In reading one of the other posts a casual point was made regarding the relative safety of flying versus driving.  It is generally assumed that flying is, by far, the safest of the two modes of travel, but is this really true?  In looking at the data, it appears that the data is being skewed because of some strange assumptions that tend to favor flying.

Let me be clear that I'm not suggesting that flying is unsafe.  In fact, I would argue that the two modes of travel can't legitimately be compared.  Why not compare flying with swimming?  or hiking?  However, since the safety argument is invariably made regarding flying and driving, it is this tacit assumption, that one is so fundamentally safer that it is beyond questioning, that I am challenging.

One of the first assumptions in evaluating safety is based on the notion that passenger miles is a legitimate metric.
"If we ignore property and bodily damage and focus on fatalities only, we should look at fatality rates per passenger mile traveled."
I would argue that this is a bogus metric. Consider that the point being made is that if 200 people travel 1000 miles together that it somehow represents 200,000 miles.  In truth, it doesn't.  It's only 1,000 miles. So using one set of statistics it is reported that the airlines experienced 572,885,732,000 passenger miles in 2005 (domestic U.S.).  The reality is a bit different.  With 10,090,274 flights averaging 867 miles, it turns out the actual distance flown is only 8,748,267,558 miles.  This is almost 66 times smaller than the original reported number.

From a safety perspective it can hardly be argued that safety is increased based on the number of passengers (since the risk isn't carried individually).  The risk is intrinsic in the actual distance traveled, so it doesn't matter if the flight is full or empty when quantifying this value.

If we consider the number of licensed drivers in the U.S. we have approximately 202.8 million (2006).  So once again, we have a factor of about 20 times difference in the number of drivers versus aircraft passengers.

It is also important to consider what constitutes a fatality in these statistics.  Clearly when an aircraft crashes there's little doubt about the fate of the passengers, however in automobile statistics this isn't as clear-cut.  Once again, using the 2005 statistics, there were 43,510 people killed1, however nearly 6,000 of those were pedestrians or bicyclists.  While this is certainly an unfortunate statistic, it can hardly be argued that it is a result of the mode of travel (unless one wants to criticize walking).

So to make a more proper comparison, we should consider the relative risk of something happening to a driver/passenger every time they enter a vehicle for transport.  After all, it doesn't really matter how long or short the trip is if we consider that it is just as likely for a plane to crash on take-off as it is on landing, just as a fatal automobile crash could occur just as readily in going to the corner store as in making a cross-country drive.

If we take some arbitrary numbers, we can begin to approximate how these numbers relate.  With 10,090,274 domestic flights in the U.S., that becomes the number of possible events that could've resulted in accidents.  With cars, I'm going to make the following assumption that taking all the licensed drivers (202.8 million) and assuming that they only drive to and from work.  This would result in two trips per day for about 261 days or 522 trips per driver.  As a result, the total number of driving events would be a staggering 105,861,600,000 events.  On this basis, we can see that an automobile driver is 10,000 times more likely to be exposed to the circumstances leading to an accident.  This comparison indicates that every time we get into a vehicle we are incurring some risk, however, we engage in such a risk 10,000 times more frequently with cars than we do with aircraft.

One of the obvious differences in assessing the two modes of travel is that automobile accidents tend to hold relatively steady from year to year, whereas aircraft fatalities can range from zero to several hundred, even with only one accident occurring.  As a result, it isn't really fair to compare the two values to each other, since any choice of aircraft fatalities is clearly arbitrary and can radically change based on the period being examined.

It should be clear that when comparing these two modes of transportation an important element that is typically ignored is in comparing the relative amount of exposure to risk that each traveler incurs.  As a result, the assumption that air travel is orders of magnitude safer than an automobile is simply urban legend.  While one can certainly make the point that increased professionalism of pilots and the oversight in place with air traffic and ground controls also offer more protection, rather than the free-for-all we typically encounter on our highways, the simple reality is that we use our cars far more often than we fly and as a result, we incur a greater risk because of it.

In the end, comparing automobiles and aircraft is an "apples and oranges" comparison, but that doesn't automatically translate into increased safety, nor a free pass in having the numbers examined. In fact, while driving has such a marketedly higher rate of participation, less qualified individuals, and approximately 30% of fatalities the result of alcohol, it's incredible that driving is actually as safe as it is.

1 This also includes motorcycles, while the aircraft statistics do not include ballooning, gliders, etc.