Planes don't interact much with other planes, and they are not alone (i.e., are directed by Controllers) when they do. Plus the pilot is still there to take over in a pinch, so planes are not fully self-directed. I don't doubt that cars will eventually become semi-autonomous (such as on a long boring stretch of interstate) but the real question is whether they will become fully autonomous in more complex settings, such as in cities. I believe that it is possible that driving a car successfully in complex environments will turn out to be a far more difficult challenge for programmers than anyone realizes. This is not a test for you (or me) but for the computer scientists who seem to think they can conquer all problems. Whether they pass the test remains an open question.