Finally, there is your own scanning technique. Sweeping your eyes across the horizon is not the best way, since it takes us a few seconds to focus. A better plan is the so-called "block" method. Divide the sky into blocks of area, each spanning roughly 10 to 15 degrees, and the same division above and below the horizon. Stop and focus on each block for a few seconds as you move up and down, and left to right across the windshield. Some suggest skipping from far left to far right, then working toward the middle. The key is to pick a block and stop, focus, then move on. Bear in mind, also, that an airplane on a collision course will not appear to move at all, but rather will "mushroom" in your field of view as it gets dangerously closer. Since our vision is most sensitive to relative movement, that means that the most dangerous target is the one that is least apparent.