How to Stay ahead of traffic
When you create an event with a location in Calendar on your iPhone, you can choose when it should alert you. “Time to Leave” assesses the traffic, and notifies you (including on your Watch) when it’s time to go.
Travel time in Maps
In the Maps app on your Watch, you can look up locations (entering text by scribbling or dictation), and then get walking, driving, or public transport directions. These will show an estimated time for each option.
Siri will help
The Siri watch face is designed to be proactive, and one thing it’s able to do is look at your work address in your contact info, work out what time you normally arrive, and start warning you if traffic conditions will affect that.