Need help with text to speech

I just got a bluetooth headset and I have it connected to my phone. My question is for when I'm driving I'd like it to be able to speak my incoming text messages to me through my headset.

Settings>Voice input & output>text-to-speech settings

Here I'm looking for the "Driving mode" option that I've seen in videos and screenshots, it does not show up for me. I believe this is what I want enabled for what I am looking for. Can anyone help? Thanks!