The numbers tell the story: Of the 308 million-plus citizens in the United States, 30% have passports.

That's just too low for such an affluent country, said Bruce Bommarito, executive vice president and chief operating officer for the U.S. Travel Association.

"Americans are comfortable in their own environment," Bommarito said.

There were 61.5 million trips outside the United States in 2009, down 3% from 2008, according to the Office of Travel and Tourism Industries. About 50% of those trips were to either Mexico or Canada, destinations that didn't require a passport until 2007.

The percentage of Americans with passports -- a number that was in the teens just a few years ago -- has spiked since the Western Hemisphere Travel Initiative was adopted. It requires American and Canadian travelers to present documents showing citizenship when entering the United States.

Despite the climbing number of American passports in circulation, 30% is still low compared to Canada's 60% and the United Kingdom's 75%.

"Not taking the leap is comforting, because this is the American life," said Matthew Kepnes, international traveler and creator of NomadicMatt.com, a blog chronicling his travels and observations. "Breaking outside anything that is your norm is scary."

Tourism experts and avid travelers attribute Americans' lack of interest in international travel to a few key factors, including: the United States' own rich cultural and geographic diversity, an American skepticism and/or ignorance about international destinations, a work culture that prevents Americans from taking long vacations abroad and the prohibitive cost and logistics of going overseas.