Doris Meissner, the former commissioner of the now-defunct US Immigration and Naturalization Service, told the BBC that the barriers can be effective are "redirecting" flows of migrants to more easily patrolled areas "The idea of having a barrier there [Starr County] is that the Border Patrol's agents and vehicles don't need to be spread out over huge distances, but distances where people are crossing," said Ms Meissner, now a senior fellow at the Washington DC-based Migration Policy Institute. "That seems to be what they're thinking about with this proposal."