The asteroids that may be of concern are those whose orbits around the sun are elliptical and have the apogee (farthest point from the Sun) and perigee (closest) on both sides of Earth's orbit. These orbits are less well known, so the only way to discover these asteroids is photographing the sky several times and see if any bright spot has moved and does not correspond to a known object. Of course, the smaller they are, the harder to detect, so that they must get the closer to be photographed, especially considering they do not have proper brightness, and the light reflected depends on its surface composition, on the distance and orientation related to the Sun, and many other factors. And it has become obvious that a diameter of 7 to 10 meters which is the estimate for the Siberian fireball don't guarantee its early detection. It is estimated that each year we are stroked by several hundred bodies of this size, but the vast majority fall into the sea and / or without damage worth mentioning. Probably a coincidence, the media focus 2012da14 has risen by, and the impact on a populated area are the causes of the enormous media impact of the Chelyabinsk fireball, which otherwise would have gone unnoticed except for a few witnesses, experts and enthusiasts.
Today we can discard a collision with a dangerous sized asteroid in the coming years, and although it may remember some films, we rely on science and technology to detect it and take appropriate action to avoid the impact. Of course, when you consider the statistics on impacts of large bodies, Humanity is rather nearer to destroy itself that perished in a catastrophic event like the one at hand. Meanwhile we can enjoy the exciting world of astronomy and the night sky closer to watch the spectacle of meteor showers that are repeated on multiple dates, year after year.