A virtual fence does virtually nothing to stop illegal crossers. The border patrol agents' problem in apprehending the illegal crossers in this video isn't the inability to detect them -- they are already detected. The problem is apprehending an overwhelming number of aliens by a few agents. This is why a double layer fence is needed. Agents could easily apprehend any aliens who managed to cross the fences while most likely others waiting their turn for the ladder needed to climb the second layer fence would turn back.