How to get the voice activated gps directions on my iphone 4s to work while driving?

How do i get the voice activated gps directions on my iphone 4s to work while driving?

Does it work while not driving?
I'll assume it doesn't work while not driving either.
Open the app (Maps) and hit the arrow button at the top left. Enter the destination in the box "End" Let it think a few a few seconds and at the top right, when the "Start" appears, tap it. It shoul start to navigate, and give you voice instructions.
You can even ask Siri "directions home" and it will go through all the motions for you.