I have never heard that tefilas haderech should be said standing. I’m not saying it’s not true, but I never heard of that before and I’ve never seen it done that way. What is the source for that?
I have occasionally seen people pull over and stop the car to say tefilas haderech (although even that, I have rarely seen), but I never saw them stand.
In any case, when people fly somewhere, don’t they usually have to travel to get to the airport first, so shouldn’t tefilas haderech be said on the way to the airport?