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Relative risk of death from ejection by crash type and crash mode
Amid. Anal. & Prev. Vol. 21. No. 5. pp. 459-468. Prmted I” Great Bntam
$3 (10 + .I10 0001-4575189 0 1989 Pergamon Press plc
1989
RELATIVE RISK OF DEATH FROM EJECTION BY CRASH TYPE AND CRASH MODE* JOY R. ESTERLITZ Insurance
Institute
for Highway (Received
Safety.
4 August
1005 N. Glebe
Road,
1988; in vevi.sed,fwm
Arlington,
30 March
VA 22201.
U.S.A
1989)
Abstract-In
virtually all circumstances, the chance of survival in a crash is much greater if the occupant is not ejected from the vehicle. Several estimates of the increased risk of death as a result of ejection (ranging from 2.5 to 25) have been made, but none were specific to the crash mode and most did not control for crash severity. The current study examined the relative risk of fatality due to ejection, by crash type and crash mode, using the Fatal Accident Reporting System data from the years 1982 through 1986. Crash type was defined as either single vehicle or multivehicle and crash mode included rollover, nonrollover, and/or direction of impact. Crash severity was controlled for using a paired comparison method of analysis. Both crash tvpe and crash mode were found to have substantial effects on the relative risk of death due to ejection. In addition. risk differences across seating position exist. Depending on crash mode or type. the risks ranged from about 1.5 to 8. Single-vehicle rollover crashes have the highest increased risk of death due to ejection: about eightfold for the driver and sevenfold for the right front passenger.
INTRODUCTION
in an automobile crash is much of the increased risk of death been specific both to the type and mode of crash. Crash type is defined here in terms of the number of vehicles involved and crash mode by the principal point of impact and whether the vehicle rolled over. Also, most studies have not controlled for crash severity. As a result, estimates of the increased risk of death from ejection range widely, from 2 112 to 25. Crashes in which an occupant is ejected are usually quite severe, and thus it is somewhat difficult to attribute the increased risk of fatality to ejection, as some of the increase may be due to differences in crash severity. Hedlund (1979), using National Crash Severity Study data, investigated ejection relative to crash mode but did not control for crash severity. The fatality rate for ejection in rollover crashes was about twice the fatality rate for ejection in frontal or side impacts. Bondy and Hart (1982), using National Center for Statistics and Analysis data, studied ejection rates by crash type, but they did not supply a crash-mode specific estimate of the increased risk of fatality due to ejection. This study also did not control for crash severity. The only estimate provided was an overall increased risk of death--the chance of death when ejected was 25 times that when not ejected. In a report on injuries associated with ejected and nonejected occupants, O’Day and Scott (1984) found that the relative risk of dying was about 2 l/2 times greater if the occupant was ejected. Although this study controlled for crash severity through a matched pairs analysis, only 121 cases were included. The most recent study of injuries and fatalities associated with ejection was conducted by the National Highway Traffic Safety Administration (NHTSA) (Sikora 1986). The researchers concluded that the risk of death due to ejection for ejected drivers was four times that for nonejected drivers, and for ejected right front passengers, the risk was about 2 l/2 times that for nonejected right front passengers compared to unejected drivers or passengers. The study design accounted for crash severity via the double pair comparison method (Evans 1985), but it did not incorporate restraint use, crash type, or crash mode as factors. The present paper reports the results of a study of the relative risk of fatality as a In virtually
all circumstances,
greater
if the occupant as a result of ejection
*This work was supported
the
chance
of survival
is not ejected. Several estimates have been made, but none have
by the Insurance
Institute
for Highway 459
Safety,
460
J. R. ESTERLITZ
result of ejection using the Fatal Accident Reporting System (FARS) data from the years 1982 through 1986 [U.S. Department of Transportation (DOT) 1982-861. The estimates of risk of death attributable to ejection are derived from an analysis that controls for crash severity, within crash type, by using a paired comparison method of analysis, and incorporates crash type and crash mode as factors. In this study, crash type is either single or multiple vehicle and crash mode includes rollover, nonrollover, and direction of impact. The likelihood of ejection and the ejection route can be expected to differ by crash type and crash mode. In side impacts, for example, the ejection path is most often through the side door or side window contacted by the occupant (Fan and Jettner 1982). In rollovers, more opportunities for ejection occur. The route of ejection for most occupants in rollover crashes is through the side window and door area, but it may also be through the vehicle roof (sunroofs, t-tops, etc.) or rear window. Because of these differences, is it necessary to control for crash type and crash mode. METHOD
Fatality data from 1982-1986 FARS records meeting the following criteria were included: a driver occupied the driver seat and a passenger occupied the right front seat; the driver and passenger were both 15 years of age or older; it was known whether the driver and right front passenger were ejected or not; it was known whether the car rolled over or what the principal point of impact was; either the driver, right front passenger, or both died; several people did not share the driver or right front seating position; there was no middle front passenger; there were no rear occupants; and both the driver and the right front passenger were reportedly unbelted. A total of 14,579 such driver-passenger pairs were identified. The double pair method is based on these pairs of two occupants, the driver and right front passenger (Evans 1985). The frequency of fatalities for a subject occupant was compared to that for the other occupant when the subject had one of two characteristics, in this case, ejected or not ejected. The ejected category includes both partial and total ejections. The subject occupant was either the driver or the right front passenger. The other occupant served as a control by estimating exposure, as both were in the same vehicle and thus exposed to the same crash conditions. The relative risk of death to a driver (or passenger) is a ratio of ratios: the numerator ratio is the number of drivers (passengers) killed and ejected divided by the number of passengers (drivers) killed but not ejected, and the denominator ratio is the number of unejected drivers (passengers) killed divided by the number of unejected passengers (drivers) killed. The relative magnitude of the ratios depends on the risk of fatality due to being ejected vs. staying in the vehicle during the crash and the greater or lesser likelihood of ejection in crashes of different types and modes, which is accounted for by determining the ratio for similar sets of crashes. After these pairs of drivers and right front passengers were selected from FARS, they were separated by crash type and crash mode into several categories: (1) singlevehicle crashes-rollover and nonrollover; (2) multivehicle car-car crashes-frontal, right side, left side, rear, and rollover; and (3) multivehicle car-other vehicle (where other vehicles include pickups, vans, tractor-trailers, and so forth)-frontal, right side, left side, rear, and rollover. Crash mode was determined as the most severe or principal impact to the vehicle for multivehicle crashes. If there was a rollover subsequent to the principal impact in the multivehicle crash, then the crash mode category was rollover. If there were enough data in each of the crash type and crash mode categories, the driver/passenger pairs in each category were separated in terms of age or sex to determine if any systematic biases were present. This separation was useful because of the possibility of confounding effects due to interactions between age or sex and behavior, vehicle use patterns, and physical responses to the same crash as a function of age or sex. Within each of these crash categories, fatality risks were calculated by forming ratios of all possible combinations of outcome for the driver and passenger; the driver or
Risk of death
461
from ejection
passenger may or may not be ejected, the driver may die, the passenger may die, or both the driver and passenger may die. Two relative risks were calculated for the driver and the passenger in each crash category. The first used an ejected driver as the subject and nonejected passenger as the control, and the second used an ejected driver as the subject and an ejected passenger as the control. These two risks were then combined by way of weighted averages to provide an overall risk of driver death from ejection, if they did not differ significantly. The weighted average was calculated by multiplying each of the two risks by the number of subject/control pairs used to determine each risk, adding these two quantities together, and dividing by the total number of subject/control pairs in both risk calculations. (For a specific example, see Table C-l in Appendix C.) To determine whether it was appropriate to combine these two risks based on an ejected and unejected control, a t-test was performed. Because the logarithm of the relative risk has a normal distribution, a standard t-test was used in the following manner: t = (log R, - log R&w,
where R, and R2 are relative risk estimates for the two different controls. The corresponding variances, u : and (J $, were calculated from the four totals of fatality counts that were used to form the ratios for the risk estimates, CT’ = l/n + l/b + l/c + l/d. (See Appendix A for detailed example.) If this test for homogeneity was not significant at the five percent level, the two relative risks were weighted and combined. If the risk estimates were significantly different, then they were not combined. The same procedure was then done for the right front passenger. RESULTS
A total of 8,626 pairs of drivers and passengers were in single-vehicle crashes that met the matched pair criteria; 4,362 were in rollovers, and 4,264 were in nonrollovers. (The raw data are shown in Appendix B .) The relative risk of driver death from ejection is two times that of nonejected drivers when they are in single-vehicle, nonrollover crashes; the increased relative risk for ejected drivers is about eight times that of nonejected drivers in rollover crashes. For ejected passengers in single-vehicle nonrollover crashes, the risk is about 1 l/2 times that of nonejected passengers, their increased relative risk is more than seven times greater in rollovers. The exact risks are given in Table 1. In the single-vehicle crash mode, there were sufficient data to disaggregate by driver and passenger age or by sex. This allows examination of whether the differences in relative risk of death attributed to ejection are not, in some part, due to age or sex differences for certain types of crashes. This separation is useful because of the possibility of confounding effects due to the interactions between age or sex and behavior or vehicle use patterns. For example, if passengers traveling with female drivers tended to be younger than passengers traveling with male drivers, the calculations of the relative risk of death due to ejection could be biased. By separating the passengers into age categories, and examining drivers of the same age, the potential confounding interactions are removed. The subjects, either drivers or the right front passengers, were divided into three age categories: 16-35,36-64, and 65 years of age or over. Because of the limited sample size, only three age categories were feasible; if finer categories were used, age-specific
Table
1. Average relative risk of death to ejection in single-vehicle crashes
Subject Driver Passenger
MP 21:5-o
Rollover 8.2 7.4
due
Nonrollover 2.2 1.6
462
.I. R. ESTERLITZ
cell sizes would be too small to calculate relative risks. There are six categories of control occupants (three age categories times two ejection categories), so for the driver or the right front passenger in each of the three age groups, there are six estimates of the relative risk of death due to ejection. In the disaggregation by sex, there are four categories of other occupant (two sex categories times two ejection categories), so for the driver or the right front passenger in each sex group, there are four estimates of the relative risk of death due to ejection. These estimates are shown in Table 2 and Table 3, respectively. To determine whether a systematic age or sex effect existed, the t-test was used again: t = (log R, -
log R>)/a,
where R, and R2 are relative risk estimates for different sex or age groups and of and a$ are the corresponding variances. For example, from Table 3, R may be the risk for an ejected female driver/unejected male passenger and R2 may be the risk for an ejected male driver/unejected male passenger; the t statistic becomes: t = [log 10.9 - log 8.0]&/0.12282
+ 0.02696 = 0.31174/0.38701 = 0.8055.
This is not significant at the five percent level. Table
2. Relative
risk of death
due to ejection, by age, nonrollover crashes
for single-vehicle
rollover
Relative Subject
(ejected)
Driver
Control
Age (yrs.)
16-35
Unejected pass. Unejected pass. Unejected pass. Ejected pass. Ejected pass. Ejected pass. Unejected driver Unejected driver Unejected driver Ejected driver Ejected driver Ejected driver Unejected pass. Unejected pass. Unejected pass. Ejected pass. Ejected pass. Ejected pass. Unejected driver Unejected driver Unejected driver Ejected driver Ejected driver Ejected driver Unejected pass. Unejected pass. Unejected pass. Ejected pass. Ejected pass. Ejected pass. Unejected driver Unejected driver Unejected driver Ejected driver Ejected driver Ejected driver
16-35 36-64 6.5 + 16-35 36-64 65+ 16-35 36-64 65 + 16-35 36-64 65 + 16-35 36-64 65+ 16-35 36-64 65+ 16-35 36-64 65+ 16-35 36-64 65 + 16-3.5 36-64 65 + 16-35 36-64 65 + 16-35 36-64 65+ 16-35 36-64 65 +
16-35
Passenger
36-64
Driver
Passenger
36-64
Driver
65+
65+
Passenger
*Sample
Age (yrs.)
size 53
Rollover
risk No rollover
7.6 10.2 _*
2.2 2.7
8.8 11.2
2.2 3.3
7.8 7.9
1.7 4.2
6.8 6.9
1.7 1.o
5.4 7.1
1.5 2.5 -
6.2 4.4
4.0 2.5
7.9 4.2
1.0 5.8
7.2 6.X
1.9
-
5.2
7.9
9.5
6.2
and
Risk of death Table
3. Relative
risk of death
463
from ejection
due to ejection, by sex, for single-vehicle nonrollover crashes
rollover
Relative Subject
(ejected)
Sex
Driver
Male
Driver
Female
Passenger
Male
Passenger
Female
*Sample
Control
Sex
Unejected pass. Unejected pass. Ejected pass. Ejected pass. Unejected pass. Unejected pass. Ejected pass. Ejected pass. Unejected driver Unejected driver Ejected driver Ejected driver Unejected driver Unejected driver Ejected driver Eiected driver
Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female
Rollover
and
risk No rollover
8.0 5.6 10.1 8.1 10.9 9.5 _*
1.9 2.3 2.0 4.2 4.4 2.6 2.x 2.8 1.5 2.1 1.4 3.4 2.3 1.3 1.3 1.2
11.0 6.7 5.3 13.4 8.1 8.1 8.0 5.7 6.9
size 53.
In both the rollover and nonrollover crash modes, no systematic age or sex biases were found. The multivehicle crashes were separated into two categories, car-car crashes and car-other vehicle crashes. There were 1,812 pairs of drivers and passengers in car-car crashes and 4,141 in car-other vehicle crashes that met the matched pair criteria. (The raw data are shown in Appendix B.) The average relative risk of death for ejected drivers and passengers by crash mode for car-car and car-other vehicle crashes is shown in Table 4. In both the car-car and car-other vehicle crashes, the right side impact crash types have the highest increased relative risk of death due to ejection, ranging from about three to about six. In both crash mode categories, frontal crashes have the lowest increased relative risk, ranging from one to two times greater risk of death for ejected drivers or passengers than for nonejected drivers or passengers. In the left side impact crash type, the risks range from about 1 l/2 to nearly 4. The rollover category for the multivehicle crashes was too small to provide meaningful estimates. (The detailed calculations performed to arrive at these average relative risks are shown in Appendix C.) Standard errors of these risk estimates were calculated by assuming that the observed fatality counts were Poisson distributed (Evans and Frick 1987). The number of ejected drivers or passengers determines the precision of the risk estimates. The standard errors vary considerably, depending on sample size, that is, on the number of subject and control occupants in the ratios that were formed to calculate the relative risks. If ejection Table 4. Average
relative risk of death multivehicle crashes Crash
Subject Car-car crashes Driver Passenger Car-other vehicle crashes Driver Passenger
Frontal
due to ejection
in
mode*
Right
side
Left side
1.6 1.8
6.1 2.5
1.8 3.7
2.0 1.3
3.1 2.4
1.6 2.6
Note: Crash mode is determined by principal point of impact. *The rear impact crash mode category had sample size 53.
464
J. R. ESTERLITZ
in a certain crash mode and type category is a rare event, then the standard error is larger. Otherwise, the standard errors are small and the risk estimates are stable. (The details of calculating the standard error estimates are found in Appendix A.)
DISCUSSION
Crash type and crash mode have substantial effects on the relative risks of death due to ejection. Seating position makes some difference, depending on crash mode, but generally the risk of death from ejection is similar for drivers and right front passengers except in right side impacts. Single-vehicle rollover crashes produce the highest risk of death due to ejection for both drivers and passengers. This high risk is attributable to the kinematics of the crash. In single-vehicle nonrollover crashes, the relative risk of death due to ejection is not as high as for rollover crashes, but ejection still increases risk of death by about a factor of two. In the multivehicle crashes, across crash type (i.e. in the car-car crashes and the car-other vehicle crashes) similar increased relative risks of death due to ejection are found in each crash mode category, except for right side impacts where the car-car risks are higher than the car-other vehicle risks. In a side impact collision, there is more occupant compartment intrusion on the near-impact side, thus increasing the likelihood that the near-side occupant would be killed yet remain in the car. Furthermore, in a collision between a car and a much larger vehicle, more intrusion would be expected than in a collision between two cars. If there is more intrusion, the passenger and driver are more likely to be killed yet remain in the car, rather than being ejected from the car. This reasoning aids in explaining the relative magnitudes of the risks of death due to ejection for passengers and drivers in the side impact crash modes. In both multivehicle crash types, and in both right side and left side impacts, the risk of death due to ejection is greater for the far side occupant than for the struck side occupant (see Table 4). That is, the relative risk of death due to ejection is higher for a passenger (than a driver) in a left side impact and for a driver (than a passenger) in a right side impact. These differences in the relative risks between passengers and drivers are greater in the car-car crash type (more than a two to one ratio) compared to the car-other vehicle crash type (about a 1 112 to 1 ratio). Again, this may be attributed to the greater crush in the car-other crashes that could cause as many deaths overall but fewer via ejection.
CONCLUSIONS
When crash severity is controlled for via the paired comparison method, both crash type and crash mode are important factors in the relative risk of death from ejection. The previously published estimates of the increased relative risks of death from ejection, which range from 2 l/2 (Sikora 1986) to 25 times (Hedlund 1979), in one case underestimate and in the other overestimate the actual relative risks by not accounting for crash type, crash mode, restraint usage, and/or crash severity. The results presented in this paper help to identify specific aspects of the ejection problem. The range of risk of death from ejection depends on crash type, crash mode, and seating position. Rollover crashes are the main candidates for high risk of death due to ejection. The primary ejection routes in rollovers are through the side window and door areas and the secondary routes are through the roof and rear window areas. The federal motor vehicle safety standards applicable to side and rear windows only require them to be tempered glass. With laminated glazing and adequate all-around anchoring of the window, many deaths by ejection could be prevented (Clark and Sursi 1985). Clark and Sursi (19X5) would have the plastic layer extending beyond the window glass and fully attached; thus, the window would not open. Prototypes for laminated glazed side windows are under development where the windows would be able to open. Also, side impact crashes have increased relative risks of death due to ejection for opposite side impacts. Upgraded
Risk of death
465
from ejection
door latches or tougher door latch standards could reduce many of these fatalities due to ejection through doors that open. Much of the increased relative risk of death due to ejection could be reduced if occupants were belted.
REFERENCES Bondy, N.; Hart, S. An analysis of the ejection problem using NCSA automated data files. Washington, DC: National Highway Traffic Safety Administration; 1982. Clark, C. C.; Sursi. P. Car crash tests of ejection reductions by glass-plastic glazing. SAE technical paper 851203. Warrendale, PA: Society of Automotive Engineers; 1985. Evans. L. Double pair comparison-a new method to determine how occupant characteristics affect fatality risk in traffic crashes. Accid. Anal. Prev. 18:217-227; 1985. Evans, L.; Frick. M. C. Seating position in cars and fatality risk. GMR-5911. Warren. MI: General Motors Research Laboratories; 1987. Fan, W. R. S.; Jettner. E. Light vehicle occupant protection-top and rear structures and interiors. SAE technical paper 820244. Warrendale. PA; Society of Automotive Engineers; 1982. Hedlund, .I. H. The National Crash Severity Study and its relationship to ESV design criteria. DOT HS 805. 199. Proceedings of the seventh international technical conference on experimental safety vehicles. Washington, DC: U.S. Department of Transportation; 1979. O’Day. J.: Scott, R. E. Corrected findings on injuries associated with ejected and non-ejected occupants. The UMTRI research review. Ann Arbor, MI: The University of Michigan Transportation Research Institute; 1984. Sikora, J. Relative risk of death for ejected occupants in fatal traffic accidents. DOT HS 807-096. Washington, DC: National Highway Traffic Safety Administration; 1986. U.S. Department of Transportation (U.S. DOT). Fatal Accident Reporting System 1982-1986. Washington, DC: National Highway Traffic Safety Administration; 1982-1986.
APPENDIX The standard
error,
A:
AR, in the estimate
STANDARD
ERROR
of relative
risk of death
AR = Rd\/a’ + l/a
+ lib
CALCULATIONS due to ejection,
+ l/c
is given by:
+ lid,
where a, b, c. and d are the number of passengers and drivers killed in the subject and control ejection categories; cr2 is an estimate of the intrinsic uncertainty analagous to a random instrumental error (Evans 1985). In this study, the oZ is assumed to be zero. As an example, consider the standard error calculation for the relative risk of death due to ejection for an ejected driver in a single-vehicle, rollover crash as the subject and an unejected passenger as the control (see Table Bl for raw data): R = 7.95 = 9.2211.16
= (904/98)/(776/669).
where 904 98 776 669
= = = =
ejected drivers killed; unejected passengers killed travelling unejected drivers killed; unejected passengers killed travelling
with ejected
drivers
with unejected
killed;
killed drivers;
and
AR = 7.95 V/11904 + 1198 + 11776 + 11669; AR = 7.95 (0.11872); AR = 0.94. This procedure was done for all of the relative risks (every subject and control in each crash mode and crash type category. The results are shown in following
Table Al.
Single-vehicle
crashes Rollover
Subject
(ejected)
Driver Driver Passenger Passenger
combination) and calculated Tables Al, A2, and A3.
No rollover
Control
Risk
SE
Risk
SE
pass. Unejected Ejected pass. Unejected driver Ejected driver
7.95 8.58 7.57 7.01
0.94 0.98 0.87 0.83
2.15 2.35 1.60 1.46
0.25 0.31 0.16 0.21
464
J. R. ESTERLITZ Table A2.
Car-car
crashes
Front Subject
(ejected)
Driver Driver Passenger Passenger *Sample
Rear
Control
Risk
SE
Risk
SE
Risk
SE
Risk
SE
pass. Unejected Ejected pass. Unejected driver Ejected driver
1.53 1.77 1.77 1.53
0.46 0.68 0.48 0.62
-* -
-
1.84 1.32 3.48 4.88
1.03 1.03 1.64 3.25
6.10 2.52
2.95 1.48
Standard
errors
for relative
risk estimates
Front (ejected)
Driver Driver Passenger Passenger
Rear
vehicle
Left
crashes
Right
Roll
Risk
SE
Risk
SE
Risk
SE
Risk
SE
Risk
SE
Unejected pass. Ejected pass. Unejected driver Ejected driver
2.09 1.42 1.28 1.89
0.46 0.34 0.23 0.51
4.80 5.31 4.14 3.73
3.16 3.87 2.40 2.96
1.60 1.78 2.60 2.35
0.57 0.61 0.63 1.02
2.97 3.84 2.43 1.88
0.75 1.63 0.88 0.62
1.31 2.63 3.56 1.78
0.96 2.23 2.56 1.53
PAIR
RAW
DATA
Table Bl.
Ejected Passenger
No No Yes Yes Total
in car-other
Control
APPENDIX
Driver
Right
size 53.
Table A3.
Subject
Left
1982-1986
matched
Driver passenger
died/ survived
Rollover
No Yes No Yes
B:
B2.
1982-1986
crashes
by crash mode
Driver survived/ passenger died Rollover
1,122 96 178 175 1,571
Driver passenger
Ejected
pair data for single-vehicle
No rollover
599 75 860 652 2,186
Table
MATCHED
No rollover
492 688 54 436 1,670
matched
Both died Rollover
1,505 254 81 132 1,972
nair data for car-car
177 36 44 249 506
crashes
Total
507 65 50 99 721
4,402 1,214 1,267 1,743 8,626
bv crash mode
Driver survived/ passenger died
died/ survived
No rollover
Both died
Driver
Passenger
Front
Rear
Left
Right
Front
Rear
Left
Right
Front
Rear
Left
Right
NO No Yes Yes Total
No
362 6 17 13 398
15
169 3 17 6 195
22 1 6 7 36
364 23 7 13 407
22 Y 1 2 34
33 5 0 5 43
266 32 3 17 31x
1x7 16 12 13 228
Y
36
4Y
vehicle
crashes
Yes No Yes
Table B3. Ejected
I 7 4 27
1982-1986
matched
Driver
died/passenger
pair data for car-other
survived
Driver
survived/passenger
died
I
6
1 1 12
4 8 54
by crash
Total
I .534
1
104 X0 94
5 5 60
I.812
mode
Both died
Driver
Passenger
Front
Rear
Left
Right
Roll
Front
Rear
Left
Right
Roll
Front
Rear
Left
Right
Roll
No No Yes Yes Total
No Yes No Yes
671 28 55 40 794
42 3 13 7 65
480 16 34 23 553
64 1 13 5 X3
39 1 5 2 47
573 38 15 25 651
44 16 2 5 67
70 14 0 4 86
588 53 11 28 680
17 5 2 I 25
424 31 I6 45 516
30
165 23 10 22 220
228 8 21 25 282
25 2
I 1 3 35
I 7 35
Total 3.460 240 19’) 242 4,141
467
Risk of death from ejection APPENDIX
C
Table Cl. Single-vehicle crashes detailed calculations of relative risk of death due to ejection Step 1. Driver and passenger fatalities by ejection Number of fatalities Ejection
Passengers killed
Drivers killed
Driver
Passenger
No No Yes Yes Total
No Yes No Yes
Rollover
Rollover
No rollover
776* 111 904 901 2692
No rollover
669 124 98 68.5 2176
1629 161 228 274 2292
*From Table Bl: 776 = 599 (driver died, passenger survived) passenger died).
2012 319 131 231 2693
+ 177 (driver died,
Step 2. Calculation of driver and passenger fatality ratios Ratio of fatalities Passengers killed/ drivers killed
Drivers killed/ passengers killed
Ejection Driver
Passenger
Rollover
No rollover
Rollover
No rollover
No No Yes Yes
No Yes No Yes
1.16* 0.15 9.22 1.32
0.81 0.51 1.14 1.19
0.86 6.52 0.11 0.76
1.24 1.98 0.57 0.84
*From Step 1: 7761669 = 1.1599 = 1.16 when rounded to two decimal places. Step 3. Calculation of relative risks from fatality ratios Relative risk of death from ejection Subiect (eiected)
Control
Rollover
No rollover
Driver Driver Passenger Passenger
Unejected pass. Ejected pass. Unejected driver Ejected driver
7.95 8.58 1.51 7.01
2.15 2.35 1.60 1.46
Example of weighted average calculation: (904 + 776)7.95 + (901 + 111)8.58 = 8.2, (904 + 776 + 901 + 111) Note: Numbers are rounded to two decimal places for presentation. were carried out to four decimal places.
All calculations
Table C2. Car-car crashes Step 1. Driver and passenger fatalities by ejection Numbei of fatalities Ejection
Drivers killed
Passengers killed
Driver
Passenger
Front
Rear
Left
Right
Front
Rear
Left
Right
No No Yes Yes Total
No Yes No Yes
549 22 29 26 626
24 2 8 5 39
205 9 21 14 249
71 2 11 12 96
551 39 19 26 635
31 10 2 3 46
69 11 4 13 97
315 33 8 22 378
468
J. R. ESTERLITZ Table C2. (continued) of driver and passenger
Step 2. Calculation
Ratio Ejection Driver
Drivers
Passenger
No
No Yes No Yes
No Yes Yes
killed/passengers
ratios
of fatalities
killed
Passengers
killed/drivers
killed
Front
Rear
Left
Right
Front
Rear
Left
Right
1.00 0.56 1.53 1.00
0.77 -* -
2.85 0.82 5.25 1.08
0.23 1.38 0.55
1.00 1.77 0.66 1.00
1.29 -
0.35 1.22 0.19 0.93
4.44
Step 3. Calculation
of relative
Relative Subject
fatality
(ejected)
Driver Driver Passenger Passenger
risks from fatality
risk of death
from ejection
0.73 1.83
ratios by crash
mode
Control
Front
Rear
Left
Right
Unejected pass. Ejected pass. Unejected driver Ejected driver
1.53 1.77 1.77 1.53
-
1.84 1.32 3.48 4.88
6.10
-
2.52
*Crash modes are excluded if any of the four fatality frequencies used to calculate Very small cell frequencies make inferences and interpretations of data unreliable.
risks is 53.
Table C3. Car-vehicle crashes Step 1. Driver and passenger fatalities by ejection Number Drivers
Ejection Driver
Passenger
No No Yes Yes Total
Front
No Yes No Yes
Right
Roll
72 4 14 10 100
645 39 44 45 773
292 9 34 30 365
64 3 6 9 82
Drivers
Passenger
No No Yes Yes
No Yes No Yes
Note:
Sample
of driver
killed/passengers
997 69 31 70 1,167
and passenger
fatality
Rear
Left
Right
Roll
74 17 3 x 102
235 37 10 26 308
816 61 32 53 962
32 7 3 x 60
ratios
Passengers
killed
killed/drivers
killed
Rear
Left
Right
Roll
Front
Rear
Left
Right
Roll
1.10 0.86 2.29 1.21
0.97 0.24 1.25
2.74 1.05 4.40 1.88
0.36 0.15 1.06 0.57
1.52 1.13
0.91 1.17 0.44 0.82
1.02 4.25 0.80
0.36 0.95 0.23 0.53
2.79 6.7X 0.94 1.77
0.66 0.89
size 53.
Relative (ejected)
Driver Driver Passenger Passenger
Front
killed
Front
Step 3. Calculation
Subject
Passengers
Left
Step 2. Calculation
Driver
killed
Rear
1,095 59 71 85 1,310
Ejection
of fatalities
of relative
risks from fatality
risk of death
from ejection
ratios by crash mode
Control
Front
Rear
Left
Right
Unejected pass. Ejected pass. Unejected driver Ejected driver
2.09 1.42 1.28 1.89
5.31 4.14
1.60 1.7x 2.60 2.35
2.97 3.84 2.43 1.8X
Roll
$3 (10 + .I10 0001-4575189 0 1989 Pergamon Press plc
1989
RELATIVE RISK OF DEATH FROM EJECTION BY CRASH TYPE AND CRASH MODE* JOY R. ESTERLITZ Insurance
Institute
for Highway (Received
Safety.
4 August
1005 N. Glebe
Road,
1988; in vevi.sed,fwm
Arlington,
30 March
VA 22201.
U.S.A
1989)
Abstract-In
virtually all circumstances, the chance of survival in a crash is much greater if the occupant is not ejected from the vehicle. Several estimates of the increased risk of death as a result of ejection (ranging from 2.5 to 25) have been made, but none were specific to the crash mode and most did not control for crash severity. The current study examined the relative risk of fatality due to ejection, by crash type and crash mode, using the Fatal Accident Reporting System data from the years 1982 through 1986. Crash type was defined as either single vehicle or multivehicle and crash mode included rollover, nonrollover, and/or direction of impact. Crash severity was controlled for using a paired comparison method of analysis. Both crash tvpe and crash mode were found to have substantial effects on the relative risk of death due to ejection. In addition. risk differences across seating position exist. Depending on crash mode or type. the risks ranged from about 1.5 to 8. Single-vehicle rollover crashes have the highest increased risk of death due to ejection: about eightfold for the driver and sevenfold for the right front passenger.
INTRODUCTION
in an automobile crash is much of the increased risk of death been specific both to the type and mode of crash. Crash type is defined here in terms of the number of vehicles involved and crash mode by the principal point of impact and whether the vehicle rolled over. Also, most studies have not controlled for crash severity. As a result, estimates of the increased risk of death from ejection range widely, from 2 112 to 25. Crashes in which an occupant is ejected are usually quite severe, and thus it is somewhat difficult to attribute the increased risk of fatality to ejection, as some of the increase may be due to differences in crash severity. Hedlund (1979), using National Crash Severity Study data, investigated ejection relative to crash mode but did not control for crash severity. The fatality rate for ejection in rollover crashes was about twice the fatality rate for ejection in frontal or side impacts. Bondy and Hart (1982), using National Center for Statistics and Analysis data, studied ejection rates by crash type, but they did not supply a crash-mode specific estimate of the increased risk of fatality due to ejection. This study also did not control for crash severity. The only estimate provided was an overall increased risk of death--the chance of death when ejected was 25 times that when not ejected. In a report on injuries associated with ejected and nonejected occupants, O’Day and Scott (1984) found that the relative risk of dying was about 2 l/2 times greater if the occupant was ejected. Although this study controlled for crash severity through a matched pairs analysis, only 121 cases were included. The most recent study of injuries and fatalities associated with ejection was conducted by the National Highway Traffic Safety Administration (NHTSA) (Sikora 1986). The researchers concluded that the risk of death due to ejection for ejected drivers was four times that for nonejected drivers, and for ejected right front passengers, the risk was about 2 l/2 times that for nonejected right front passengers compared to unejected drivers or passengers. The study design accounted for crash severity via the double pair comparison method (Evans 1985), but it did not incorporate restraint use, crash type, or crash mode as factors. The present paper reports the results of a study of the relative risk of fatality as a In virtually
all circumstances,
greater
if the occupant as a result of ejection
*This work was supported
the
chance
of survival
is not ejected. Several estimates have been made, but none have
by the Insurance
Institute
for Highway 459
Safety,
460
J. R. ESTERLITZ
result of ejection using the Fatal Accident Reporting System (FARS) data from the years 1982 through 1986 [U.S. Department of Transportation (DOT) 1982-861. The estimates of risk of death attributable to ejection are derived from an analysis that controls for crash severity, within crash type, by using a paired comparison method of analysis, and incorporates crash type and crash mode as factors. In this study, crash type is either single or multiple vehicle and crash mode includes rollover, nonrollover, and direction of impact. The likelihood of ejection and the ejection route can be expected to differ by crash type and crash mode. In side impacts, for example, the ejection path is most often through the side door or side window contacted by the occupant (Fan and Jettner 1982). In rollovers, more opportunities for ejection occur. The route of ejection for most occupants in rollover crashes is through the side window and door area, but it may also be through the vehicle roof (sunroofs, t-tops, etc.) or rear window. Because of these differences, is it necessary to control for crash type and crash mode. METHOD
Fatality data from 1982-1986 FARS records meeting the following criteria were included: a driver occupied the driver seat and a passenger occupied the right front seat; the driver and passenger were both 15 years of age or older; it was known whether the driver and right front passenger were ejected or not; it was known whether the car rolled over or what the principal point of impact was; either the driver, right front passenger, or both died; several people did not share the driver or right front seating position; there was no middle front passenger; there were no rear occupants; and both the driver and the right front passenger were reportedly unbelted. A total of 14,579 such driver-passenger pairs were identified. The double pair method is based on these pairs of two occupants, the driver and right front passenger (Evans 1985). The frequency of fatalities for a subject occupant was compared to that for the other occupant when the subject had one of two characteristics, in this case, ejected or not ejected. The ejected category includes both partial and total ejections. The subject occupant was either the driver or the right front passenger. The other occupant served as a control by estimating exposure, as both were in the same vehicle and thus exposed to the same crash conditions. The relative risk of death to a driver (or passenger) is a ratio of ratios: the numerator ratio is the number of drivers (passengers) killed and ejected divided by the number of passengers (drivers) killed but not ejected, and the denominator ratio is the number of unejected drivers (passengers) killed divided by the number of unejected passengers (drivers) killed. The relative magnitude of the ratios depends on the risk of fatality due to being ejected vs. staying in the vehicle during the crash and the greater or lesser likelihood of ejection in crashes of different types and modes, which is accounted for by determining the ratio for similar sets of crashes. After these pairs of drivers and right front passengers were selected from FARS, they were separated by crash type and crash mode into several categories: (1) singlevehicle crashes-rollover and nonrollover; (2) multivehicle car-car crashes-frontal, right side, left side, rear, and rollover; and (3) multivehicle car-other vehicle (where other vehicles include pickups, vans, tractor-trailers, and so forth)-frontal, right side, left side, rear, and rollover. Crash mode was determined as the most severe or principal impact to the vehicle for multivehicle crashes. If there was a rollover subsequent to the principal impact in the multivehicle crash, then the crash mode category was rollover. If there were enough data in each of the crash type and crash mode categories, the driver/passenger pairs in each category were separated in terms of age or sex to determine if any systematic biases were present. This separation was useful because of the possibility of confounding effects due to interactions between age or sex and behavior, vehicle use patterns, and physical responses to the same crash as a function of age or sex. Within each of these crash categories, fatality risks were calculated by forming ratios of all possible combinations of outcome for the driver and passenger; the driver or
Risk of death
461
from ejection
passenger may or may not be ejected, the driver may die, the passenger may die, or both the driver and passenger may die. Two relative risks were calculated for the driver and the passenger in each crash category. The first used an ejected driver as the subject and nonejected passenger as the control, and the second used an ejected driver as the subject and an ejected passenger as the control. These two risks were then combined by way of weighted averages to provide an overall risk of driver death from ejection, if they did not differ significantly. The weighted average was calculated by multiplying each of the two risks by the number of subject/control pairs used to determine each risk, adding these two quantities together, and dividing by the total number of subject/control pairs in both risk calculations. (For a specific example, see Table C-l in Appendix C.) To determine whether it was appropriate to combine these two risks based on an ejected and unejected control, a t-test was performed. Because the logarithm of the relative risk has a normal distribution, a standard t-test was used in the following manner: t = (log R, - log R&w,
where R, and R2 are relative risk estimates for the two different controls. The corresponding variances, u : and (J $, were calculated from the four totals of fatality counts that were used to form the ratios for the risk estimates, CT’ = l/n + l/b + l/c + l/d. (See Appendix A for detailed example.) If this test for homogeneity was not significant at the five percent level, the two relative risks were weighted and combined. If the risk estimates were significantly different, then they were not combined. The same procedure was then done for the right front passenger. RESULTS
A total of 8,626 pairs of drivers and passengers were in single-vehicle crashes that met the matched pair criteria; 4,362 were in rollovers, and 4,264 were in nonrollovers. (The raw data are shown in Appendix B .) The relative risk of driver death from ejection is two times that of nonejected drivers when they are in single-vehicle, nonrollover crashes; the increased relative risk for ejected drivers is about eight times that of nonejected drivers in rollover crashes. For ejected passengers in single-vehicle nonrollover crashes, the risk is about 1 l/2 times that of nonejected passengers, their increased relative risk is more than seven times greater in rollovers. The exact risks are given in Table 1. In the single-vehicle crash mode, there were sufficient data to disaggregate by driver and passenger age or by sex. This allows examination of whether the differences in relative risk of death attributed to ejection are not, in some part, due to age or sex differences for certain types of crashes. This separation is useful because of the possibility of confounding effects due to the interactions between age or sex and behavior or vehicle use patterns. For example, if passengers traveling with female drivers tended to be younger than passengers traveling with male drivers, the calculations of the relative risk of death due to ejection could be biased. By separating the passengers into age categories, and examining drivers of the same age, the potential confounding interactions are removed. The subjects, either drivers or the right front passengers, were divided into three age categories: 16-35,36-64, and 65 years of age or over. Because of the limited sample size, only three age categories were feasible; if finer categories were used, age-specific
Table
1. Average relative risk of death to ejection in single-vehicle crashes
Subject Driver Passenger
MP 21:5-o
Rollover 8.2 7.4
due
Nonrollover 2.2 1.6
462
.I. R. ESTERLITZ
cell sizes would be too small to calculate relative risks. There are six categories of control occupants (three age categories times two ejection categories), so for the driver or the right front passenger in each of the three age groups, there are six estimates of the relative risk of death due to ejection. In the disaggregation by sex, there are four categories of other occupant (two sex categories times two ejection categories), so for the driver or the right front passenger in each sex group, there are four estimates of the relative risk of death due to ejection. These estimates are shown in Table 2 and Table 3, respectively. To determine whether a systematic age or sex effect existed, the t-test was used again: t = (log R, -
log R>)/a,
where R, and R2 are relative risk estimates for different sex or age groups and of and a$ are the corresponding variances. For example, from Table 3, R may be the risk for an ejected female driver/unejected male passenger and R2 may be the risk for an ejected male driver/unejected male passenger; the t statistic becomes: t = [log 10.9 - log 8.0]&/0.12282
+ 0.02696 = 0.31174/0.38701 = 0.8055.
This is not significant at the five percent level. Table
2. Relative
risk of death
due to ejection, by age, nonrollover crashes
for single-vehicle
rollover
Relative Subject
(ejected)
Driver
Control
Age (yrs.)
16-35
Unejected pass. Unejected pass. Unejected pass. Ejected pass. Ejected pass. Ejected pass. Unejected driver Unejected driver Unejected driver Ejected driver Ejected driver Ejected driver Unejected pass. Unejected pass. Unejected pass. Ejected pass. Ejected pass. Ejected pass. Unejected driver Unejected driver Unejected driver Ejected driver Ejected driver Ejected driver Unejected pass. Unejected pass. Unejected pass. Ejected pass. Ejected pass. Ejected pass. Unejected driver Unejected driver Unejected driver Ejected driver Ejected driver Ejected driver
16-35 36-64 6.5 + 16-35 36-64 65+ 16-35 36-64 65 + 16-35 36-64 65 + 16-35 36-64 65+ 16-35 36-64 65+ 16-35 36-64 65+ 16-35 36-64 65 + 16-3.5 36-64 65 + 16-35 36-64 65 + 16-35 36-64 65+ 16-35 36-64 65 +
16-35
Passenger
36-64
Driver
Passenger
36-64
Driver
65+
65+
Passenger
*Sample
Age (yrs.)
size 53
Rollover
risk No rollover
7.6 10.2 _*
2.2 2.7
8.8 11.2
2.2 3.3
7.8 7.9
1.7 4.2
6.8 6.9
1.7 1.o
5.4 7.1
1.5 2.5 -
6.2 4.4
4.0 2.5
7.9 4.2
1.0 5.8
7.2 6.X
1.9
-
5.2
7.9
9.5
6.2
and
Risk of death Table
3. Relative
risk of death
463
from ejection
due to ejection, by sex, for single-vehicle nonrollover crashes
rollover
Relative Subject
(ejected)
Sex
Driver
Male
Driver
Female
Passenger
Male
Passenger
Female
*Sample
Control
Sex
Unejected pass. Unejected pass. Ejected pass. Ejected pass. Unejected pass. Unejected pass. Ejected pass. Ejected pass. Unejected driver Unejected driver Ejected driver Ejected driver Unejected driver Unejected driver Ejected driver Eiected driver
Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female
Rollover
and
risk No rollover
8.0 5.6 10.1 8.1 10.9 9.5 _*
1.9 2.3 2.0 4.2 4.4 2.6 2.x 2.8 1.5 2.1 1.4 3.4 2.3 1.3 1.3 1.2
11.0 6.7 5.3 13.4 8.1 8.1 8.0 5.7 6.9
size 53.
In both the rollover and nonrollover crash modes, no systematic age or sex biases were found. The multivehicle crashes were separated into two categories, car-car crashes and car-other vehicle crashes. There were 1,812 pairs of drivers and passengers in car-car crashes and 4,141 in car-other vehicle crashes that met the matched pair criteria. (The raw data are shown in Appendix B.) The average relative risk of death for ejected drivers and passengers by crash mode for car-car and car-other vehicle crashes is shown in Table 4. In both the car-car and car-other vehicle crashes, the right side impact crash types have the highest increased relative risk of death due to ejection, ranging from about three to about six. In both crash mode categories, frontal crashes have the lowest increased relative risk, ranging from one to two times greater risk of death for ejected drivers or passengers than for nonejected drivers or passengers. In the left side impact crash type, the risks range from about 1 l/2 to nearly 4. The rollover category for the multivehicle crashes was too small to provide meaningful estimates. (The detailed calculations performed to arrive at these average relative risks are shown in Appendix C.) Standard errors of these risk estimates were calculated by assuming that the observed fatality counts were Poisson distributed (Evans and Frick 1987). The number of ejected drivers or passengers determines the precision of the risk estimates. The standard errors vary considerably, depending on sample size, that is, on the number of subject and control occupants in the ratios that were formed to calculate the relative risks. If ejection Table 4. Average
relative risk of death multivehicle crashes Crash
Subject Car-car crashes Driver Passenger Car-other vehicle crashes Driver Passenger
Frontal
due to ejection
in
mode*
Right
side
Left side
1.6 1.8
6.1 2.5
1.8 3.7
2.0 1.3
3.1 2.4
1.6 2.6
Note: Crash mode is determined by principal point of impact. *The rear impact crash mode category had sample size 53.
464
J. R. ESTERLITZ
in a certain crash mode and type category is a rare event, then the standard error is larger. Otherwise, the standard errors are small and the risk estimates are stable. (The details of calculating the standard error estimates are found in Appendix A.)
DISCUSSION
Crash type and crash mode have substantial effects on the relative risks of death due to ejection. Seating position makes some difference, depending on crash mode, but generally the risk of death from ejection is similar for drivers and right front passengers except in right side impacts. Single-vehicle rollover crashes produce the highest risk of death due to ejection for both drivers and passengers. This high risk is attributable to the kinematics of the crash. In single-vehicle nonrollover crashes, the relative risk of death due to ejection is not as high as for rollover crashes, but ejection still increases risk of death by about a factor of two. In the multivehicle crashes, across crash type (i.e. in the car-car crashes and the car-other vehicle crashes) similar increased relative risks of death due to ejection are found in each crash mode category, except for right side impacts where the car-car risks are higher than the car-other vehicle risks. In a side impact collision, there is more occupant compartment intrusion on the near-impact side, thus increasing the likelihood that the near-side occupant would be killed yet remain in the car. Furthermore, in a collision between a car and a much larger vehicle, more intrusion would be expected than in a collision between two cars. If there is more intrusion, the passenger and driver are more likely to be killed yet remain in the car, rather than being ejected from the car. This reasoning aids in explaining the relative magnitudes of the risks of death due to ejection for passengers and drivers in the side impact crash modes. In both multivehicle crash types, and in both right side and left side impacts, the risk of death due to ejection is greater for the far side occupant than for the struck side occupant (see Table 4). That is, the relative risk of death due to ejection is higher for a passenger (than a driver) in a left side impact and for a driver (than a passenger) in a right side impact. These differences in the relative risks between passengers and drivers are greater in the car-car crash type (more than a two to one ratio) compared to the car-other vehicle crash type (about a 1 112 to 1 ratio). Again, this may be attributed to the greater crush in the car-other crashes that could cause as many deaths overall but fewer via ejection.
CONCLUSIONS
When crash severity is controlled for via the paired comparison method, both crash type and crash mode are important factors in the relative risk of death from ejection. The previously published estimates of the increased relative risks of death from ejection, which range from 2 l/2 (Sikora 1986) to 25 times (Hedlund 1979), in one case underestimate and in the other overestimate the actual relative risks by not accounting for crash type, crash mode, restraint usage, and/or crash severity. The results presented in this paper help to identify specific aspects of the ejection problem. The range of risk of death from ejection depends on crash type, crash mode, and seating position. Rollover crashes are the main candidates for high risk of death due to ejection. The primary ejection routes in rollovers are through the side window and door areas and the secondary routes are through the roof and rear window areas. The federal motor vehicle safety standards applicable to side and rear windows only require them to be tempered glass. With laminated glazing and adequate all-around anchoring of the window, many deaths by ejection could be prevented (Clark and Sursi 1985). Clark and Sursi (19X5) would have the plastic layer extending beyond the window glass and fully attached; thus, the window would not open. Prototypes for laminated glazed side windows are under development where the windows would be able to open. Also, side impact crashes have increased relative risks of death due to ejection for opposite side impacts. Upgraded
Risk of death
465
from ejection
door latches or tougher door latch standards could reduce many of these fatalities due to ejection through doors that open. Much of the increased relative risk of death due to ejection could be reduced if occupants were belted.
REFERENCES Bondy, N.; Hart, S. An analysis of the ejection problem using NCSA automated data files. Washington, DC: National Highway Traffic Safety Administration; 1982. Clark, C. C.; Sursi. P. Car crash tests of ejection reductions by glass-plastic glazing. SAE technical paper 851203. Warrendale, PA: Society of Automotive Engineers; 1985. Evans. L. Double pair comparison-a new method to determine how occupant characteristics affect fatality risk in traffic crashes. Accid. Anal. Prev. 18:217-227; 1985. Evans, L.; Frick. M. C. Seating position in cars and fatality risk. GMR-5911. Warren. MI: General Motors Research Laboratories; 1987. Fan, W. R. S.; Jettner. E. Light vehicle occupant protection-top and rear structures and interiors. SAE technical paper 820244. Warrendale. PA; Society of Automotive Engineers; 1982. Hedlund, .I. H. The National Crash Severity Study and its relationship to ESV design criteria. DOT HS 805. 199. Proceedings of the seventh international technical conference on experimental safety vehicles. Washington, DC: U.S. Department of Transportation; 1979. O’Day. J.: Scott, R. E. Corrected findings on injuries associated with ejected and non-ejected occupants. The UMTRI research review. Ann Arbor, MI: The University of Michigan Transportation Research Institute; 1984. Sikora, J. Relative risk of death for ejected occupants in fatal traffic accidents. DOT HS 807-096. Washington, DC: National Highway Traffic Safety Administration; 1986. U.S. Department of Transportation (U.S. DOT). Fatal Accident Reporting System 1982-1986. Washington, DC: National Highway Traffic Safety Administration; 1982-1986.
APPENDIX The standard
error,
A:
AR, in the estimate
STANDARD
ERROR
of relative
risk of death
AR = Rd\/a’ + l/a
+ lib
CALCULATIONS due to ejection,
+ l/c
is given by:
+ lid,
where a, b, c. and d are the number of passengers and drivers killed in the subject and control ejection categories; cr2 is an estimate of the intrinsic uncertainty analagous to a random instrumental error (Evans 1985). In this study, the oZ is assumed to be zero. As an example, consider the standard error calculation for the relative risk of death due to ejection for an ejected driver in a single-vehicle, rollover crash as the subject and an unejected passenger as the control (see Table Bl for raw data): R = 7.95 = 9.2211.16
= (904/98)/(776/669).
where 904 98 776 669
= = = =
ejected drivers killed; unejected passengers killed travelling unejected drivers killed; unejected passengers killed travelling
with ejected
drivers
with unejected
killed;
killed drivers;
and
AR = 7.95 V/11904 + 1198 + 11776 + 11669; AR = 7.95 (0.11872); AR = 0.94. This procedure was done for all of the relative risks (every subject and control in each crash mode and crash type category. The results are shown in following
Table Al.
Single-vehicle
crashes Rollover
Subject
(ejected)
Driver Driver Passenger Passenger
combination) and calculated Tables Al, A2, and A3.
No rollover
Control
Risk
SE
Risk
SE
pass. Unejected Ejected pass. Unejected driver Ejected driver
7.95 8.58 7.57 7.01
0.94 0.98 0.87 0.83
2.15 2.35 1.60 1.46
0.25 0.31 0.16 0.21
464
J. R. ESTERLITZ Table A2.
Car-car
crashes
Front Subject
(ejected)
Driver Driver Passenger Passenger *Sample
Rear
Control
Risk
SE
Risk
SE
Risk
SE
Risk
SE
pass. Unejected Ejected pass. Unejected driver Ejected driver
1.53 1.77 1.77 1.53
0.46 0.68 0.48 0.62
-* -
-
1.84 1.32 3.48 4.88
1.03 1.03 1.64 3.25
6.10 2.52
2.95 1.48
Standard
errors
for relative
risk estimates
Front (ejected)
Driver Driver Passenger Passenger
Rear
vehicle
Left
crashes
Right
Roll
Risk
SE
Risk
SE
Risk
SE
Risk
SE
Risk
SE
Unejected pass. Ejected pass. Unejected driver Ejected driver
2.09 1.42 1.28 1.89
0.46 0.34 0.23 0.51
4.80 5.31 4.14 3.73
3.16 3.87 2.40 2.96
1.60 1.78 2.60 2.35
0.57 0.61 0.63 1.02
2.97 3.84 2.43 1.88
0.75 1.63 0.88 0.62
1.31 2.63 3.56 1.78
0.96 2.23 2.56 1.53
PAIR
RAW
DATA
Table Bl.
Ejected Passenger
No No Yes Yes Total
in car-other
Control
APPENDIX
Driver
Right
size 53.
Table A3.
Subject
Left
1982-1986
matched
Driver passenger
died/ survived
Rollover
No Yes No Yes
B:
B2.
1982-1986
crashes
by crash mode
Driver survived/ passenger died Rollover
1,122 96 178 175 1,571
Driver passenger
Ejected
pair data for single-vehicle
No rollover
599 75 860 652 2,186
Table
MATCHED
No rollover
492 688 54 436 1,670
matched
Both died Rollover
1,505 254 81 132 1,972
nair data for car-car
177 36 44 249 506
crashes
Total
507 65 50 99 721
4,402 1,214 1,267 1,743 8,626
bv crash mode
Driver survived/ passenger died
died/ survived
No rollover
Both died
Driver
Passenger
Front
Rear
Left
Right
Front
Rear
Left
Right
Front
Rear
Left
Right
NO No Yes Yes Total
No
362 6 17 13 398
15
169 3 17 6 195
22 1 6 7 36
364 23 7 13 407
22 Y 1 2 34
33 5 0 5 43
266 32 3 17 31x
1x7 16 12 13 228
Y
36
4Y
vehicle
crashes
Yes No Yes
Table B3. Ejected
I 7 4 27
1982-1986
matched
Driver
died/passenger
pair data for car-other
survived
Driver
survived/passenger
died
I
6
1 1 12
4 8 54
by crash
Total
I .534
1
104 X0 94
5 5 60
I.812
mode
Both died
Driver
Passenger
Front
Rear
Left
Right
Roll
Front
Rear
Left
Right
Roll
Front
Rear
Left
Right
Roll
No No Yes Yes Total
No Yes No Yes
671 28 55 40 794
42 3 13 7 65
480 16 34 23 553
64 1 13 5 X3
39 1 5 2 47
573 38 15 25 651
44 16 2 5 67
70 14 0 4 86
588 53 11 28 680
17 5 2 I 25
424 31 I6 45 516
30
165 23 10 22 220
228 8 21 25 282
25 2
I 1 3 35
I 7 35
Total 3.460 240 19’) 242 4,141
467
Risk of death from ejection APPENDIX
C
Table Cl. Single-vehicle crashes detailed calculations of relative risk of death due to ejection Step 1. Driver and passenger fatalities by ejection Number of fatalities Ejection
Passengers killed
Drivers killed
Driver
Passenger
No No Yes Yes Total
No Yes No Yes
Rollover
Rollover
No rollover
776* 111 904 901 2692
No rollover
669 124 98 68.5 2176
1629 161 228 274 2292
*From Table Bl: 776 = 599 (driver died, passenger survived) passenger died).
2012 319 131 231 2693
+ 177 (driver died,
Step 2. Calculation of driver and passenger fatality ratios Ratio of fatalities Passengers killed/ drivers killed
Drivers killed/ passengers killed
Ejection Driver
Passenger
Rollover
No rollover
Rollover
No rollover
No No Yes Yes
No Yes No Yes
1.16* 0.15 9.22 1.32
0.81 0.51 1.14 1.19
0.86 6.52 0.11 0.76
1.24 1.98 0.57 0.84
*From Step 1: 7761669 = 1.1599 = 1.16 when rounded to two decimal places. Step 3. Calculation of relative risks from fatality ratios Relative risk of death from ejection Subiect (eiected)
Control
Rollover
No rollover
Driver Driver Passenger Passenger
Unejected pass. Ejected pass. Unejected driver Ejected driver
7.95 8.58 1.51 7.01
2.15 2.35 1.60 1.46
Example of weighted average calculation: (904 + 776)7.95 + (901 + 111)8.58 = 8.2, (904 + 776 + 901 + 111) Note: Numbers are rounded to two decimal places for presentation. were carried out to four decimal places.
All calculations
Table C2. Car-car crashes Step 1. Driver and passenger fatalities by ejection Numbei of fatalities Ejection
Drivers killed
Passengers killed
Driver
Passenger
Front
Rear
Left
Right
Front
Rear
Left
Right
No No Yes Yes Total
No Yes No Yes
549 22 29 26 626
24 2 8 5 39
205 9 21 14 249
71 2 11 12 96
551 39 19 26 635
31 10 2 3 46
69 11 4 13 97
315 33 8 22 378
468
J. R. ESTERLITZ Table C2. (continued) of driver and passenger
Step 2. Calculation
Ratio Ejection Driver
Drivers
Passenger
No
No Yes No Yes
No Yes Yes
killed/passengers
ratios
of fatalities
killed
Passengers
killed/drivers
killed
Front
Rear
Left
Right
Front
Rear
Left
Right
1.00 0.56 1.53 1.00
0.77 -* -
2.85 0.82 5.25 1.08
0.23 1.38 0.55
1.00 1.77 0.66 1.00
1.29 -
0.35 1.22 0.19 0.93
4.44
Step 3. Calculation
of relative
Relative Subject
fatality
(ejected)
Driver Driver Passenger Passenger
risks from fatality
risk of death
from ejection
0.73 1.83
ratios by crash
mode
Control
Front
Rear
Left
Right
Unejected pass. Ejected pass. Unejected driver Ejected driver
1.53 1.77 1.77 1.53
-
1.84 1.32 3.48 4.88
6.10
-
2.52
*Crash modes are excluded if any of the four fatality frequencies used to calculate Very small cell frequencies make inferences and interpretations of data unreliable.
risks is 53.
Table C3. Car-vehicle crashes Step 1. Driver and passenger fatalities by ejection Number Drivers
Ejection Driver
Passenger
No No Yes Yes Total
Front
No Yes No Yes
Right
Roll
72 4 14 10 100
645 39 44 45 773
292 9 34 30 365
64 3 6 9 82
Drivers
Passenger
No No Yes Yes
No Yes No Yes
Note:
Sample
of driver
killed/passengers
997 69 31 70 1,167
and passenger
fatality
Rear
Left
Right
Roll
74 17 3 x 102
235 37 10 26 308
816 61 32 53 962
32 7 3 x 60
ratios
Passengers
killed
killed/drivers
killed
Rear
Left
Right
Roll
Front
Rear
Left
Right
Roll
1.10 0.86 2.29 1.21
0.97 0.24 1.25
2.74 1.05 4.40 1.88
0.36 0.15 1.06 0.57
1.52 1.13
0.91 1.17 0.44 0.82
1.02 4.25 0.80
0.36 0.95 0.23 0.53
2.79 6.7X 0.94 1.77
0.66 0.89
size 53.
Relative (ejected)
Driver Driver Passenger Passenger
Front
killed
Front
Step 3. Calculation
Subject
Passengers
Left
Step 2. Calculation
Driver
killed
Rear
1,095 59 71 85 1,310
Ejection
of fatalities
of relative
risks from fatality
risk of death
from ejection
ratios by crash mode
Control
Front
Rear
Left
Right
Unejected pass. Ejected pass. Unejected driver Ejected driver
2.09 1.42 1.28 1.89
5.31 4.14
1.60 1.7x 2.60 2.35
2.97 3.84 2.43 1.8X
Roll