The United States Border Patrol operates 71 traffic checkpoints, including 33 permanent traffic checkpoints, near the southern border of the United States. The stated primary purpose of these inspection stations is to deter illegal immigration and smuggling activities. After the September 11 attacks in 2001, they apparently took on the additional role of terrorism deterrence. These checkpoints are located between 25 and 75 miles (40 and 121 km) of the Mexico – United States border along major U.S. highways. Their situation at interior locations allow them to deter illegal activities that may have bypassed official border crossings along the frontier.