Is GA Flying Safer Than Driving?
Is it true? Is our "little plane" flying safer than driving a car?
Raw statsFor GA accident stats, we'll use the 2005 Nall Report (available at http://www.aopa.org/asf/publications/nall.html), which reports on accidents from calendar year 2004.
For driving, we'll use 2004 statistics from the National Highway Traffic Safety Administration (NHTSA), available here.
According to the Nall report, in 2004:
Let's compare the rate of fatal accidents to the amount of flying/driving done.
For GA, there were 11.2 fatal accidents and 19.7 fatalities per million hours of flying.
For motor vehicles, there were 1.32 fatal accidents and 1.47 fatalities per 100M miles.
Note, though, that about 13% of these fatalities were pedestrians, and so weren't actually engaged in driving. Also note that motorcycles have a much higher fatality rate than cars/trucks, namely 38.8 fatalities per 100M miles, over 25 times higher than the average, so these numbers are a bit inflated when it comes to determining the risk of an occupant of a car or truck.
Of course, the next hurdle we face is that our flying accident rate is based on the number of hours, while the driving accident rate is based on miles driven. To directly compare them, we have to either pick an average speed for GA aircraft and compare by hours, or else pick an average speed for cars, and compare by miles. We'll try both of these approaches.
GA flying covers small training aircraft capable of cruising at 100mph, and business jets capable of cruising at several hundred miles per hour, so choosing an average cruise speed is difficult, but for the sake of argument, we'll choose 150mph. This gives us a comparison of:
How about if we compare on an "hour to hour" basis? That requires an assumption of an average speed for autos. We'll choose 40mph. This leads to the following numbers:
How do the GA numbers compare to commercial airline travel?The NTSB provides accident statistics for "part 121" air carriers (scheduled airline travel) at http://www.ntsb.gov/Publictn/2001/SR0101.pdf Airline fatalities are concentrated into a small number of accidents. It's not uncommon for an entire year to pass with 0 airline fatalities, so it's not really possible to generate meaningful statistics by looking at just one year. Instead, we'll sum up all the fatalities and estimated flight hours and miles flown for US carriers over the ten years from 1991-2000. Since we're evaluating the risk to passengers, we'll only consider the fatalities of those aboard the plane (and not victims on the ground, the airliner equivalent to "pedestrians").
The results are:
What about commercial vehicle travel?The GA flying data above excludes results from airline travel. The safety record for airline travel is so far superior to GA, that to combine the stats together would skew the results for GA travel, boosting the appearance of GA's safety record.
The motor vehicle stats above, however, do include commercial motor vehicle travel. Is it possible that commercial motor vehicle travel is similarly skewing the results for motor vehicles, making cars seem safer than they really are?
The FHWA results for 2004 can be found summarized here: http://www.fhwa.dot.gov/policy/ohim/hs04/htm/vm1.htm
Out of 2.96 trillion vehicle miles driven, .226 trillion of these miles were by "SINGLE-UNIT 2-AXLE 6-TIRE OR MORE TRUCKS" and "COMBINATION TRUCKS". This would include all tractor trailers, buses, etc.
First, note that this represents 7% of all vehicle miles traveled. So if commercial truck and bus travel was unfairly improving the motor vehicle safety record, the maximal possible impact would be if these vehicles had 0 fatal accidents in 2004, in which case the impact couldn't exceed 7% of the overall fatal accident rate.
So in the case of motor vehicles, the percentage of travel that could be described as "commercial" is such a small percentage of the whole, that it's really not possible for it to significantly boost the motor vehicle stats, assuming commercial motor vehicle travel was even shown to have a better safety record than cars. On the other hand, if the truck accident rate was much higher than the overall average, it could be unfairly raising the motor vehicle accident stats, making cars appear to be more dangerous.
The NHTSA data lists 761 occupants killed in "Large Trucks" and 556 in "Other Vehicles" (which includes buses and farm and construction equipment). Together, these account for about 4% of overall motor vehicle fatalities.
But while the NHTSA report lists 761 occupants killed in Large Trucks, in accidents involving these vehicles, 4006 "other vehicle occupants" were killed, as well as 423 "non-occupants" (pedestrians, presumably).
So accidents involving large trucks account for 12.6% of fatalities, while only accounting for 7% of vehicle miles traveled. This suggests that commercial motor vehicle travel is actually worsening the stats for cars slightly. However, in accidents involving trucks, the casualties were mostly the occupants of other vehicles and pedestrians, so commercial motor vehicles are, in themselves, a significant risk to car travelers, but the numbers just aren't large enough to make a significant difference.
ConclusionChoosing "mile to mile" as the more appropriate comparison for differing modes of transportation (and overlooking that small planes often takeoff and land at the same airport, without ever really "going anywhere"), let's review the fatality rates:
But what's the important stat?The numbers above provide both the rate of fatal accidents, i.e. accidents in which there are 1 or more fatalities, and also the rate of occurence of fatalities. When we consider these numbers, what we're typically hoping to come to terms with is something like "what's the relative risk of me, a single passenger, taking a trip via each of these modes of transportation?"
Neither of these stats perfectly encapsulates what we're looking for. The best approximation to this would be the product of:
It may be possible to get a better idea of risk to a single passenger by comparing fatalities per fatal accident to average number of passengers on-board. This assumes that there's no correlation between fatal accident rate and passenger counts, which may not be true (e.g. is the probability of fatal accident equal for a large jet as for the smallest planes still used for scheduled airline service?)
CaveatsWhat are the assumptions that went into this?
The miles driven data are compiled by the FHWA from data provided from individual states. I'm not sure what methodology is used to generate these numbers.
Harry Mantakos / firstname.lastname@example.org