I have never been to US national park, but I've visited some in Eastern Europe and Scotland. I can't imagine myself traveling over 4 thousand miles by any other transportation (trains, buses, bicycles, walking) within given timeframe, budget and convenience. Now this is the "old world", where you travel 500 miles and find yourself in an another country; I can only imagine what traveling in the US feels like. Maybe, one day, I will know - one can always hope...