These alerts will be provided as you drive, which will be updated as and when Google is able to do so. Suggested route alternatives will be supplied when creating a journey too. An interesting feature of the upcoming update is Google Maps will be able to estimate how long you may be stuck in said traffic should you ignore alerts and continue on your affected journey.
"While you're on the road, Google Maps will give you a heads up if congestion lies ahead, and how long you'll be stuck in a jam. You'll also get the option to take alternate routes, including explanations for why one is recommended—whether it's the fastest or avoids an incident"
It'll be a welcomed update and one that will surely help you remain on the move and avoid hold-ups. As noted already, this update will hit Google Maps in the coming days.
Source: Google
