The ditch is just to avoid laterally flying debris (it's the stuff in the air that kills ya). Anything that's dropped will squish in a ditch just as easily as in a field. And if you're in a ditch, you also have to watch out for water levels.
Avoiding bridges is basically just an extension of the "Avoid anything that can crush you".
That being said, though I'm living at the other edge of the end of Tornado Alley, since we're in the Great Lakes region, we don't usually have to concern ourselves too much. The lakes provide enough water to the funnels to make them too heavy to continue for too long in this area.