Riley, K. J. (2006). Border Control. RAND Corporation.The borders of the United States can be conceptualized as four segments, or points of entry. Three segments—airports, ports, and guarded land points—are official. The fourth—unguarded land borders and shoreline—is unofficial and is used primarily by migrants, smugglers, traffickers, and perhaps terrorists. Each segment is to some degree porous, because of the volume of activity and the amount of physical space that must be protected. Physical space is particularly important at unofficial points of entry. The need to improve control over the airport border was highlighted dramatically by 9/11, but the attacks also indicated a need for increased control over the other segments.