The average American travels 2.9 times/year, defined as an overnight trip >100 miles from home. The average European travels 4.9 times a year by this definition. So yes, a huge gap - add to this that the majority of trips Americans take are to visit family rather that to visit somewhere new. The early stage travel startup scene in Europe is much more vibrant than the US, but fewer of them get funding because European VC is so cautious and travel is universally acknowledged to be a tricky industry. I guess this means there should be more American startups that target European travel markets...