There is no single answer save one STAY OUT OF DITCHES. I will explain.
If you have to time to do so safely, get out of dodge, the problem with tornados is that they sometimes turn on you, the one in Oklahoma did just that it turned and surprised a bunch of folks .(Well one of them did at least)
Underground might be OK, if it is a strong shelter, but I've seen basement walls fall in and that was not anywhere near an F-5.. I might add I was a bit too close to that one for comfort.
Ive seen a bicycle (mine) sitting on it's kick stand, ride out said storm still sitting on it's kick stand (Same tornado).
And that is just what I have seen, with my own eyes.
There is no single answer.. You have to take each storm as it comes and decide what to do based on what is happening.
The best thing you can do is take weather spotter training, they teach you both storm identification, tracking and SURVIVAL.
Now, why stay out of ditches?
Because in a heavy rain they can flood and you can drown.
