Flying and Driving after the September 11 Attacks
As all those stock prospectuses say, these figures are descriptive for the time period studied and are not predictions of future performance. Making predictive statements about the relative risk of flying and driving after the attacks of September 2001 is indeed tough. In particular, it requires some assumptions about whether such aberrant events will be repeated and, if so, how often. Because the frequency of such episodes cannot be reliably estimated, we instead decided to calculate the frequency needed for the two travel modes to become equally risky.
As we explained above, the risk of a fatality while driving the length of an average nonstop flight is 5,091 X 10–9. For nonstop flights to have had the same estimated risk, there would have to have been 28,046 flight fatalities over the 10-year period studied (based on 54,061,237 nonstop segments and 101.9 passengers per nonstop segment). That translates to 27,845 flight fatalities in addition to the 201 people who actually died over those years (not counting those on the four hijacked flights). In turn, dividing 27,845 by 232 (the number of passengers who died on the four hijacked planes) we obtain the following: For flying to become as risky as driving, disastrous airline incidents on the scale of those of September 11th would have had to occur 120 times over the 10-year period, or about once a month.
Two conclusions follow. First, without diminishing the tragedy of September 11th (which also involved many deaths of people on the ground) or its political ramifications, from the perspective of personal safety it is important to consider that the annual number of lives lost in road traffic accidents in the United States is enormous in comparison (42,119 fatalities in 2001). Second, the relative safety of domestic flying on the major airlines over driving is so strong that the direction of the advantage would be unchanged unless the toll of terrorism in the air became, almost unthinkably, many times worse than it has been.