time on the way to work. I called my boss immediately and then called one of my co-workers to see if they could fill in for me for the first couple hours of my shift."