A CALCULATION OF THE EFFECTIVENESS OF CAR SEAT
BY A COMPARISON OF UTILIZATION BY DAYTIME CAR DRIVER FATALITIES WITH
BY THE PARENT POPULATION.
The following changes have been introduced since the
last (2000) study.
- Professor Elliott Levine of the Philosophy
me by making keystone suggestions
the production of this paper over the past two years. It
his crucial suggestion that since NOPUS parent data was obtained during
daylight, then the derivative FARS data (of fatalities) should also be
limited to daylight, rather than presuming (as I had) that the daylight
wearing trend would extend to 24 hours. He was also responsible
drawing my attention to a discrepancy which led to my discovery of the
document in heading (2).
- The earlier study had to be revised following publication of a document
explaining details on the use of the FARS database.
- The police reported "unknown" and police "not reported" data for
affected fatalities have been allocated.(Table 3 Notes)
- It is argued that ignoring the belt status "Unknown"
I have not extended the study beyond 1994 - 1996 because air
became a possibly significant influence on data collected after that
The novel procedure employed in this study is to
the belt utilization of live car drivers (the parent population) with
belt utilization of car driver fatalities. This procedure avoids
problems found in time-step and pair comparison methods. The
procedure is susceptible to chance disturbances caused by such factors
as engineering improvements (such as collapsing steering columns),
influences (such as petrol prices) legislative action (such as police
on alcohol usage) or even the weather. The pair comparison procedure relies upon self
reporting. Evidence indicates that even in the absence of penalties, drivers will report that they were wearing a belt
when they were not.
Belt utilization by the parent population, defined
"daytime car drivers on urban or rural roads" was obtained from NOPUS
Occupant Protection Use Survey) studies. The NHTSA commissioned
to estimate the proportion of car occupants that wear belts.
has stated that "NOPUS are not inconsistent with the national use rate
estimates as calculated from state surveys."
The data on belt utilization by daytime car driver fatalities was
from FARS (Fatality Analysis Reporting System) which is an online
of all road fatalities in the USA provided by NHTSA. This
of the data mining procedure from FARS.
Table 1 was constructed using three 1994 and two
NOPUS (NHTSA) research notes.
- a "moving traffic" study in
the shoulder and lap & shoulder belt use of 167,000 car drivers by
- a "controlled intersection study" in
determined the shoulder and lap & shoulder belt use at
of 50,000 car drivers according to age, race and urbanization in 1994.
- a "shopping center study" where
ratios of use of lap, shoulder and lap & shoulder belt use of 5,500
car drivers as they alighted from vehicles in a shopping center.
Lap belt alone was found to be used by 1% of total drivers in 1994.
- a second "moving traffic" study
in 1996 where
the shoulder and lap & shoulder belt 176,651 car drivers by region.
- a "controlled intersection" study 1996
the shoulder and lap & shoulder belt use at intersections of 49,387
car drivers according to age, race and urbanization in 1996.
TABLE 1 - NOPUS ESTIMATES.
TABLE 1 NOTES
- The data given in parentheses (nn%) are
- The first six rows of data (columns 1994 & 1996) were
the five NOPUS papers listed above.
- The "shopping center" study
drivers used a lap belt. Only shoulder or shoulder
& lap belts
were counted in the other four studies.
- "Urban Adjusted" was calculated by combining city
intersection wearing rates, (using linear interpolation weighted by
and increased by 1% to account for lap belts.
- "Rural Adjusted" is from rural intersection
1% to account for lap belts.
- NOPUS studies were carried out during daylight
Following is an extract from the NHTSA web site:
To obtain as near a match as possible with NOPUS, the
data was extracted for daytime (8am - 6pm) fatalities for drivers of
cars on urban and rural roads.
The Fatality Analysis Reporting System
contains data on a census of fatal traffic crashes within the 50
the District of Columbia, and Puerto Rico. To be included in FARS, a
must involve a motor vehicle traveling on a trafficway customarily
to the public and result in the death of a person (occupant of a
or a non-occupant) within 30 days of the crash. FARS has been
since 1975 and has collected information on over 989,451 motor vehicle
fatalities and collects information on over 100 different coded data
that characterizes the crash, the vehicle, and the people involved.
Urban results for 1994 - 1996 are
in Table 2a, rural results are on Table 2b. Later years were not
considered because of the increasing use of airbags.
TABLE 2a DAYTIME
URBAN CAR DRIVER FATALITIES
TABLE 2b DAYTIME RURAL CAR DRIVER FATALITIES
TABLE 2 NOTES.
ALCOHOL EFFECT TABLES.
- In approximately 12% of urban fatalities and 6%
fatalities the police reported driver belt wearing status is marked as
This failure to obtain data might have been due to intervention by a
party who removed the safety belt (perhaps in an attempt to administer
first aid to the victim.) The high (12%) urban value supports
possibility. Another suggestion is that the quality of the
might have been such that it was not possible to determine with any
whether the driver was originally belted.
- "Belted fatalities %" was calculated by
column value for each year by the sum of "Belted" and "Unbelted" column
values. The Unknown
row was ignored.
factor of E
was determined by the formula E
= 1 (Nb/Nu) x (1 - Nopus)/Nopus where Nb & Nu are the
count of belted & unbelted fatalities, and Nopus is the relevant
observed belt wearing ratio.
According to the NHTSA, drivers under the influence
alcohol are much less likely to wear safety belts.
The proportion of drivers in the parent population
are alcohol affected is estimated as less than 1%.
However it is variously estimated that over 20% of
drivers involved in fatal accidents are alcohol affected.
It follows that a small (< 1%) proportion of
who rarely wear belts produces a large proportion (>20%) of
This imbalance generates an error, known to statisticians as Simpson's
Paradox. To correct for this error, driver
that were reported by the police to be alcohol affected were isolated
the FARS data collected.
The calculated result is shown in Tables 2a and 2b. If the
"failure to report" entries in the FARS data are random, then seat
provide an effectiveness of around 35%. This is somewhat lower
than the figure of 45% suggested by the NHTSA, but establishes the
efficacy of safety belts.
- Adams - John Adams' "Risk" (UCL 1995)
- Evans - Leonard Evans' book "Traffic Safety and the Driver" (Van
- Wilde - Gerald J S Wilde's "Target Risk" (PDE 1994) also
(NOTE February 2012. The links below are now dead. However
enquiries at NOPUS
might - or might not- be productive.)
- FARS. Fatality
Analysis Reporting System
- NHTSA Estimating
Lives saved by Restraint Use in Potentially Fatal Crashes. 1995
- NOPUS 1994
- Controlled Intersection Study ~ US Department of
Transportation. (I have placed
a copy here)
- NOPUS 1995 - Shopping Center Study~ US Department of
- NOPUS 1995
- Observed Safety Belt Use in 1994 ~ Moving Traffic Study US
- NOPUS 1996
- Controlled Intersection Study ~ US Department of Transportation.
- NOPUS 1997
- Observed Safety Belt Use in 1996" ~ research Note US
- NHTSA Alcohol
Traffic Safety Facts 1996