Re: Why?
"Traffic avoidance. Routing round accidents, roadworks and events."
Funny thing about this. What it does is re-route all the through traffic (freeways, etc.) onto surface streets, which aren't designed to handle the volume of the freeway. And every single piece of software doing the re-routing uses the same basic algorithm, thus all the freeway traffic winds up on the same surface streets, leading to even more congestion. I see it all the time on the Hwy 101 corridor between the Golden Gate Bridge and roughly Sonoma County Airport.
Adding to the problem is locals taking the freeway one or two exits to get across town. Seems to be faster, but try timing it sometime. (I'm squining at you, Marinites and Santa Rosans). This has been quite obviously getting progressively worse as more people "trust the car" to tell them where to go.
Computers are supposed to be a tool to enhance human thought, not a replacement for it!