Sixty five kilometers (about 40 miles) from the center of the explosion an eyewitness stated that "[...] I suddenly saw [...] the sky split in two and fire appeared high and wide over the forest. The split in the sky grew larger, and the entire northern side was covered with fire. At that moment I became so hot that I couldn't bear it, as if my shirt was on fire; from the northern side, where the fire was, came strong heat. I wanted to tear off my shirt and throw it down, but then the sky shut closed, and a strong thump sounded, and I was thrown a few metres. I lost my senses for a moment, but then my wife ran out and led me to the house. After that such noise came, as if rocks were falling or cannons were firing, the earth shook, and when I was on the ground, I pressed my head down, fearing rocks would smash it. When the sky opened up, hot wind raced between the houses, like from cannons, which left traces in the ground like pathways, and it damaged some crops. Later we saw that many windows were shattered, and in the barn a part of the iron lock snapped."*
That asteroid was only about 50 meters (160 feet) in diameter. If it had hit close to a populated area millions of lives would have been lost.
When ATLAS becomes operational in a few years it will identify asteroids like the Tunguska impactor about a week in advance of impact; assuming we can identify a source for the $3,000,000 to build it. There would be nothing we could do about avoiding the impact but ATLAS will provide early warning of the danger and allow people to be evacuated to safety. That was not the case for perhaps 10,000 people who perished in 1490 A.D. due to a shower of 'falling stones' according to Chinese records.#
* from N. V. Vasiliev, A. F. Kovalevsky, S. A. Razin, L. E. Epiktetova (1981). Eyewitness accounts of Tunguska (Crash)., Section 6, Item 4 as recorded in http://en.wikipedia.org/wiki/Tunguska_event
# Yau, K., Weissman, P., and Yeomans, D. "Meteorite Falls in China and Some Related Human Casualty Events" in Meteoritics 29: 864-871.