I recently purchased a Google Pixel XL 32g phone. It is by far the best phone I've owned.
It is great company on my daily commute. The Android Auto app warns me of traffic, plays my podcasts for me, and lets me dictate text messages. I've combined it with ReadItToMe so that my incoming text messages are read aloud.
I noticed on my first afternoon commute though the Do Not Disturb icon was turned on on my Android watch. At the next traffic light I discovered that Do Not Disturb was on on the phone as well.
A Google search at home found I was not alone in noticing/suffering this. The Android Product Forum had mention of it.
People first reported this back in November 2016, so hopefully a fix will be coming.
Meanwhile, I worked out a workaround using AutoMagic.
The flow below detects the launch of Android Auto and then turns off Do Not Disturb by setting the Ringer on, and turning the audio volume up. I have tested it briefly here at home, but not yet on a commute.
Update: 3/10/2017 A link to the XML file to make this flow in AutoMagic.
Update: 3/14/2017: I am finding this does not always work reliably. Use at your own risk. It may also interfere with navigation. Testing continues...