Funnily enough I was thinking of posting something similar!
Don't use the nav too much but have wondered before about the built in systems traffic re-routing.
My interest was peaked last week when I had an early Monday morning trip from Gravesend to Gatwick Airport. I was using the
Cupra nav and it took me far away from the M25 due to traffic. It took me down the A25, even routing me off that road and through side roads. Time was starting to get away from me so I swung back round to the nearby M25 entry slip. There was a bit of traffic stop/starting but it kept moving once near Clacketts and Gatwick was reached without any more hold ups.
On the return (Thursday rush hour) the first thing the
Cupra nav said was that there was a better route available faster than M25 due to traffic. I wasn't taking A and B roads all the home so fired up Waze (which I use on a daily basis in my van). Waze said M25 was still best route despite traffic delays and would take 58 minutes instead of 45-50 minutes normal run time. Indeed that is exactly how long it took.
So in future its Waze all the way for me from now on.
I will say that my 17 plate Range Rover I owned previously had by far the best Sat nav I have ever used so not all built in nav are the same!