Call the number that you used to schedule your phone interview with and explain the situation to the person who answers the phone. Just tell them you were unaware of something on your record and you're unsure if it's a traffic violation or not and you just want to cover your bases. During the interview they don't tend to let you talk about specifics, but they'll make you fill out some paperwork. I think this is for a background check for before you get down there. It's better to disclose all information ahead of time.
Something similar happened to a roommate of mine. She was unaware of a charge on her record when they did her background check. She ended up having to go down to casting and straighten it out later on. So nothing terrible happened, but it was a big mess and inconvienant.
