A ditch or culvert is potentially better than taking refuge under an overpass - the reason being - anything greater than an EF-2, the winds getting funneled into a smaller space actually increase the intensity and getting sucked out and up into a multi-vortex tornado is lethal.
Most folks have a tendancy to stay in their cars under an overpass during a tornado. Much of this is the result of a video of a family and a television crew hiding under an overpass while a weak tornado passed right through them in 1991, and they survived without so much as a scratch.
In anything of EF4 or 5 strength - even reinforced concrete homes can get ripped from their foundations, so hiding under an overpass during such a tornado is not going to provide any more safety. In those cases, prayers are all we got to keep us safe if something on the magnitude seen today again in Moore hits you.
That said - if I’m in a car with a giant wedge heading for me, and there is no ditch or culvert I can get under - I would try to get up under the girders of an overpass if I had no ability to evade an oncoming tornado by angling to the left of it’s approach. Remaining in my car is not an option. it will end up as a 200 MPH missile that most likely will get wrapped around and imbedded into a tree or building.
Wouldn’t a better approach be to try and escape it? Hopefully traveling at a 90 degree angle.
Now that everybody has a smartphone it shouldn’t be difficult to discern which direction to go.
You’re definitely correct about a ditch or culvert being the best, but the problem is in some cases there’s been 2-3 inches of rain in the previous few hours.
I was driving an empty 12-passenger van between Tuscaloosa and Birmingham one April and was faced with thinking about that one. Tornado watch, visibility almost zero, I’d pulled over and van was rocking, and I thought I saw the wind just shift - but the ditch beside the interstate was full of water. I just stayed in the van (turned out no tornado).
Getting up into the girders goes without question, as little exposure as you can manage, and brace yourself. Cover your face as best you can. And pray.