Yes, it's just so much safer to remain exposed on open highway with no shelter whatsoever than to seek the reinforced concrete and steel of a highway overpass. /s
This "myth-busting" seems to be generating a myth of it's own. Where would you suggest individuals stuck on a highway with an F4 bearing down upon them go?
Realistic choices, please. You're talking largely flat country. Heavy rain and large hail, near constant lightning. You might drown in a ditch or culvert, and still have no protection from 200 mph winds with airborn debris. Staying in your car can be suicidal, but what the heck, you always loved Thelma & Louise, whee!
Seriously, what on earth do you suppose people stuck on a highway are supposed to do, here?
Several years ago, I was driving my little Saturn back from Fredericksburg to Fort Worth when a storm whipped up and caught me on I-35 (back before I learned about 287). I heard a couple of hailstones ping off my hood/roof and stopped under the next overpass where I spent the next several minutes hiding in the base of the girders with some other folks, until the worst of it passed. Guess we were lucky it didn’t reach tornado level. Otherwise, I can’t really think of where else we could have gone, because it sure was flat as all get out around there.
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.