We made flying nearly collision-proof decades ago. Why are intersections still so dangerous?

On average, 11 car crashes occur every minute in the U.S. By the time you finish reading this sentence, several vehicle collisions will have happened across the country, some of which were likely fatal. In the world of aviation, the number of crashes involving a U.S. civilian aircraft is about 1,200 per year, and very few of those result in fatalities.  Despite the 5,500 American planes that are in the air at any given moment during peak times, collisions are rare, because airspace is...