<P> As a result of the September 11 attacks, the United States began implementing a range of measures to increase the security of its borders and its identity documents . One result of this was WHTI, which mandates that a smaller, more secure number of documents verifying both identity and citizenship be used to facilitate identification and international border crossing . Previous to WHTI, many different types of documents were acceptable to cross the border, including birth certificates issued by thousands of different authorities within the United States and Canada . As a result of WHTI, American citizens traveling to Canada would have been required to obtain a passport booklet in order to cross the border . In order to offer a less expensive and more portable alternative to border communities and frequent travelers, the passport card was developed . </P> <P> In an effort to improve efficiency at land crossings, the passport card also includes a vicinity - read radio frequency identification chip with a unique identifying number tied to government databases . Unlike the passport book, the RFID chip in the passport card is designed to be readable at a greater distance, allowing border agents to access traveler information before they pull up to the inspection station . While a biometric passport contains a chip containing all of the traveler's information in electronic format, the RFID chip in a passport card does not contain any personal information beyond the identifying number, which is used to locate records in secure government databases . To prevent the RFID chip from being read when the card is not being used, the passport card comes with a sleeve designed to block RFID while inside . </P> <P> The passport card is a limited travel document, valid only for land and sea travel within North America (Canada, the United States, Mexico, the Caribbean, and Bermuda). It cannot be used for international air travel . The Department of State indicates that this is because "designing a card format passport for wide use, including by air travelers, would inadvertently undercut the broad based international effort to strengthen civil aviation security and travel document specifications to address the post 9 / 11 threat environment". </P> <P> Aside from being unable to be used for international air travel, the passport card is treated as a passport for all other purposes . A United States passport card can be used as primary evidence of United States citizenship, just like a passport booklet, and can be used as a valid proof of citizenship and proof of identity both inside and outside the United States . However, the acceptance of the passport card as an identity document by both private and governmental entities within the U.S. varies greatly, due to its unfamiliarity with many people . </P>

Where can you travel with the passport card