I'm not aware of any studies, although I think it is clear that when the United States imposed the western hemisphere travel initiative, which meant that all Canadians and U.S. residents had to have passports to cross the border, that definitely had, and probably still has, an impact on tourism.
I grew up 16 miles from the U.S. border in Manitoba. We used to cross it every day, back and forth, and nobody paid any attention to it. If you wanted to, you could go a half mile and phone in to the U.S. customs and tell them you were coming. If you didn't want to do that, you just crossed over. Those days, unfortunately, are gone forever, and as I said, they've gone because of the terrible reality that the world did change on September 11 and we do have very serious security threats. Therefore, travel is never going to be as easy and as simple as it once was. That's a reality we have to face, I'm afraid.