Apple Maps now re-routes with the best route.
I was using Apple Maps to get from Boulder, CO to Longmont, CO and the commute time was 32 minutes. Siri found me a better route and cut my travel time to 20 minutes. Down a whole 12 minutes is impressive!
Edit: this was during 5 o clock traffic mind you!
edit 2: I see lots of confusion.
This all happened automatically, without any prompts or anything of the like.