The changes to United States security after 11 September have impacted Canada in several ways. Before 2001, travel and trade between the two countries was much easier. The North American Free Trade Agreement of 1991 had removed many of the barriers to trade and allowed people from the United States and Canada to work in each others’ countries easily. After 2001, trade became more difficult because of increased security at the border, resulting in massive traffic backups. Now a passport is required to travel from Canada to the United States, where earlier all you needed was a driver’s license. The United States has also requested lists of all passengers on planes that fly through U.S. airspace to Canada even if they do not land in the U.S.