FILE - In this Aug. 8, 1996 file photo, a sign warns motorists of the possibility of undocumented immigrants crossing the freeway near the border in San Diego. To many Republican presidential contenders in Thursday night’s debate, the U.S. border with Mexico must be secured for millions in the country illegally to stay. To many commuters at the nation’s busiest crossing, their border already is secure - even thriving by historical standards. (AP Photo/Damian Dovarganes)