FUN WITH SCIENCE. In answer to Mike's motion sensor query, the reason the border has been largely bereft of such Flashdance-era technology is that motion sensors aren't very smart. Separating a person from a squirrel, or a bird, or a tumbleweed is tricky. The number of false positives, which would force our already understaffed border guard to constantly dart out into the desert, would be staggering, and would probably lead to less effective enforcement than we have currently. To be sure, there are better systems out there, capable of separating man from marsupial, but they're expensive and we're cheap.