Driving Mode Setting on iPhone

I don't want to be distracted by text messages while driving. It is also against the law text while driving. Good idea - driving mode. However, it's OK to call, as I use a handfree device, just like chatting with a passenger sitting next to me. I allow anyone to call, in my address book or not. But, if I understand it correctly, I won't receive any indication that someone is trying to call me unless the phone is unlocked. To preserve my battery, the screen is automatically locked in 1 minute. I would think, most of the people have this setting. So, what good would "allow call from anyone" when I won't know when to pick up the call??