I have a question on the stock nav system...hopefully someone can help.

I recently took a driving trip through a couple states (in US). The Nav seems to keep wanting to take me off the interstates and take smaller "parkways" and two lane secondary routes that might be slightly more direct, but a LOT slower. It's extremely annoying when you're not familiar with the area and you're following the thing somewhat blindly.

How can I force it to favor interstates and major highways? I know you can do this with my old Garmin.