[Omah]

http://en.wikipedia.org/wiki/Identit..._United_States

For many years, passports were not required for US citizens to re-enter from countries near the United States (including Canada, Mexico, Bermuda, and most Caribbean and Central American nations.) In light of this, and given the country's immense size and the great distances which the average citizen lives from an international border, passport possession in the United States had remained relatively low. Indeed, most Americans normally did not obtain passports or carry them regularly unless traveling abroad, and as of 2006, only 60 million (20% of Americans) had passports. As of 2011, approximately 37% of Americans have Passports or Passport cards.

However, in response to recommendations in the 9/11 Commission Report, the U.S. Department of Homeland Security now requires proof of citizenship for people entering the United States from neighboring countries. This requirement is known as the Western Hemisphere Travel Initiative, and was implemented in stages:

On January 23, 2007, a passport, U.S. Coast Guard Merchant Mariner's Document, or NEXUS card became mandatory when re-entering from those locations when traveling by air, with a few exceptions.
On 31 January 2008, officers at land and sea ports of entry stopped taking oral declarations of citizenship from travelers; all individuals entering the U.S. are now required to present documentary proof of identity and citizenship.
Beginning 1 July 2009, people entering the United States by land or sea must present a passport, passport card, or other document proving citizenship or permanent resident status.