is it possible to train the 2010 prius nav to prefer specific roads?

1. Not that I know of. Other than the short/alternate/quick route selections or the Route Preference settings I do not know of a way to select a specific highway. If you were to drive on the specific highway that you want the system should recalculate a new route based on your present position.
2. In the Nav manual, using the Detour feature, you can select a specific mileage distance and it will return to the original route. I have found that if I leave the original route and then return it will automatically continue on the original route. Of course it will be trying to recalculate a new route the entire time you are off of the original route.
3. Toyota typically updates the maps once per year. The updates are expensive at about $200. I have a 2010 and they have had one new update since I have had mine. I have the 9.1 rev and they have a new 10.1 rev. My dealership notified me that an update was available.

 

I hear this guy can do it - but it's a costly time consuming process...

 

If you want to force particular routing, insert additional destinations along the way, and then chain the destinations together. With marine and aviation systems we can do this with waypoints, but most automotive systems lack this feature, so we have to use destinations.

Tom