Estimate the probability that an individual US airline would end in mid-air collision:

1. Above the continental united states and
2. In the vicinity of major airports

How would the accident rate of uncontrolled air traffic compare to that of the space shuttle and the real statistics for US airline traffic?

A probability is a number between 0 and 1. Multiplying a probability by 100% gives the percentage chance that a given flight ends in disaster. To start, you'll want to estimate (within an order of magnitude) several important quantities, including the size and average speed of an airliner and the volume of the relevant airspace. The concept of a 'mean free path' is helpful.