Go to Settings, focus the search text box at the top of the screen. Enter “driving”, it should show “DND while driving”. Turn that on. Not much of a deterrent, because there's a "I'm not driving button" when you try and use your phone. That's understandable, since I doubt "Fuck everybody else, I've got toots to post!" would fit in the modal, and localization would be difficult.

I also thought it was the default, but obviously not.