The reason people in the US are less traveled is because you essentially have to cross an ocean to go anywhere. Some people like to go to Canada, but that is pretty similar to the US. Most of the people that I know that go to Mexico stay at resorts for safety concerns.
Edit: I would assume the median number of times an individual has crossed an ocean is still 0. Even for citizens of rich countries. I don't think this number is going to change with the current technology branch being developed by Boeing and Airbus. Maybe in a century Hyper loop, Electric Airplanes, Space Elevators, or trains on floating bridges will have advanced enough to change this to 1 or 2. Maybe a seemingly low-tech solution like Zeplins or Boats can be done fast enough, cheap enough and safe enough for broad appeal.