Speed limits pop up in Android Auto when using Google Maps
Reddit (via XDA Developers), new speed limit icons should pop up in the bottom right corner of the map.
Some claim they've had the feature in the United States for several months now, while others outside the US have just noticed it. The way Android Auto rolls out new features and improvements is very weird, so it's hard to say when and where speed limits will show up for everyone.
One thing is clear though, Google has started a wider rollout of this Android Auto feature, which means it will eventually arrive for everyone, so keep checking the app if you don't have it yet.