Auto response to text messages while driving?

All the other phones can do this. How do I set it up on an iPhone? I am sure it is something simple I am just over looking. AT&T has a safe driving app that has all sorts of handy features, but I know we had to wait for apple to build it in since they won't let apps access that functionality of the phone. I thought it was part of the last upgrade, but I can't find it.