Google Maps announced this week on their Google+ page that you can now use "dynamic re-routing" on your Google Maps app while you are driving with GPS directions.
Google said, "start navigating using dynamic re-routing and voice-guided GPS navigation with Google Maps for mobile."
I am not 100% sure if this is a new feature but this is the first I've seen of it myself, although other GPS software does do this. In short, as you are driving, Google will tell you if there is a faster route and give you the option if you want to take it.
You have the option to reroute or stay on your current path.
Here is a video of it in action:
Forum discussion at Google+.
This post was written earlier this week and scheduled to be posted today.