LETTERS TO THE EDITORS
Measuring Risks
Measuring Risks
To the Editors:
The interesting article by Leonard Evans in the May–June issue
("Traffic Crashes") leaves me with questions. When comparing the risk of
dying on the road in different countries, he measures risk in terms of the
number of vehicles driven. The data presented do not really address
differences in risk while driving a car for different lengths of time in the
various countries. Risk is usually assessed in terms of exposure to the
hazard involved; thus, airplane safety is usually stated in terms of
casualties per million passenger miles—
not in terms of the number of aircraft.
George A. Paulikas
Palos Verdes, California
To the Editors:
Leonard Evans states that airbags "were mandated by a lawyer-led
safety agency that claimed benefits far in excess of published technical
estimates and ignored technical information documenting their harmful
effects." But published technical estimates do state significant benefits
from airbags. From www.nhtsa.gov, one finds: In purely frontal crashes, air
bags have a fatality reducing effectiveness of 34 percent. And for people who
are concerned about harmful effects of airbags, on-off switches are
available.
Don Sellers
Dallas, Texas
Dr. Evans replies:
Airplane safety is commonly measured by fatal crashes per million
departures and not casualties per million passenger miles. Since air crashes
generally occur soon after takeoff or just prior to attempted landing, flight
distance has little influence on risk. For ground transportation, risk is
approximately proportional to travel distance (or time). My article focused
on changes in fatality rates over the years—so most conclusions based on
deaths per vehicle apply similarly to deaths per mile of travel because
distance of travel per vehicle per year does not change much from year to
year. Britain and the United States are among only a few countries that
systematically estimate national vehicle travel—their fatality rates based
on travel distances are compared in the figure below. If the U.S. rate change
had matched that in Britain, then 14,700 fewer Americans would have died in
2000, compared to the estimate of 14,400 fewer deaths derived in Figure 12 of
my article.
The 34 percent effectiveness cited by Don Sellers is for the type of
crash in which airbag effectiveness is greatest, namely center-front impacts
by illegally unbelted drivers. Crashes with left-front or right-front impacts
are excluded, as are crashes in all other directions and all rollover
crashes. Safety belts reduce driver fatality risk by 82 percent when rollover
is the first harmful event. However, it is not this highest value but the 42
percent effectiveness averaged over all crashes that is invariably quoted.
For airbags the corresponding overall value is 11 percent. The airbag mandate
was introduced based on the absurd claim of 40 percent overall effectiveness.
More than 200 people have been killed by airbags in crashes they would
otherwise have survived with no more than minor injuries. Airbags increase
harm, on average, to belted female drivers. Drivers known to be at increased
risk may indeed petition for government permission to pay extra to disconnect
a device government forced them to buy. Even then, businesses are often
unwilling to honor such permissions for fear that the same legal system that
spearheaded the airbag mandate will aim to profit from liability litigation
against them for legally disconnecting airbags.
