I just noticed that Google Maps introduced the ability to show historical "average traffic" for any given time on particular day of the week. I just used it to confirm my commuting schedule, and it seems to correlate well with my experiences and validate that the times I drive are pretty reasonable.
There are definitely some systemic traffic slowdowns in the Portland area. For example, 26 west bound between Murray and Bethany is nearly always a problem from 4PM-7PM, probably stemming from the fact that the road moves from 3 lanes down to 2 just after the Bethany exit. This is especially unfortunate since the third lane was just added...although I do admit that there are much more serious traffic concerns in the area to focus our resources on.
It would be nice to know that those that plan road improvements take a look at this type of data to prioritize their efforts.