I had a friend who went through a hell of a time with a pulmonary embolism (a very common life-threatening and commonly life-ending event in all ages) four days after a flight from Rome to New York. I would have thought that the stats were higher, but it appears that the occurrence of deep vein thrombosis (clot formation, in this case usually in the legs, which, when they get loose, are carried to the lungs) is only doubled during travel of four hours or more - whether car, plane, bus, etc.
The solution might be a baby aspirin, but best is to keep those legs moving a bit instead of sitting immobile for long trips. Get up, walk around, stop the car and walk in a circle, whatever. Or just twitch your legs restlessly the entire trip, as if you had ADHD. That will help prevent this terrible problem.