I started noticing this very abruptly - that Google no longer redirects me from congested roadways to low-traffic parallel roads. Now, as soon as I take the exit to a less congested road, my estimated drive time might drop by 5-10 minutes, so clearly Google knows that it isn't sending me down the optimal route. I don't think this used to be the case.

Interesting - I noticed recently, for the first time, that when I modified a 'fastest' proposed route using local knowledge, I found the predicted time dropped, and that has happened several time since.

This may be the result of better predictive modeling, and particularly in recognizing that dumping a lot of traffic into a low-capacity route is pointless. It is probably also a political response to the fact that residential areas don't like having a lot of traffic routed through them, and are pressing for the power to regulate it.

Better predictions could also explain toast0's observations even when global optimization is not being attempted.