U.S. Department

of Transportation

National Highway

Traffic Safety

Administration

DOT HS 807 067 February 1987

NHTSA Technical Report

A Preliminary Evaluation

of Seat Back Locks

for Two-Door Passenger Cars

with Folding Front Seatbacks

This document is available to the public from the National Technical Information Service, Springfield, Virginia 22161.
The United States Government does not endorse products

or manufacturers. Trade or manufacturers' names appear

only because they are considered essential to the object

of this report.
Technical Report Documentation Page

1. Report No.

DOT HS 807 067

2. Government Accession No. 3. Recipient's Catalog No.

4'. Title and Subtitle

A PRELIMINARY EVALUATION OF SEAT BACK LOCKS

for Two-Door Passenger Cars with

Folding Front Seatbacks

5, Report Date

February 1987

6. Performing Organization Code

7. Author's)

Charles J . Kahane, Ph.D.

8. Performing Organization Report No.

9. Performing Organization Name and Address

Office of Standards Evaluation

National Highway Traffic Safety Administration

Washington, D.C. 20590

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

12. Sponsoring Agency Name and Address

Department of Transportation

National Highway Traffic Safety Administration

Washington, D.C. 20590

13. Type of Report and Period Covered

NHTSA Technical Report

14. Sponsoring Agency Code

15. Supplementary Notes

An agency staff review of an existing Federal regulation performed in response to

Executive Order 12291.

16. Abstract

Federal Motor Vehicle Safety Standard 207 specifies strength requirements for

automotive seats and their attachment assemblies, so as to minimize the possibility

of their failure by forces acting on them as a result of vehicle impact. The

standard includes a requirement that front seats with folding seatbacks be equipped

with a locking device, designed to limit the forward motion of the seatback in a

collision and to keep the seatback away from front seat occupants. These seat back

locks were installed in domestic two-door passenger cars during 1967-68. The

objectives of this preliminary evaluation are to determine if seat back locks are

effective in reducing deaths or injuries and to measure the actual cost of the

locks. The evaluation is based on statistical analyses of Washington, Texas, New

York, Fatal Accident Reporting System and Multidisciplinary Accident Investigation

data (with special emphasis on back seat occupants, frontal crashes, and crashes

involving occupant ejection or vehicle fire), sled test analyses and a cost study

of production lock assemblies. It was found that the locks hold seatbacks in place

in crashes when the back seat is unoccupied, but locks or other seat components

often separate at moderate crash speeds when there are unrestrained back seat

occupants. No statistically significant injury or fatality reductions were found

for seat back locks in any of the accident data files or in the sled tests. The

locks add about $14 (in 1985 dollars) to the lifetime cost of owning and operating

a car.

17. Key Words

seat back lock; seating system;

back seat; sled test; ejection;

crashworthiness; evaluation; fire;

biomechanics; accident analysis;

statistical analysis;

IS. Distribution Statement

Document is available to the public

through the National Technical

Information Service,

Springfield, Virginia 22161

19. Security Clossif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. of Pages

253

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

TABLE OF CONTENTS

Acknowl edgments xi i i

Executive Summary xv

1. INTRODUCTION AND BACKGROUND 1

1.1 Evaluation of NHTSA regulations and programs 1

1.2 Evaluation of Standard 207 2

1.3 Why evaluate seat back locks? 3

1.4 SBL implementation dates 4

1.5 Costs of SBL 6

1.6 Potential benefits of SBL 8

1.7 Evaluation methods and their limitations 12

1.8 Review of Ball's study of SBL 17

2. STATISTICAL ANALYSES OF SLED TEST DATA 21

2.1 Planning and running the sled tests 21

2.1.1 Sled buck, test setup and crash modes 22

2.1.2 Dummies and injury parameters 24

2.1.3 Sled test matrix 25

2.1.4 Running the sled tests 28

2.2 Discussion of sled test results .32

2.3 Statistical analysis of sled test results 41

2.3.1 Average values of the injury criteria -

wi th and wi thout SBL 41

2.3.2 Significance of the results: t tests of matched pairs 45

2.3.3 Nonparametric significance tests 50
3. ANALYSES OF NASHINGTON, TEXAS AND NEW YORK STATE ACCIDENT DATA 55

3.1 Analysis method .55

3.1.1 The control group: 4 door cars 56

3.1.2 Limiting the data set to reduce bias ...59

3.1.3 Statistical significance testing 62

3.2 Washington State data ...,..,.. 64

3.2.1 Analysis of injuries - full data set 65

3.2.2 Analysis of ejections - full data set 68

3.2.3 Analysis of injuries - matching makes and models ...73

3.2.4 Analysis of ejections - matching makes and models 76

3.2.5 Effect of SBL on injuries by seat position -

full data set 77

3.2.6 Effect on injuries of restrained occupants -

full data set 81

3.2.7 When children ride in the back seat 82

3.3 Texas data on drivers' injury risk 85

3.3.1 All crashes 87

3.3.2 Frontal crashes 89

3.4 New York State data - effect of SBL by injury type ...89

4. ANALYSES OF THE FATAL ACCIDENT REPORTING SYSTEM 97

4.1 Analysis method ,. .97

4.1.1 Fatal i ty data 98

4.1.2 Exposure data 99

4.1.3 Statistical significance testing ..105

4.2 Effect on fatalities 105

4.3 Effect on ejections 110

IV
4.4 Fatalities and ejections, by seat position 114

4.5 Effect for restrained occupants 119

4.6 Effectiveness in frontal crashes 121

4.7 Effect on fatal accidents involving fire 125

4.8 Effectiveness, by car size and manufacturer 128

4.8.1 Effect on fatalities. 129

4.8.2 Ejections 131

4.9 When children ride in the back seat 134

5. ANALYSES OF THE PERFORMANCE OF CAR DOORS AND SEATS

IN NHTSA ACCIDENT FILES 137

5.1 Description of the data files 137

5.2 Analysis of ejections 139

5.3 Analysis of door integrity in crashes 141

5.4 Seat performance in crashes of 2 door cars with SBL 146

6. SUMMARY AND COMPARISON OF FINDINGS 153

6.1 Do SBL hold seatbacks in place during crashes? 153

6.2 Do SBL reduce fatalities and injuries in crashes? 156

6.2.1 All occupants - all crashes 156

6.2.2 Frontal crashes 159

6.2.3 By injury type 162

6.2.4 By seat position 164

6.2.5 Restrained occupants. 169

6.2.6 Summary 171

6.3 Do SBL affect the risk of occupant ejection? 172

6.4 Do SBL affect fatality risk in car fires? 176
APPENDIX A: Sled test results 179

APPENDIX B: Detailed State data tabulations .185

APPENDIX C: Detailed FARS data tabulations 213

References 235
LIST OF TABLES

Table 2-1 Sled test matrix (in chronological order) 29

2-2 Sled test matrix (in schematic order) ....30

2-3 Performance of unrestrained dummies relative to

Standard 208 criteria 38

2-4 Performance of unrestrained dummies relative to the

head and chest criteria of Standard 208 40

2-5 Sled test results - all dummies 42

2-6 Sled test results - dummies in the front seat with

no dummy positioned behind them 44

2-7 Sled test results - dummies in the front seat with

a dummy positioned behind them 46

2-8 Sled test results - dummies; in the back seat.... 47

2-9 Nonparametric analysis of sled test results - all dummies...52

2-10 Nonparametric analysis of sled test results -

by seat position 53

3-1 Washington State 1973-77: K + A (fatal or serious) injury

rates - all occupants 57

3-2 Washington State 1973-77: K + A + B (moderate/serious)

injury rates - all occupants 67

3-3 Washington State 1973-77: overall injury rates -

all occupants 69

3-4 Washington State 1973-77: ejectees per 1000 occupants,

all seating positions 70

3-5 Washington State 1973-77: net effect of SBL on injury risk

(by severity level) and ejection risk 72

3-6 Washington State 1973-77: net effect of SBL on injury risk

(by severity level) and ejection risk: 2 door vs. 4 door

sedans and hardtops of the same makes and models 74

3-7 Washington State 1973-77: net effect of SBL on injury risk

(by severity level) - by seat position , 78

vri
3-8 Washington State 1973-77: net effect of SDL on injury risk

(by severity level) - belted occupants 83

3-9 Washington State 1973-77: net effect of SBL on nonejected

occupants' injury risk (by severity level) when unrestrained

children are in the back seat -...84

3-10 Texas 1972-74: net effect of SBL on drivers' injury risk

(by severity level) 88

3-11 New York State 1974: net effect of SBL on injury risk -

by injury type ..92

3-12 New York State 1974: net effect of SBL on injury risk,

nonejected occupants in frontal impacts - by injury type 94

4-1 FARS 1975-85: fatality rates per million vehicle years,

all occupants 106

4-2 FARS 1975-85: fatality rates per million vehicle years,

all occupants - 2 door vs. 4 door sedans and hardtops of

the same makes and models 109

4-3 FARS 1975-85: ejectees per million vehicle years,

all seating positions .111

4-4 FARS 1975-85: ejectees per million vehicle years,

all seating positions - 2 door vs. 4 door sedans and

hardtops of the same makes and model s 113

4-5 FARS 1975-85: net effect of SBL on fatality risk -

by seat position 116

4-6 FARS 1975-85: net effect of SBL on occupant ejection

rates - by seat position 118

4-7 FARS 1975-85: fatality rates per million vehicle years,

all restrained occupants 120

4-8 FARS 1975-85: fatality rates in frontal crashes,

per million vehicle years, all occupants 122

4-9 FARS 1975-85: ejectees per million vehicle years,

in frontal crashes, all seating positions 123

4-10 FARS 1975-85: nonejection fatality rates in frontal

crashes, per million vehicle years, all occupants 124

4-11 FARS 1975-85: net effect of SBL on fatality risk

in crashes involving fires 127
4-12 FARS 1975-85: net effect of SBL on fatality risk -

by car size and manufacturer 130

4-13 FARS 1975-85: net effect of SBL on occupant ejection

rates - by car size and manufacturer 132

4-14 FARS 1975-85: net effect of SBL on fatality risk

when children are in the back seat 135

5-1 NCSS and MDAI files: ejectees per 1000 occupants,

all seating positions 140

5-2 NCSS and MDAI files: incidence of doors opening

during Impacts ;. 143

5-3 MDAI data: front seat separation rates,

2 door cars equipped with SBL, as a function of Delta V,

in frontal crashes - lower estimate of Delta V 148

5-4 MDAI data: front seat separation rates, by size of

back seat occupant, 2 door cars equipped with SBL,

as a function of Delta V, 1n frontal crashes 150

5-5 MDAI data: front seat separation rates,

2 door cars equipped with SBL, as a function of Delta V,

in frontal crashes - higher estimate of Delta V 152

6-1 Front seat separation rates, 2 door cars equipped with SBL,

as a function of Delta V, 1n frontal crashes 155

6-2 Overall casualty reduction for SBL 157

6-3 Casualty reduction for SBL in frontal crashes 160

6-4 Casualty reduction for SBL - by Injury type .163

6-5 Casualty reduction for SBL - front seat occupants

with nobody sitting behind them 165

6-6 Casualty reduction for SBL - front seat occupants

with somebody sitting behind them 167

6-7 Casualty reduction for SBL - back seat occupants 168

6-8 Casualty reduction for SBL - belted occupants 170

6-9 Observed "effects" of SBL on occupant ejection and on

doors opening during Impacts 174

6-10 Effect of SBL on fatality risk in crashes involving fires..177

IX
LIST OF ABBREVIATIONS

ABC

ACIR

AIS

AMC

bleed.

CDC

Co.

Corp.

CPI

CPIR

CRASH

Crit.

CSI

df

FARS

fpc

g

GM

HIC

HPR

HSRI

HYGE

Inj.

Install.

police reported nonfatal injury severity scale

Automotive Crash Injury Research

Abbreviated Injury Scale

American Motors Corporation

bleeding

Collision Deformation Classification

Company

Corporation

Consumer Price Index

Collision Performance and Injury Report

Computer Reconstruction of Accident Speeds on the Highway

Criterion

Chest Severity Index

degrees of freedom

Fatal Accident Reporting System

finite population correction

unit of acceleration approximately equal to 32.2 ft/sec

General Motors Corp.

Head Injury Criterion

High Penetration Resistant windshield

Highway Safety Research Institute, now called UMTRI

trade name for an accelerator sled

Injury

Installation
KA, K + A

KAB, K + A + B

L

Lacerat.

MDAI

mph

MSC

MVMA

MY

NC

NCSS

NHTSA

No.

NY

NYSICS

Occ.

R

Red.

SAE

Sev.

SBL

TAD

TLI

Tot.

unch.

VIN

Yrs.

police-reported serious or fatal injuries

police-reported moderate, serious or fatal injuries

left

Laceration

Multidisciplinary Accident Investigation

miles per hour

Mean Strain Criterion

Motor Vehicle Manufacturers Association

Model Year

North Carolina

National Crash Severity Study

National Highway Traffic Safety Administration

number

New York State

New York State Injury Coding System

Occupants

right

Reduction

Society of Automotive Engineers

Severity

Seat Back Locks

Traffic Accident Data project accident severity scale

Total Laceration Index

total

unchanged

Vehicle Identification Number

Years

XI

ACKNOWLEDGMENTS

Arnold Johnson developed and managed the sled testing project,

which provided essential data for this evaluation as well as much insight

on the performance of unrestrained dummies in frontal crashes. Larry

Blincoe, Dan Cohen, Mark Edwards, Jerome Kossar, Sue Partyka, Rob Sherrer

and John Van Dyke reviewed the manuscript and/or provided helpful suggestions on data sources and analysis methods. John Garrett, Ernie Grush,

Marie Hansen, Richard Humphrey, Anil Khadilkar and Joseph C. Marsh IV

provided information on seat and door installation methods, facilitated

access to data files or explained details in the sled test procedure.

xm

EXECUTIVE SUMMARY

Federal Motor Vehicle Safety Standard 207, which took effect

for passenger cars on January 1, 1968, is one of the National Highway

Traffic Safety Administration's initial vehicle regulations. It specifies

strength requirements for automotive seats and their attachment assemblies, so as to "minimize the possibility of their failure by forces

acting on them as a result of vehicle impact." Many of the specific

requirements of Standard 207 are based on the Society of Automotive

Engineers' Recommended Practice 0879, which had already been in place

since November 1963. The one significant difference between Standard 207

and Recommended Practice J879 - the one tangible modification of vehicle

seating systems in model years 1967-68 - was the introduction of seat-back

locks in the folding front seatbacks of passenger cars with two doors.

In a two-door car without seat back locks, the seatback may

fold over without restraint in a frontal crash and press on the backs of

front seat occupants while they are impacting the steering assembly or

instrument panel, possibly increasing the severity of those impacts. The

purpose of seat back locks is to limit the forward motion of folding

seatbacks in crashes and keep the seatback away from the front seat

occupant at the time they impact the steering assembly or instrument

panel. Seat back locks are potentially even more important when there are

unrestrained back seat passengers in the car (although, actually, this is

not a crash situation addressed by Standard 207). If the locks hold the

seatback upright and in place, they can help prevent back seat passengers

xv
from being thrown into the front half of the compartment, where they might

injure a front seat occupant, impact the windshield or instrument panel,

or be ejected from the car through open doors or windows.

Current imports as well as older domestic cars have manually

operated seat back locks which must be disengaged each time a back seat

passenger enters or leaves the car. Current domestic vehicles have an

automatic lock which allows the seatback to move freely except during

impacts or other sudden decelerations. A possible side effect of manual

seat back locks is that they might slow down back seat passengers' egress

from the car in emergency situations such as fires or immersion in water.

Executive Order 12291 (February 1981) requires agencies to

evaluate their existing regulations. The objectives of an evaluation are

to determine the actual benefits - lives saved, injuries prevented, damage

avoided - and costs of safety equipment installed in production vehicles

in connection with a standard. This report is a preliminary evaluation of

seat back locks for two-door passenger cars - i.e., the specific piece of

safety equipment installed in connection with Standard 207.

The cost of seat back locks was estimated by analyzing the

components of the locks that were actually installed in six production

cars. It is estimated that manual seat back locks add $12.83 (in 1985

dollars) to the lifetime cost of purchasing and operating a two-door car.

Automatic locks add $14.14 to lifetime cost. In 1985, 3.2 million

two-door cars were sold with automatic locks and 1.3 million with manual

xvi
locks; thus the total consumer cost of the locks was about $62 million.

The effectiveness of seat back locks was estimated by analyzing

sled test results and highway accident data. There were 28 frontal sled

tests with an average of three instrumented, unrestrained dummies per

test: 14 runs with a seat assembly that included seat back locks and 14

matching runs under identical conditions and with the same type of seat,

except that the locks were disabled to allow free movement of the seatback

as 1n a pre-standard car. Well over a million accident cases from

Washington State, Texas, New York, the Fatal Accident Reporting System,

Multidisciplinary Accident Investigation and the National Crash Severity

Study were statistically analyzed. The analysis strategy is to calculate

the reduction in casualty risk in two-door cars in the model years

immediately after the introduction of seat back locks; since many safety

devices besides the locks were installed 1n those years, the reduction in

the two-door cars 1s compared to the corresponding reduction in four-door

cars of the same makes, models and model years. Since four-door cars

received the same safety equipment as two-door cars except seat back

locks, they act as a control group for the purpose of this evaluation.

They are not always a perfect control group, as will be documented in the

evaluation.

The effectiveness analysis addresses two questions. Do seat

back locks prevent the loading of the seatback on the front seat occupants

and retain back seat passengers within the rear half of the passenger

compartment? Do seat back locks reduce deaths and injuries? The answer

x vn
to the first question 1s fundamentally, "No." Specifically, when there

are unrestrained back seat passengers, seat back locks or other seating

system components were torn loose 1n every sled test at 26.5 mph and 1n a

large percentage of frontal crashes of moderate severity on the

Mult1d1sc1pl1nary Accident Investigation file:

Frontal Percent of Cars with

Delta V (mph) Seats Torn Loose

Less than 10 6

10-14.9 21

15-19.9 35

20-29.9 47

30 or more 88

At lower speeds, even when seating components were not torn loose,

seatbacks were deflected forward to the point where they could not be

relied on to keep back seat passengers In the rear half of the passenger

compartment and away from front seat occupants. Seat back locks did

perform better when the back seat was unoccupied: all seats remained

Intact in the sled tests, while 12 percent of seats tore loose at 20-25

mph and 26 percent at 25-30 mph in the accident file. Nevertheless, when

the back seat 1s unoccupied, the sled tests showed that, even without seat

back locks, the seatback only makes a minimal contribution to Impact

forces on front seat occupants.

Under these circumstances, 1t is unreasonable to expect seat

back locks to have much effect on fatality or injury rates. Indeed, no

significant casualty reduction was found in the sled tests or any of the

accident data files, in spite of the large samples of data analyzed and

the strenuous efforts to avoid biases in the analyses. Positive results

xvni
were not obtained for any seating position (front seat occupants with

nobody sitting behind them, front seat occupants with somebody sitting

behind them, back seat passengers) or any specific injury type. No

benefits were found in frontal crashes, or for restrained occupants, or

unrestrained occupants.

One consistent statistical finding was that occupant ejection

decreased dramatically 1n four-door cars in the model years that seat back

locks were first Installed in two-door cars, but that such a reduction did

not take place 1n the two-door cars. Analyses of the National Crash

Severity Study and the Mult1disc1pl1nary file suggest that these effects

may largely 1f not entirely be due to changes in door locks and latches

that merely coincided with the Introduction of seat back locks. These

door latch modifications greatly Improved door integrity in crashes for

four-door cars, but not for two-door cars. In other words, four-door cars

are not an appropriate control group for the analysis of ejections.

Nevertheless, the possibility cannot be ruled out that seat back locks may

slightly Increase front seat occupants' risk of ejection, because they

prevent seatbacks from folding over and blocking avenues of ejection.

The Fatal Accident Reporting System data at least appear to be

consistent with a tentative conclusion that manual seat back locks may

quite possibly have increased the risk, as feared, of back seat passengers

dying in vehicle fires, but the increase may have been offset by a

decrease of front seat occupants' risk in those accidents. None of the

results on vehicle fires, however, were statistically significant.

xix
This evaluation is called a "preliminary" one because, statistically speaking, it is difficult to prove definitively that a safety

device is not effective. That is because the absence of statistically

significant positive findings does not constitute proof that a safety

device Is completely ineffective. There might still be some narrowly

defined crash types where the locks are, in fact, beneficial but there are

simply not enough data of that type of crash for a statistically meaningful effectiveness analysis. Specifically, it seems logical that SBL might

be beneficial for children riding in the back seat 1n moderate speed

frontal crashes (Delta V 20 mph or less): the SBL may withstand the

relatively light impact load of a child and the seatback may retain the

child within the safer rear half of the passenger compartment, whereas in

a car without SBL, the child might be propelled over the folded seatback

and contact the windshield, header. Instrument panel or a front seat

occupant. Even though the analyses of this evaluation do not show any

fatality or Injury reductions with SBL for child back seat occupants, the

sample size for this limited crash situation 1s too small for the results

to be convincing evidence that, 1n fact, SBL are not effective there.

Similarly, it seems logical that SBL might account for a modest reduction

of nonfatal injuries in low to moderate speed frontal crashes (Delta V 15

mph or so), especially when there are adults in the back seat; as above,

the available data did not show a significant positive effect but do not

preclude the possibility that there is, in fact, a modest positive effect. These crash situations could be possible topics for further study.

What has been accomplished, though, is that a large number of

xx
accident cases from various files were analyzed by techniques that are

believed to be unbiased. If seat back locks had a positive overall effect

even as small as a few percent, it should have appeared in at least some

of the analyses, but no such effects were found. The frequent tearing

loose of the locks at Delta V over 15 mph when there are back seat

occupants and the minimal amelioration of occupant trajectories, as

observed in the sled tests, even when the locks remain intact are important additional justifications for a preliminary conclusion, based on

available data, that seat back locks have generally not been effective in

reducing deaths or injuries.

xxi
CHAPTER 1

INTRODUCTION AND BACKGROUND

1•1 Evaluation of NHTSA regulations and programs

Executive Order 12291, dated February 17, 1981, requires

Federal agencies to perform evaluations of their existing regulations

[9]. The evaluations should determine the actual costs and actual

benefits of existing rules. More recently, Executive Order 12498, dated

January 4, 1985, requires agencies to develop a regulatory planning

process including publication of plans to review existing regulations

pursuant to Executive Order 12291 [10].

The National Highway Traffic Safety Administration began to

evaluate its existing Federal Motor Vehicle Safety Standards in 1975

[28]. Its goals have been to monitor the actual benefits and costs of

safety equipment installed in production vehicles in response to standards. More generally, evaluations compare a standard's actual on-theroad performance and effectiveness with goals that may have been specified

when the rule was initially promulgated - e.g., in its preamble, regulatory impact analysis, or other supporting documents - including analyses of

possible benefits or impacts that had not been originally anticipated.

The agency has published 14 comprehensive evaluations of safety standards

or other vehicle programs to date. NHTSA intends to evaluate every one of

its safety standards that can be associated with a tangible, clearly

defined modification in production vehicles and whose costs and benefits

can be measured by analyzing data on production vehicles.
1.2 Evaluation of Standard 207

Federal Motor Vehicle Safety Standard 207 specifies strength

"requirements for automotive seats, their attachment assemblies, and their

installation to minimize the possibility of their failure by forces acting

on them as a result of vehicle impact [4]." It took effect for passenger

cars on January 1, 1968 and for multipurpose passenger vehicles, trucks

and buses on January 1, 1972. Standard 207 is essentially associated with

one tangible vehicle modification: the introduction of seat back locks

.t

(SBL) in the folding front seatbacks of passenger cars with two doors.

Moreover, Standard 207 requires SBL to withstand a force equal to 20 times

the weight of the seatback, applied in a forward direction.

The other requirements of Standard 207 are largely based on SAE

Recommended Practice J879 [40] pp. 954-955, which had been in place since

November 1963. It required that the front seat must not become detached

from the adjuster attachment bolts, the adjuster mechanisms must not

separate and the seat adjusters must not become detached from the car's

floorpan, when a static load of 20 times the weight of the seat assembly

is applied in a forward or a rearward direction. It also required

seatbacks to remain intact when subjected to a rearward moment of 4250

inch-pounds at each seating position, applied at the center of gravity of

the seatback and calculated about the rear attachments of the seat frame

to the seat adjusters. Standard 207 kept the requirements essentially

without change. (The last requirement was reworded, however. The

rearward moment was reduced to 3300 inch-pounds, but calculated about the

seating reference points rather than the rear attachments of the seat
frame to the seat adjusters. Since the distance between tho height of the

applied load and the axis about which the moment is calculated is less in

Standard 207 than in the SAE practice, a smaller bending moment is created

even though the same force level is applied. In July 1968, after Standard

207 took effect, the SAE revised Recommended Practice 0879 and issued

0879b, making it consistent with the NHTSA standard, including the SBL

requirement [39].) A NHTSA contractor examined seating systems of 12

domestic cars from model year 1969, especially the adjuster mechanisms and

tracks and the portion of the floorpan where the tracks are attached, and

concluded that no reinforcements were made in those structures in response

to Standard 207 1201, p. 16. The conclusion has to be viewed with a

degree of caution because the contractor did not examine pre-Standard 207

vehicles for comparison, nor imported cars which, perhaps, might not have

followed the SAE practice before Standard 207. Nevertheless, the preponderant evidence suggests that the only tangible car seat modification in

the 1966-69 era (other than head restraints) was the introduction of SBL

in the folding seatbacks of two door cars. An evaluation of Standard 207

is primarily an evaluation of SBL.

1-3 Why evaluate seat back locks?

Although SBL are not as prominent,as some other safety modifications since 1960, they meet the criteria for NHTSA evaluations. They

are a well defined piece of safety equipment, added at a specific, known

date to a particular group of passenger cars - see Section 1.4. There is

no problem, unlike with many of the safety standards, of determining "if"

something was done to cars and if so, "what" and "when"? SBL had significant, measurable cost, adding $12-15 to the lifetime cost of a 2 door car
- see Section 1.5. The price of SBL per car easily exceeds the incremental cost of High Penetration Resistant windshields over pre-Standard

glazing [27], pp. 222-223, and 1s comparable to energy absorbing steering

columns [23], pp. 119-129. In addition to raising costs, the manually

operated SBL that were originally used in cars had a small but evident

effect on vehicle performance as perceived by consumers: they had to

disengage the locks before they could get into or out of the back seat.

NHTSA does not possess survey data indicating vehicle owners' attitudes

toward the locks. The domestic manufacturers' subsequent shift to

automatic inertial SBL (see Section 1.4) could be a sign, however, that

owners complained about having to operate the locks manually.

The potential benefits of SBL, like the costs, are capable of

being quantified and evaluated. Specific hypotheses can be formulated

about the effect of SBL on occupant kinematics and injury risk - see

Section 1.6. The hypotheses can be tested experimentally by running sled

tests with and without SBL or by analyzing accident data: here, the fact

that SBL were introduced only in 2 door cars makes it possible to use 4

door cars as a control group and compare the casualty reduction in 2 door

cars (post vs. pre-Standard 207) to the reduction in 4 door cars.

Finally, the evaluation is needed because, to date, there has been only

one study [2] of the effectiveness of SBL and it did not have clear-cut

results - see Section 1.8.

1.4 SBL implementation dates

Costenoble reported in 1978 that SBL were introduced in all
General Motors 2 door cars 1n model year 1967 and 1n the other domestic

cars in model year 1968 [7], pp. 49-50. She also stated that many

imported cars contained SBL before Standard 207 took effect, Including

Volkswagen and Opel by 1966 and Flat, Renault, Datsun and Sunbeam by

1967. These statements were recently checked with Multidisciplinary

Accident Investigation (MDAI) data, by tabulation of the variables

"left/right front seat back locks equipped" against model year. The MDAI

data confirmed that SBL were implemented at GM in 1967 and Ford and

Chrysler in 1968. The sample was too small for clear results on American

Motors or imported cars. AMC and imports are excluded from the accident

data analyses of Chapters 3-5 for that reason and also because most

imports of the mid to late 1960's were 2 door cars and there would not be

an adequate sample of control group vehicles (4 door cars) for comparison

purposes.

At first all SBL were of the manual type. Persons desiring to

enter the back seat of a 2 door car could not fold over the front seatback

until they disengaged the lock by operating a lever or pressing a button.

Circa 1980, the domestic manufacturers switched to automatic inertial SBL,

which operate much like Inertial safety belt retractors. The front

seatback folds over freely as in a pre-SBL car except during the moments

when the car is subjected to decelerations by impacts, road bumps or

emergency braking. The major importers continued to use manual SBL

through 1986 [3].

The accident data analyses of this evaluation are all based on
cars with manual SBL, produced during the first 4 model years after the

SBL implementation date (1967-70 for GM and 1968-71 for Ford and Chrysler). The sled tests used front seats from Chevrolet Citations, which had

automatic SBL (although the inertial actuation device was bypassed for the

tests). Thus, both types of SBL were studied in the evaluation.

1.5 Costs of SBL

The cost of manual SBL was analyzed by a NHTSA contractor in

1979 [203 and the cost of automatic inertial SBL, by another contractor in

1986 [33. The contractors calculated the weight added to cars by SBL and

the "purchase price increase," which is based on the value of materials,

labor, tooling, assembly, overhead, manufacturer's and dealer's markups

and taxes. The results in [20] for four manual SBL are:

Weight Purchase Price

(pounds) Increase (1979 Dollars)

69 Ford Mustang 2.95 6.46

69 Ford Thunderbird 3.32 7.34

69 Chevrolet Nova 2.93 9.84

69 Pontiac Firebird 1.10 on p. 18, 1.89 on p. A-3 2.67

The results for the first three cars are reasonable and consistent,

especially since the Mustang had a simpler lever for operating the SBL

than did the Thunderbird and Nova, as evidenced by the photographs in

[20]. The much lower results for the Firebird are anomalous, especially

since the photographs suggest it had almost the same hardware as the

Mustang; in addition, the report gives two conflicting values for added

weight, as noted above. It is possible that the cost estimates for the

Firebird were incorrectly calculated or transcribed in several categories. Only the results for the first 3 vehicles are used here for
computing the average. The contractor's cost estimates in 1979 dollars

can be converted to 1985 prices by multiplying them by 1.30, the Consumer

Price Index (CPI) multiplier for new cars. Each pound of weight added to

a car results in a 1.17 gallon increase in the lifetime fuel consumption

of a car [11].. The net present value of the increase in fuel consumption

is $0.72 for each pound of weight added to the car [11]. The lifetime

cost for the manual SBL in 1985 dollars is:

69 Ford Mustang

69 Ford Thunderbird

69 Chevrolet Nova

AVERAGE

Weight

(pounds)

95

32

93

3.07

Purchase

Price

Increase

(1985 $)

8.40

9.54

12.79

$10.24

Lifetime

Cost

Increase

(1985 $)

10.89

12.34

15.26

$12.83

Weights and price increases were estimated for automatic

inertial SBL in 3 model year 1986 cars, one from each of the large

domestic manufacturers [3]. The results are highly consistent. The

amounts in [3] are stated in 1984 prices. Here, they are converted to

1985 prices by a CPI multiplier of 1.03:

86 Chevrolet Camaro

86 Dodge 400

86 Ford Tempo

AVERAGE

Weight

(pounds)

3.95

3.84

4.08

3.96

Purchase

Price

Increase

(1985 $)

10.41

10.32

11.70

$10.81

Lifetime

Cost

Increase

(1985 $)

13.74

13.55

15.14

$14.14
It can be concluded with confidence that the lifetime cost per

car of SBL of either type is not far from $13, since the six cars above

had fairly similar costs. On the other hand, it cannot be concluded with

confidence that automatic SBL are more expensive than manual, since the

observed average difference ($1.31) is small compared to the variation

within each type, given the small and not completely representative

samples of cars of each type and the possibility of differences in the

cost estimation methods of the two contractors.

The total annual cost of SBL in the United States is obtained

by multiplying sales of 2 door cars by the cost per car. The calculation

is performed separately for domestic vehicles, which use automatic SBL and

imports, which have manual SBL. In 1985, 8 million North American cars

were sold in the United States. The proportion of domestic cars with 2

doors has remained at 40 percent since 1982 [45], [46], [47]. In 1985,

2.8 million cars were imported from overseas. The proportion of imports

with 2 doors has remained at 47 percent since 1983 [45], [46], [473.

Thus, the total annual cost of SBL is:

Percent 2 Door Cost

Cars with Cars per Total

Sold 2 Doors Sold Car Cost

Domestic 8,000,000 40 3,200,000 $14.14 $45.2 M

Imported 2,800,000 47 1,316,000 $12.83 $16.9 M

TOTAL $62.1

(1985 $) million

1.6 Potential benefits of SBL

Many of the potential benefits that occupants could obtain from
SBL are outlined in a 1969 paper by Severy, Brink, Baird and Blaisdell

[42], pp. 323-325. In a 2 door car without SBL, the folding front

seatback can move forward without restraint, especially in a frontal

crash. The load of the seatback is added to the weight of front seat

occupants as they contact structures at the front of the passenger

compartment, increasing the severity of those contacts. "Effective

latching [of the seatback by SBL] for the front seat occupants relieves

them of seatback inertial loading that might be derived from the hinged

backrest as it abruptly shifts forward during impact, pressed against

their backs." SBL are of potentially even greater value when there are

unrestrained back seat passengers as well as front seat occupants. It

should be noted, though, that the preamble of Standard 207 does not

explicitly claim benefits for SBL in this- situation, nor does the standard

itself contain tests simulating a crash where the back seat is occupied.

In a car without SBL, the back seat passenger will move forward with the

seatback and the combined load will be applied to the front seat occupant,

magnifying the effect described above. Even worse, as the seatback tips

over forwards, it can act as a ramp for the back seat passenger, who will

then vault head first into the front half of the passenger compartment,

contacting structures in that area or making head to head impacts with

front seat occupants. According to Severy, "Effective latching of hinged

seatbacks provides protection for unbelted rear seat passengers, preventing them from being ramped against front seat occupants or into the

windshield or header."

The potential benefits of SBL, then, include a reduction of
thoracic injuries in frontal crashes for front scat occupants with nobody

sitting behind them; a larger reduction of such injuries for front seat

occupants with somebody sitting behind them; and a significant reduction

of head injuries for front seat occupants with somebody sitting behind

them and for back seat passengers. It would be appropriate to perform

separate analyses for the three types of occupants (front seat with nobody

behind them, front seat with somebody behind them, back seat) since

different hypotheses apply to each group.

Severy et al add an important caveat to their list of potential

benefits: the SBL must perform in crashes up to a reasonably high level of

severity. If the SBL separate during the impact or even if they hold

together but allow large amounts of forward deflection of the seatback,

they will not prevent seatback loads on the front seat occupant or ramping

by back seat passengers. The actual requirements of Standard 207 must be

examined in this context. The standard requires SBL to remain intact when

exposed to a load of 20 times the weight of the seatback. In frontal

crashes with an unoccupied back seat, that load is produced by a deceleration of 20 g's, a level that typically occurs in frontal crashes with

Delta V of, say, 25-35 mph - a high level of severity for an unrestrained

occupant. But when there are one or more back seat occupants, the level

of protection set by Standard 207 is much lower. Typical values for the

weight of a seatback are 26 pounds for a 1980 Chevrolet Citation [16], pp.

A_28 - A-30 and 36 pounds for a 1981 Plymouth Reliant [17], pp. A-29 -

A-32. If a back seat occupant weighs 150 pounds, the combined load of the

seatback and the back seat occupant will exceed 20 times the weight of the

10
seatback at deceleration levels well below 10 g's. Unless the SBL

actually installed in vehicles far exceed the standard's minimum requirements, SBL separation can be expected at crash severities well delow those

that are dangerous for unrestrained occupants. Moreover, Standard 207

only requires that seat components remain intact, allowing an unlimited

amount of seatback deformation which, as stated above, could defeat the

potential advantage of SBL. For these reasons, Severy et al believed that

Standard 207 should have much higher test loads (e.g. application of a

bending moment of 100,000 inch-pounds rather than 3,300) and a limitation

on the amount of deflection that is allowed in the tests [42], p. 325.

SBL could have additional effects, not necessarily positive, in

two specific accident situations: fire/immersion and occupant ejection

from the vehicle. In an emergency egress situation, such as a fire or

immersion in water, the back seat occupant of a 2 door car has a difficult

task of getting out of the car: waiting, if necessary, for the front seat

occupants to exit, folding over the seatback and leaving by the front

door. Ball presented the hypothesis that manual SBL could further

complicate the process, since the back seat occupant must remember to

release them [2]. Moreover, on many cars it is impossible to disengage

the SBL while someone presses the seatback forward, as is likely to happen

to a person who is in a panic to leave the car. Thus, manual SBL might

increase the likelihood of a back seat passenger burning or drowning.

Automatic inertial SBL would not have that shortcoming, since the seatback

moves freely except during an impact. On the other hand, the SBL could

help a front seat occupant exit more rapidly: it might hold seatbacks in

11
place and enable the occupant to get to the door immediately, rather than

having to push the seatback out of the way or extricate himself from being

pinned under the seatback or a back seat passenger. Intuitively, the

positive effect for the front seat occupant seems much smaller than the

negative one for the back seat passenger - but that might be compensated

for by the much higher occupancy rate of the front seat.

The logic for occupant ejection is the reverse. Occupants are

burned or drowned when they cannot exit the vehicle quickly enough,

whereas an ejectee is one who is caused to exit the vehicle too quickly.

A seatback held upright by SBL could act as a barrier between back seat

occupants and front doors or glazing. But front seat occupants might fare

better without SBL, since the pressure of the unrestrained seatback on

their backs could hold them away from open doors, while a folded over

seatback on the opposite side of the car could act as a barrier between

them and the door on the opposite side. Here, unlike fires and immersions, there ought to be no difference between manual and automatic SBL,

since the phenomena take place during an impact. Intuitively, any

negative effects for the front seat occupant would probably be small,

since the seatback is unlikely to press on the front seat occupant or

remain folded over for a large proportion of the impact sequence. If the

data analyses show large negative "effects" on occupant ejection risk, it

is appropriate to suspect that something other than SBL is to blame.

1.7 Evaluation methods and their limitations

The principal difficulty in evaluating SBL is that the potential benefits, as described in the preceding section, are not likely to be

12
large in absolute terms and therefore not easy to measure in a statistically reliable way. There are some safety devices (e.g., adhesive bonding

of the windshield [27]) whose benefits, although not large in absolute

terms, are relatively great and readily measurable in one or two specific

crash situations. Unfortunately, SBL are not in that category.

The difficulty is compounded in analyses of highway accident

data by the fact that many important crash protection devices were

introduced at the same time, or nearly the same time as SBL:

HPR windshields 1966

padded instrument panels [26], p.123 1966-68

energy absorbing steering assemblies 1967-68

SBL 1967-68

improved door latches [14] 1967-68

head restraints [24], p. 104 1967-69

shoulder belts 1968

How is the effect of SBL to be distinguished among the effects of the

other devices?

The approach used throughout the evaluation is to use control

groups or controlled experiments as much as feasible. In the sled tests

of Chapter 2, two identical series of sled test are run: one with a sled

buck containing seats equipped with SBL, the other with the same sled

buck, except that the SBL are disengaged and the seat can fold over

freely. In the accident data analyses of Chapters 3, 4 and 5, cars with 4

doors are used as the control group. That approach, originally developed

by Ball [2], is based on the fact that all the safety standards of the

13
1960's, except SBL, were implemented in 4 door cars at the same time as in

2 door cars - and the assumption that the relative effect of all the

safety standards, except SBL, is the same in 4 door cars as in 2 door

cars. Thus, the net effectiveness of SBL is the amount by which the

casualty reduction in ,2 door cars (post vs. pre-SBL) exceeds the reduction

in 4 door cars (after vs. before the date of SBL implementation in 2 door

cars of the same make and model). Unfortunately, the assumption that the

effects of the other standards is the same in 4 door and 2 door cars is

not foreordained to be valid and, specifically, will be shown incorrect in

the case of the door latch improvements (see Section 5.3). Moreover, the

analyses can be biased by the market shift that took place in the years

just before and after SBL: 2 door cars accounted for 41 percent of sales

in 1963 [50] and 55 percent in 1971 [48]. If the growth in 2 door car

sales was comprised especially of younger, less experienced drivers who

had more severe crashes, the casualty rate comparison could be biased

against 2 door cars and SBL. Section 3.1 describes the additional

analysis techniques aimed at identifying and removing those kinds of bias.

Another guideline for the analysis is that it should conform to

the hypotheses on effectiveness stated in Section 1.6. For example, since

SBL may have different effects for front seat occupants with nobody

sitting behind them, front seat occupants with somebody behind them and

back seat passengers, separate analyses should be performed for the three

types of occupants. Crashes with fires or ejections should be analyzed

separately to test the hypotheses in Section 1.6. The sled test films and

accelerometer data are reviewed in detail in Section 2.2 to see if SBL

14
have the specific effects on occupant kinematics that were proposed by

Severy et al [42]. Severy also warned that the minimum strength requirements of Standard 207 are too low to assure seating system integrity in

moderately severe crashes; the actual performance of SBL will be observed

in sled tests (Section 2.2) and accident data (Section 5.4) to test the

validity of Severy's concerns.

This preliminary evaluation analyzes numerous sled tests and

over a million accident cases, but none of the data showed significant

benefits for SBL. Statistically, it is easy to prove definitively that a

safety device is effective but difficult to prove that it is not. That is

because statistical methods typically test a null hypothesis that a safety

device is not effective. If the statistically significant positive

effects are found, the null hypothesis is rejected - i.e., there is

convincing evidence that the safety device is effective. But the lack of

statistically significant positive results is not, by itself, convincing

evidence that the null hypothesis is true and the device is not effective. For that reason, this study, none of whose results show effectiveness, is called a "preliminary" evaluation. There might still be some

limited, narrowly defined portion of the crash environment where SBL are,

in fact, beneficial, but there are simply not enough data of that type of

crash for a statistically meaningful effectiveness analysis. Specifically, it seems logical that SBL might be beneficial for children riding in

the back seat in moderate speed frontal crashes: the SBL may withstand the

relatively light impact load of a child (see Table 5-4) and the seatback

may retain the child within the safer rear half of the passenger

15
compartment, whereas 1n a car without SBL, the child might be propelled

over the folded seatback and contact the windshield, header, instrument

panel or a front seat occupant. Even though the analyses of this evaluation do not show any fatality or injury reductions with SBL for child back

seat occupants, the sample size for this limited crash situation is too

small for the results to be convincing evidence that, in fact, SBL are not

effective there. Similarly, it seems logical that SBL might account for a

modest reduction of nonfatal injuries in low to moderate speed frontal

crashes (Delta V 15 mph or so), especially when there are adults in the

back seat; as above, the available data did not show a significant positive effect but do not preclude the possibility that there is, in fact, a

modest positive effect. These crash situations could be possible topics

for further study. Nevertheless, the data make a strong case that SBL

have little or no overall effect:

o The sled tests showed that SBL definitely modified

occupant kinematics, but not in a way that would reduce

injuries significantly, primarily because seatbacks are

substantially deflected even with SBL.

o Accident data showed that SBL or other seat components

separate even in medium severity crashes when there are

back seat occupants. Thus, as Severy feared, SBL have

limited effectiveness in the type of crashes where they

might have been most useful.

o A massive amount of accident data was analyzed by

techniques that are believed to be unbiased. If SBL

had a positive effect even as small as a few percent,

1t should have appeared in at least some of the

analyses, but no such effects were found.

The most prominent effect seen in the accident data was a

dramatic reduction of occupant ejection in 4 door cars at the time that

16
SBL were introduced in 2 door cars, but no comparable reduction in the 2

door cars - suggesting, at first glance, a strong negative "effect" for

SBL. The data in Section 5.3 make a rather convincing case that the

phenomenon is primarily if not entirely due to factors other than SBL.

Nevertheless, the possibility cannot be ruled out that SBL may have had a

slight negative effect on ejection.

The sample size of crashes involving fires was too small for

definitive, statistically significant results. The data at least appear

to be consistent with a tentative conclusion that SBL increased the risk

of fire fatalities in the back seat, while possibly reducing them in the

front seat, consistent with the hypotheses in Section 1.6.

1 • 8 Review of Ball's study of SBL

The only previous statistical analysis of accident data

pertaining to SBL was performed in 1980 by Ball et al under a contract

sponsored by NHTSA's Office of Program Evaluation and managed by Kahane

[2]. It is based on statistical analyses of State accident files from

Texas (1972-74), North Carolina (1973-75) and New York (1974). Records

were extracted for dn.ve.rs of any domestic 2 door or 4 door passenger car

of any known model year involved in a frontal crash. The cases were

initially cross-tabulated by number of doors (2 vs. 4) , pre vs. post SBL

implementation date, and injury severity (police reported K + A vs. B + C

+ uninjured, or K + A + B vs. C + uninjured, or any injury vs. uninjured). Injury rates were computed for the 2 door cars, pre and post-SBL and

for the 4 door cars (the control group) before vs. after the implementation date for SBL in 2 door cars of the same makes and models. The

17
effectiveness of SBL was defined the same way as in Section 3.1.1 of this

report:

effectiveness - 1 - (inj. rate. 2 dr. 1 ) / (

inj. rate, 2 door, pre inj. rate, 4 door, pre

The resulting effectiveness estimates [2], p. 2-4, were:

Net Inj. Red. for SBL (%)

State

TX

NY

NC

Calendar

Year

1972

1973

1974

1974

1973

1974

1975

K + A

+5

-7

-3

-27

-50

-29

+20

K+A+B

-3

-1

-16

-15

-7

-27

+ 12

Any Injury

CM •— CO

1 + •

1

— -8

-10

-19

+1

Sixteen of the 21 observed effectiveness values are negative, suggesting

at first glance that SBL significantly increased injury risk. Since the

calculations are based on cars of all ages, however, they may be biased,

especially, by the market shift from 4 to 2 door cars that took place in

the years just before and after SBL (see Section 1.7). If the growth in 2

door car sales was comprised especially of younger, less experienced

drivers who had more severe crashes, the casualty rate comparison could be

biased against 2 door cars and SBL. More generally, with so many model

years of cars included in the calculations and with so many things besides

SBL happening to cars over those years, it is difficult to believe that

the observed effects are really due to SBL.

Ball acknowledged the possibility of biases in the results and

18
attempted to control for them by using control variables such as driver

age, crash mode, roadway type, damage severity, vehicle manufacturer or

vehicle weight, as suggested by Kahane, the contract manager. Essentially, separate analyses were performed for each combination of values of the

selected control variables and the results averaged. The use of control

variables did control for a small portion of the biases and resulted, on

the average, in just slightly less negative effectiveness estimates [2],

p. 2-5:

Net Inj. Red. for SBL (7.)

(Adjusted by Control Variables)

State

TX

NY

NC

Calendar

Year

1972

1973

1974

1974

1973

1974

1975

K + A

+5

-13

+2

-18

-44

-19

+27

K+A+B

-1

-4

-10

-12

-4

-20

+ 15

Any Injury

_2

-1

-8

-7

-8

-15

+6

The differences between the adjusted results and the earlier table of

simple effectiveness estimates is usually'not more than a few percent.

Just as before, 16 of the 21 effectiveness estimates are negative,

suggesting that SBL significantly increased injury risk.

These negative results cannot be accepted as valid because it

is not evident that the use of control variables successfully removed the

biases. In 1980, control variables were considered a good way to adjust

data files (as, indeed, they are in the case of detailed NHTSA accident

files such as the National Crash Severity Study - see, for example, the

19
evaluations of energy absorbing steering assemblies [23], pp. 164-193, and

side door beams [26], pp. 183-252). Subsequent experience showed that

control variables are not useful for removing biases in State accident

files. NHTSA's 1982 evaluation of head restraints [24], Sections 5.3 and

5.6, provided insights on analysis of State data. It showed that the use

of a wide range of model years creates a risk of vehicle age related

biases and that, with State data, control variables only remove a minimal

portion of the vehicle age biases. Instead, the most suitable control

techniques with State data are to restrict the range of model years as

much as possible (as in Section 5.6.2 of [24]) or to perform regressions

(as in Section 5.6.3 of [24]). An especially satisfactory technique for

controlling vehicle age biases in large State data files is to limit the

analysis to cars built just one model year before or after the implementation of the safety device, then expand it to 2 model years before and

after, then ±3 and ±4 model years, generating a sequence of effectiveness

estimates. This year by year approach was developed in the 1983 evaluation of side marker lamps [25] and again applied in studying windshield

modifications [27]. It is the critical difference between Ball's analyses

and the work in Chapters 3 and 4 of this report, some of which covers the

same data files used by Ball.

20
CHAPTER 2

STATISTICAL ANALYSES OF SLED TEST DATA

The most precise way to determine if seat back locks (SBL) are

effective in crashes is to compare the occupant injury experience in a car

with SBL to the experience of identical occupants in an identical car -

except that this car does not have SBL - in an identical crash situation.

Only in the laboratory, with simulated crashes and anthropomorphic

dummies, can such a close match be obtained. A NHTSA contractor performed

14 frontal sled tests with a sled buck that simulated the passenger

compartment of a 1981 Chevrolet Citation with operational SBL [34], [35],

[36], [43]. Speeds ranged from 15 to 30 mph. The sled bucks were

occupied by 2 to 4 unrestrained 50th percentile Part 572 dummies. The

contractor performed 14 other sled tests that were identical in every

respect - speed, crash pulse, dummy positioning - except that the SBLs

were removed to allow the seatbacks to pivot freely in the crash, simulating a pre-Standard 207 car. The injury predictions for the 42 dummies in

the cars with SBL were compared to the values for the corresponding

dummies in the cars without SBL. No statistically significant differences

(two-sided alpha • .05) were found for any of the injury criteria.

2.1 Planning and running the sled, .tests

The principal guideline for the sled testing program was to

obtain matching pairs of crashes, identical in all respects except that

one of them was with SBL and the other without them. Every precaution had

to be taken to avoid sources of variation other than the presence/absence

21
of SBL.

2.1.1 Sled buck, test setup and crash modes

The sled buck had to be the passenger compartment of a passenger

car, in order that the dummies' injury measurements realistically simulate

the injury producing contacts experienced by car passengers in highway

accidents. One possibility would have been to construct a non-SBL sled

buck from a pre-Standard 207 car and an SBL-equipped sled buck from a

post-Standard 207 car. That approach had to be rejected because it was

inconsistent with the principal guideline: there are many differences

between pre and post-Standard 207 cars besides SBL (e.g., energy-absorbing

steering columns were introduced in most cars at the same time as SBL).

Instead, a single sled buck was constructed from a post-standard, SBL

equipped passenger car and the non-SBL tests were run with the same buck,

but after disabling the locks so that the seatback would pivot freely in

the crash. It would have been desirable to build sled bucks from a

representative selection of passenger cars, but that would have been well

beyond the scope of available resources. Instead, a single buck was

developed from a 1981 Chevrolet Citation sedan with a split-bench front

seat. The 1981 Citation is equipped with automatic inertial SBL (see

Section 1.4). Since it was unknown whether the crash sensing device would

respond in a sled test as it does in a real crash, the contractor bypassed

the emergency locking device and assured that the seats were locked from

the start, in those tests that were to be run with functioning SBL. The

Citation was close to the median - in terms of mass, interior room and

component stiffness - among cars produced and sold in the United States at

22
the time of the sled test study (1984-85). The passenger compartment

structure was severed from the hood and trunk regions and mounted on the

sled as described in [32], pp. 10-12.

The sled itself was of the decelerator type. The sled buck was

gradually accelerated to the desired impact speed by pneumatic pistons.

The crash event was then simulated by allowing the sled buck to be stopped

by a system of steel bands and rollers programmed to deform at a rate

which reproduces the deceleration pulse seen in vehicle-to-barrier

impacts. Although this type of sled will generally not have the same

level of repeatability of crash pulses as a HYGE (accelerator) sled, the

contractor achieved an acceptable repeatability: peak sled g's had a 3

percent coefficient of variation, at any given sled speed, throughout the

project. That is as good as what has been achieved in some projects with

HYGE sleds [30]. It is also a negligible source of variation compared to

other factors affecting injury criteria in this study.

The study was limited to frontal and frontal-oblique impacts

because they were believed to be the type of crash where SBL are most

likely to be of value (see Section 1.6). The limitation to frontal

crashes made it possible to seat up to 4 unrestrained dummies in the sled

buck on each test, 2 in the front and 2 in the back, because there is

little or no interaction between the left and right side dummies.

Experience with 30 degree oblique vehicle-to-barrier crash tests

indicates that dummies tend to impact with the passenger compartment on a

23
line about 11 degrees to the side of the longitudinal axis (since the car

is rotated during the impact). Thus, the oblique impacts were simulated

by mounting the' sled at an 11 degree angle (see also [32], pp. 60-65).

Only a single series of oblique tests was run during the project and it

was arbitrarily decided to run 1:00 (right corner) impacts.

2.1.2 Dummies and injury parameters

Since all available Hybrid III dummies were committed to other

NHTSA studies, the sled tests were performed with Part 572 dummies

simulating the 50th percentile adult male [53. The unrestrained front

seat dummies were considered likely to contact the windshield and were

furnished with chamois face coverings to allow measurement of the laceration index. The back seat dummies were not equipped with the coverings.

The injury criteria measured in the tests corresponded with the instrumentation that is customarily supplied for Part 572 dummies:

o Head Injury Criterion (HIC)

o Head resultant g's (3 millisecond peak values were measured

by the contractor)

o Mean Strain Criterion (MSC) for the head

o Total Laceration Index (TLI) for the face (front seat dummies

only)

o Chest g's, the 3 millisecond peak of the resultant upper

spine acceleration

o Chest Severity Index (CSI), also known as the Gadd Severity

Index

o Left femur load in pounds

o Right femur load in pounds

In the tests with back seat dummies, it was found that the front seat

24
dummies suffered two impacts of comparable severity and opposite directions of force: first, when they impacted the front of the passenger

compartment and later, when the back seat dummies propelled the seatbacks

into the front dummies' backs. Since the second impact directly involves

the seatback, it was considered especially likely to be influenced by the

presence or absence of SBL and consequently of particular interest to this

study. Thus, for the 30 front seat dummies that had another dummy behind

them, peak head and chest g's were measured for the whole crash, the first

impact alone, and for subsequent impacts alone.

2.1.3 Sled test matrix

The objective of the sled test program was to evaluate the

effectiveness of SBL. Consistent with that objective, the range of test

speeds should include any where SBL have a reasonable chance of affecting

the risk of serious injuries. Another consideration is that the speeds

should not be so high as to cause irreparable damage to the sled buck or

so low that the dummies' injury criterion measurements cannot be meaningfully related to levels of injury risk (e.g., HIC of 100 or less). Since

the project was one of the first to include a large number of sled tests

with unrestrained adult dummies and, especially, unrestrained back seat

dummies, the suitable speed ranges were not known in advance and had to be

established by trial and error. It was found that impact speeds above 27

mph were likely to result in significant damage to structural elements of

the sled buck, such as the A pillar and windshield header. Such damage

must be avoided because, even if repaired, it could affect the windshield's response to impact on subsequent tests. Impact speeds under 20

25
mph did not produce injury criterion measurements large enough for

meaningful estimates of serious injury risk. Therefore, almost all of the

tests were conducted impact speeds of 22.5 or 26.5 mph, except for a pair

of tests at 15 mph and another pair at 30 mph, at the end of the project,

when the buck was expendable. The range appears narrow at first glance

but is reasonable for unrestrained occupants: it is in the 20-30 mph

rangeof Delta V where the unrestrained occupant, in cars of the 1960-85

era, had a substantial likelihood of serious injury yet that likelihood

could be reduced by safety improvements that were built into the vehicle

(e.g., energy absorbing steering columns, High Penetration Resistant

windshields).

The contractor sought to design the system of steel bands and

rollers so as to reproduce the crash pulses actually observed in frontal

barrier impacts of Chevrolet Citations, many of which had been performed

in earlier NHTSA contracts [36], pp. 5-7. These crash pulses achieved

peak decelerations of 8 g's in 15 mph impacts, 12 g's at 22.5 mph, 15 g's

at 26.5 mph and 18 g's at 30 mph, based on interpolation of the graphs on

p. 7 of the contractor's October 1984 report [36]. The contractor's sled

pulses generally averaged a little bit below that: 9 g's at 15 mph, 11 g's

at 22.5 mph, 14 g's at 26.5 mph and 15 g's at 30 mph (see Appendix A of

this report). Those peaks are mild in comparison with the barrier test

experience of other cars. For example, NHTSA compliance tests for

Standard 213, which are intended to represent barrier crashes of the

average car of the mid-to-late 1970's, develop close to 15 peak g's at

18.5 mph and 22 g's at 27.5 mph. The shape and intensity of the sled

26
pulse usually has a large influence on dummies1 injury criteria, but is

not so important in this study of exclusively unrestrained dummies. Since

the dummies do not make early contact with a restraint system, they get

relatively little benefit from a soft "ride-down" of the vehicle. By the

time unrestrained dummies have moved forward and contacted the instrument

panel and other structures, those structures will have close to zero

velocity, even with the soft crash pulses used by the contractor.

The sled test study had to be limited to unrestrained dummies,

because resources were unavailable for running both restrained and

unrestrained sled test matrices. Since most occupants were unrestrained

in the 1960-85 era, the study concentrated on them. The effectiveness of

SBL for belted occupants is studied in the accident data only (Sections

3.2.6 and 4.5).

The actual sled test matrix was based on 29 tests, since that

was as many as available resources allowed. Since each test without SBL

had to have a corresponding test with SBL, that meant 14 pairs of tests

(the 29th test was used in the process of determining the appropriate

speed range). Rather than using a complete block design or always varying

the test parameters from pair to pair, NHTSA directed the contractor to

emphasize the 3 most "fundamental" test conditions where many unrestrained

occupants are injured yet SBL might have significant benefits: the 22.5

mph frontal impact with dummies in the front and back seats, the 26.5 mph

frontal with the front and back seats occupied and the 26.5 mph impact

with only the front seat occupied. Three pairs of tests were run for each

27
of these conditions. The remaining tests were run at speeds from 15 to 30

mph, with one pair of tests for each of 5 parameter combinations; all were

frontal except for two pairs of oblique tests at 22.5 mph.

Table 2-1 lists the 29 tests in the order that they were run,

showing the contractor's test number (which follows chronological order),

speed, direction of force, back seat occupancy and SBL status. Table 2-2

shows the experimental design matrix for the study. The three left

columns indicate the parameters for each pair of tests: speed, direction

of force and occupancy. The last two columns show, respectively, the

number of the test(s) that was run without SBL and with SBL.

The test matrix must not allow biases such as a tendency to

perform the SBL test with only certain dummies or only at the early (late)

part of the project. Those biases were avoided. As Table 2-1 shows, the

SBL and non-SBL tests were scattered throughout the project, in no

particular order. The same dummies were used at the same seat positions

in all the tests; since every test with SBL has a companion test without

SBL, the same dummy will be exposed to identical crash parameters with and

without SBL.

2.1.4 Running the sled tests

Prior to the entire sled test series, the contractor performed

the standard thoracic impact test (at 14 feet per second) on each of the 4

Part 572 dummies [5], p. 440, Because little was known about the potential effects of unrestrained impacts on the dummies, the calibration test

28
TABLE 2-1

Contractor's

Test

Number

2915*

2918

2921

2924

2927

2935

2938

2941

2944

2947

2950

2957

2962

2967

2972

2977

2982

2994

2995

2996

3002

3003

3004

3008

3009

3016

3017

3018

3021

(IN

Targeted

Impact

Speed

(mph)

12.5

22.5

15

15

30

26.5

26.5

26.5

26.5

26.5

26.5

26.5

26.5

26.5

26.5

26.5

26.5

22.5

22.5

22.5

22.5

22.5

22.5

22.5

22.5

22.5

22.5

22.5

30

SLED TEST MATRIX

CHRONOLOGICAL ORDER)

Principal

Direction

of Force

frontal

frontal

frontal

frontal

frontal

frontal

frontal

frontal

frontal

frontal

frontal

frontal

frontal

frontal

frontal

frontal

frontal

frontal

frontal

frontal

frontal

oblique

oblique

oblique

oblique

frontal

frontal

frontal

frontal

Back

Seat

Occupied?

no

no

yes**

yes**

no

no

no

no

no

no

no

yes

yes

yes

yes

yes

yes

yes

yes

yes

no

no

no

yes

yes

yes

yes

yes

no

Seat

Back

Locks?

no

no

no

yes

no

yes

yes

no

no

no

yes

yes

yes

no

no

no

yes

yes

yes

no

yes

yes

no

no

yes

yes

no

no

yes

*Not used in the analyses - initial run to help establish speed range

**Dummies in the driver's and left rear seats only

29
15

22.5

22.5

22.5

22.5

26.5

26.5

TABLE 2-2

SLED TEST MATRIX

(IN SCHEMATIC ORDER)

Targeted

Impact

Speed

(mph)

Principal

Direction

of Force

Back

Seat

Occupied?

Contractor's

Test Numbers

Without With

SBL SBL

frontal

frontal

frontal

oblique

oblique

frontal

frontal

yes'

no

yes

no

yes

no

yes

2921

2918

2924

3002

2996

3017

3018

3004

3008

2941

2944

2947

2967

2972

2977

2994

2995

3016

3003

3009

2935

2938

2950

2957

2962

2982

30 frontal no 2927 3021

''Dummies in the driver's and left rear seats only

30
was initially repeated after each use of a dummy; in the last half of the

project, the calibration was performed after every third use of a dummy.

The results were satisfactory: the dummies always met the Part 572

criteria for peak impactor force, peak chest deflection and hysteresis.

Moreover, even when the test results for the 4 different dummies were

pooled, only 1 of the 66 values of peak impactor g's was more than 10

percent away from the average value.

The sled buck had to be refurbished after each test, because it

was damaged by the unrestrained dummies. The front seat assembly,

instrument panel, dashboard, glove compartment, heater core, sun visors,

windshield and steering assembly (including brackets) were replaced after

each test in which they were contacted by a dummy, even a minor contact.

They were replaced by used but undamaged Chevrolet Citation parts (except

the windshields, which were new). The replacement seat assemblies

included the tracks for the seats. Each new seat was anchored in the sled

buck by the same procedure as the original seat. The replacement steering

assemblies were diagnosed by the procedure described in GM shop manuals to

assure that the energy-absorbing column would function as designed. The

sled buck was reinforced at the firewall, steering column support structure, roof, A pillar and windshield header areas in order to minimize the

possibility of damage to those structures; since dummies did not directly

contact them in these moderately severe frontal sled tests, their reinforcement probably had little influence on the injury severity measurements. It was also necessary to reinforce the floor under the rear seat

and anchor the rear seat in place more firmly. (For more details about

31
the test setup see [51], pp. 2-8 - 2-13; however, NHTSA and the contractor

agreed to modify some of the procedures in that work plan, as explained by

Khadilkar [31]).'

The sled tests were filmed primarily by onboard cameras,

operating at 1000 frames per second, shooting through the open side

windows. Dummy faces were coated with colored chalk to leave a record of

contacts with vehicle interior surfaces.

The sled tests were successful in gathering data on the dummy

responses. The 28 sled tests used for the analysis involved 84 dummies.

HIC and the other head injury measurements were obtained for all of the

dummies; so was the right femur load. Peak chest g's and left femur load

were unknown for just one dummy; Chest Severity Index could not be

calculated in 2 cases. All of the accelerometer traces are documented in

[34], [35], [36], or [43]. Appendix A of this report lists the injury

criterion measures and test conditions for each dummy.

2.2 Discussion of sled test results

One of the most valuable aspects of the sled test study is that

it is possible to see how seatbacks actually perform in crashes. The

following discussion of what happened in the sled test films, relating the

events in the films to injury severity results, will show that SBL did

have a noticeable effect on seatback performance and dummy motion, but

little effect on injuries.

32
In each of the crashes there was an initial impact of the front

seat dummies into the frontal components of the passenger compartment*. If

the back seat was unoccupied, the initial impact was the only one that

made a significant contribution to injury risk. But if the back seat was

occupied, there was an important second impact when the unrestrained back

seat dummies contacted the front seatback, propelling it forward into the

front seat dummies just when they were rebounding from the initial

impact. While SBL significantly change the motion of the seatback, they

do little to alleviate the severity of either impact.

What is most evident from the films is that the front seatback,

properly "locked" in place with SBL, is not even remotely a "rigid"

structure. Even without any loading by a back seat occupant, the locked

front seatback is carried, by its own momentum, about halfway from its

original position to the instrument panel before it is "caught" by the SBL

in a 22.5 mph frontal crash. (Of course, without SBL, the seatback moves

forward without any restraint in frontal and frontal-oblique crashes.)

When, in addition, the front seatback is loaded by the back seat dummy,

the seatback is deformed and deflected all the way to contact with the

front seat dummy, notwithstanding the SBL. In fact, SBL do not appear to

reduce the speed at which the front seatback contacts the front seat

occupant, after it has been contacted by the back seat occupant.

These observations apply to 22.5 mph crashes. But in 26.5 mph

crashes, the effect of the SBL was even less. In the 3 tests with locked

seatbacks and back seat dummies, the front seatback did not stay in

33
place. On one of the tests (no. 2962), both SBL were sheared off. On the

other two (nos. 2957 and 2982), the front seat was pulled completely out

of its track on one side.

The preceding phenomena (extensive deflection of the seatback at

22.5 mph and separation of the SBC or track at 26.5 mph) are perhaps a

reflection of the specific hardware used in the 1981 Chevrolet Citation or

the manner in which the hardware was installed in the sled buck and might

not be representative of other cars. The MDAI data (Section 5.4),

however, suggest that separation of the SBL or the track is common for all

types of cars when there are back seat occupants.

The films make it possible to see the influence of seatback

motion on dummy trajectories in detail. Here is what happens in the

initial impact of the front seat dummies into the front of the passenger

compartment, which is the only important impact when there are no back

seat dummies. As the sled buck decelerates, the dummies continue straight

forward in a sitting posture and the front seatback rotates forward with

them. When the car has SBL, the seatback stops moving about half way to

the instrument panel and has no further effect on dummy trajectories in

this phase of the impact. When there are no SBL, the seatback continues

forward with the occupant, but not really touching the occupant. Next,

the dummy in the driver's seat engages the steering assembly with its

chest and gets one of its strongest deceleration spikes of the initial

impact phase. At that moment, however, the seatback is barely in contact

with the dummy's back and has little effect on the dummy's trajectory. As

34
the steering column is compressed and begins to decelerate the dummy, the

seatback gradually "catches up" with the dummy and begins to load it in

the upper thorax. That process takes some time because the front of the

seatback is soft and is deflected several inches. By the time the

seatback bottoms out on the dummy, the dummy has more or less bottomed out

on the steering assembly and instrument panel. Thus, the additional load

of the seatback may have added slightly to the loads experienced by the

dummy's thorax, but over an extended time period and without any noticeable spike. The dummy in the right front seat is even less influenced by

SBL. This dummy continues forward until it contacts the instrument panel,

which is well forward of the steering assembly. By the time the dummy

engages the instrument panel, the seatback has rotated about as far

forward as it can go and is beginning to move more downward than forward.

Only the upper part of the seatback scrapes against the dummy's back while

the dummy is bottomed out on the instrument panel.

When there are dummies in the back seat, they continue to move

forward as the sled buck decelerates. If the car has SBL, the locks

"catch" the front seatback, as mentioned above, and hold it in a more or

less upright position. The back seat occupant contacts the front seatback

with his knees first and then with his head and chest. The seatback

deflects forward easily and only decelerates the back seat occupant a

small amount. The back seat dummy and seatback together plow into the

front seat dummy just as it is rebounding from the steering assembly or

instrument panel, resulting in a severe blunt impact to the front seat

dummy's upper spine or the back of its neck.

35
When the car does not have SBL, the films suggest that the

results are a bit less severe for the front seat dummy but more severe for

the back seat dummy. Without SBL, as mentioned above, the seatback

continues to rotate forward until it bottoms out on the front seat dummy

and the front of the passenger compartment. By that time, the top of the

seatback ii well to the front of the lower part of the seatback. The back

seat dummy initially engages the seatback with its knees only and rotates

forward head first. When the dummy1s head reaches the front seatback, it

has some of the weight of the upper torso behind it, possibly sustaining a

more severe impact than it would have with SBL. On the other hand, the

second impact seems a little less severe for the front seat dummies: they

are not rebounding as rapidly at the moment of impact as they are in the

case with SBL because the earlier motion of the unlocked seatback retarded

their rebound. Also, with the seatback in a more forward and less

vertical position, the back seat dummy appears to strike more of a

glancing impact. The injury parameters measured on the dummies (described

in the next section) are consistent with what appeared in the films

(decreased head injury for the back seat dummy and increased 2nd impact

thorax injury for the front dummy in the SBL runs), although the observed

differences in the injury parameters were not statistically significant

(two-sided alpha - .05).

In a few tests, the back seat dummy's head and thorax bypassed

the bucket style front seatback and entered the front portion of the

passenger compartment. That is usually undesirable because the back seat

dummy can strike hard surfaces such as the windshield and instrument panel

36
or make direct head to head contact with a front seat dummy (although in

some cases the back seat dummy avoids head contacts and is better off).

In the oblique tests, not surprisingly, bypassing occurred in the test

without SBL. Since the front seatback is further forward when there are

no SBL, the back seat dummy must travel a greater distance to reach the

seatback. In an oblique crash, that means enough lateral travel to miss

the seatback. In the straight frontal crashes, however, bypassing

occurred only on some of the tests with SBL, at 22.5 mph. It appears that

the back seat dummies twisted the top of the seatback and then slid past

it. Bypassing did not happen in the 26.5 mph tests with SBL, perhaps

because the SBL sheared off or the seat came loose from its track before

much twisting occurred.

The tests revealed some other phenomena, unrelated to SBL but of

more general interest. They showed that unrestrained dummies score much

higher on the injury parameters than restrained dummies would have under

similar test conditions. In particular, the unrestrained Part 572 dummies

could not repeatably meet the criteria of Standard 208 (HIC below 1000,

chest g's below 60 and femur load below 2250 pounds), even though the

tests were run at well below the 30 mph and roughly 22 peak sled g's used

for Standard 208 testing. Table 2-3 shows that both front seat dummies

met the Standard 208 criteria on just 4 of the 12 tests at 22.5 mph, one

of the 12 tests at 26.5 mph and neither of the 2 tests at 30 mph. (Chest

g's are tabulated only for the initial impact of the front seat dummy into

the steering assembly or instrument panel, since the second impact of the

back seat dummy into the front seatback and front seat dummy would not

37
TABLE 2-3

PERFORMANCE OF UNRESTRAINED DUMMIES

RELATIVE TO STANDARD 208 CRITERIA

•(HIC-1000, Chest g's-60, Femur Load=2250:

values exceeding these criteria are underlined)

Test

No.

2918

2994

2995

2996

3002

3003

3004

3008

3009

3016

3017

3018

2935

2938

2941

2944

2947

2950

2957

2962

2967

2972

2977

2982

2927

3021

HIC

213

293

385

457

234

235

111

111

185

738

139

167

All

351

321

433

543

762

578

397

640

228

450

326

533

696

D r i v

Chest

g's*

41.2 ,

41.7

35.0

30.0

46.5

29.0

39.0

36.7

35.0

38.0

40.0

37.1

53.9

51.0

47.7

fiLJ

67.7

54.7

72.6

50.0

60.0

40.3

59.5

56.7

54.8

68.8

e r s

Femur

L

1152

872

1513

1276

2925

3445

3841

2108

4403

1344

2192

1280

1745

1965

2062

1803

2622

2831

1629

2564

4039

1866

2056

1450

865

1841

Load

R

22.5

1178

642

581

712

780

515

323

509

567

440

624

359

26.

1935

998

1536

1079

725

530

721

562

650

987

767

777

30

1428

738

Right Front

HIC

mph tests

146

376

477

474

546

394

234

337

413

253

1111

3068

5 mph tests

218

298

459

345

428

376

327

4307

934

819

2723

334

mph tests

1958

1629

Chest

g's*

50.3

20.0

24.9

27.8

29.5

40.2

36.6

46.4

39.0

45.0

49.1

48.9

50.9

75.8

1SL1

67.8

73.5

55.6

74.1

I&A

80.0

53.2

55.8

60.3

mi

84.6

Passengers

Femur

L

497

1103

1829

1922

1355

3136

1738

3524

4J11

2692

3814

2899

1349

2323

1418

2779

3668

2322

2702

M52

3647

2875

2459

2775

2213

3859

* Load

R

513

901

485

1364

2436

1798

2148

2356

3378

2156

3195

2120

1276

2251

2921

1873

2983

2133

1535

2901

3320

2036

2933

2279

1745

5129

Pass

208?

Yes

Yes

Yes

Yes

No

No

No

No

No

No

No

No

Yes

No

No

No

No

No

No

No

No

No

No

No

No

No

*In tests where there was a back seat dummy, accelerations of the front seat

dummy which were attributable to the back seat dummy were not considered in

the calculation of peak chest g's.

38
have occurred in Standard 208 testing.) The table shows, however, that

many of the dummies were unable to meet the Standard 208 criteria for

femur load, while they had acceptable head and chest scores. The high

severity of the right front passenger's left femur load is primarily due

to contacts with the heater core. It is also possible, however, that the

femur loads in the sled tests may have increased in some cases because the

contractor had to reinforce the firewall in order to maintain the structural integrity of the sled buck: some of the dummies' knees may have

contacted hardened spots in the firewall beyond the instrument panel.

Table 2-4 compares the dummy performance relative to Standard 208 criteria

for HIC and chest g's, only. Ten of the 12 tests at 22.5 mph had passing

head and chest scores for both dummies, but 2 right front passengers had

HIC over 1000. At 26.5 mph, only 3 of 12 tests had passing head and chest

scores for both dummies; most of the right front dummies had more than 60

chest g's. Neither test at 30 mph met Standard 208 criteria.

It was also noteworthy that the Citation's energy absorbing

steering columns were compressed all the way or almost all the way (i.e.,

4-5 inches), in every test at 22.5 mph or more, even in the oblique tests

at 22.5 mph. Nevertheless, the successful compression of the column in

almost every case did not fully dissipate the kinetic energy of the driver

dummy's thorax. The chest accelerometer traces all show nearly the same

pattern: an initial peak of 40-50 g's at about 100 milliseconds, apparently representing a "threshold" force needed to initiate compression of the

column; deceleration then rapidly drops off to a trough of close to 10 g's

as the column compresses, absorbing relatively little energy during the

39
TABLE 2-4

PERFORMANCE OF UNRESTRAINED DUMMIES

RELATIVE TO THE HEAD AND CHEST CRITERIA OF STANDARD 208

<HIC=1000, Chest g's=60: values

exceeding these criteria are underlined)

Test

No.

2918

2994

2995

2996

3002

3003

3004

3008

3009

3016

3017

3018

2935

2938

2941

2944

2947

2950

2957

2962

2967

2972

2977

2982

2927

3021

D r i

HIC

213

293

385

457

234

235

111

227

185

738

139

167

477

351

321

433

543

762

578

397

640

228

450

326

533

696

v e r s

Chest

g's*

41.2

41.7

35.0

30.0

46.5

29.0

39.0

36.7

35.0

38.0

40.0

37.1

53.9

51.0

47.7

SL1

£U

54.7

72.6

50.0

60.0

40.3

59.5

56.7

54.8

Right Front

HIC

22.5 mph tests

146

376

477

474

546

394

234

337

413

253

Jill

3068

26.5 mph tests

218

298

459

345

428

376

327

4307

934

819

2723

334

30 mph tests

1958

1629

Passengers

Chest

g's*

50.3

20.0

24.9

27.8

29.5

40.2

36.6

46.4

39.0

45.0

49.1

48.9

50.9

75.8

70.1

67.8

73.5

55.6

74.1

78.1

80.0

53.2

55.8

60.3

79.2

84.6

Pass

208?

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

No

No

Yes

No

No

No

No

Yes

No

No

No

Yes

No

No

No

No

*In tests where there was a back seat dummy, accelerations of the front

seat dummy which were attributable to the back seat dummy were not

considered in the calculation of peak chest g's.

40
main portion of Its stroke; finally, at 125 milliseconds, deceleration

reaches a second and usually worse peak as the dummy bottoms out the

column and 1s brought to an abrupt stop.

2.3 Statistical analysis of sled test results

2.3.1 Average values for the Injury criteria - with and without SBL

Appendix A of this report shows the injury criterion scores for

each of 42 dummies in tests without SBL as well as the scores for 42

dummies in corresponding tests with SBL. The best overall measure of the

performance of SBL is to compute the average scores for the 42 dummies

without SBL and compare them to the averages with SBL.

Table 2-5 displays the average scores on the 7 injury criteria

that were measured on every dummy. The 42 dummies without SBL had an

average HIC of 464. (In the case of HIC, Chest Severity Index and Mean

Strain Criterion, the "average" 1s not the simple arithmetic average but

the antilog of the average of the logs of the values. The logarithm

transformation was employed because the distributions are highly skewed

and the arithmetic averages are not good indicators of central tendency

because they are pulled to the right by an outlier or two.) The 42

dummies with SBL had an average HIC of 408, which is 12 percent lower than

the non-SBL dummies. In Table 2-5 as well as the remaining tables of this

chapter, positive numbers in the reduction columns indicate that the

dummies with SBL had better results than the non-SBL dummies; negative

numbers indicate worse results. The reduction, however, is not statistically significant (two-sided alpha - .05), as will be shown in the next

41
TABLE 2-5

SLED TEST RESULTS - ALL DUMMIES

(42 dummies in tests with SBL vs.

42 dummies in identical tests without SBL)

HIC*

Head g's

Mean Strain

Chest g's

Chest Sev.

Left femur

Riaht femur

Cr'it.*

Index*

load

load

Average

Severity

Without

Locks

464

71.6

0.0058

45.4

235

1689

1167

With

Locks

408

70.4

0.0051

47.3

242

1689

1173

Reduction

for SBL

Absolute

56

1.2

0.0007

-1.9

-7

none

-6

Relative

(%)

12

2

12

-4

-3

none

-1

Significance Test

for Matched Pair

Comparison**

t

0.84

0.22

1.13

-0.85

-0.27

0

-0.04

Significant?

no

no

no

no

no

no

no

*Values shown are the antilogs of the average of the logs of these criteria.

**t test with two-sided alpha « .05

42
section. The SBL dummies also had slightly lower scores than the non-SBL

cases on the other two measures of head injury: a 2 percent reduction of

peak head g's and a 12 percent drop in Mean Strain Criterion. They were,

however, a little worse off on both chest injury scores: a 4 percent

increase on peak chest g's and a 3 percent increase on Chest Severity

Index. There was hardly any difference between the two groups on femur

load.

Each dummy is given equal weight in computing the simple

averages of Table 2-5. Since the dummies were distributed about equally

between three seat position conditions - in the front seat with nobody

behind them, in the front seat with somebody behind them, and in the back

seat - the average is not necessarily representative of highway crashes,

where the vast majority of occupants sit in the front seat and nobody is

sitting behind them. But with the small samples of dummies available

here, there is little to be gained by "weighting" the cases to be moire

representative of highway crashes.

Table 2-6 is limited to the subset of dummies that were in the

front seat and had no dummy sitting behind them: 12 with SBL and a

corresponding 12 without SBL. Here, the dummies with SBL had slightly

worse results on two of four head injury measures (HIC and peak g's) but

better on the other two (Mean Strain Criterion and laceration index).

They had fractionally lower scores on the chest injury measures but worse

scores on femur load. Any comparison, however, needs to be tested for

statistical significance before it can be accepted as a genuine indication

43
TABLE 2-6

SLED TEST RESULTS - DUMMIES IN THE FRONT SEAT

WITH NO DUMMY POSITIONED BEHIND THEM

(12 dummies in tests with SBL vs.

12 dummies in identical tests without SBL)

HIC*

Head

Mean

Tot.

Chest

Chest

Left

Riant

g's

Straiti Crit.*

Lacerat. Index

g's

Sev.

femur

Index*

load

femur load

Average

, Severity

Without

Locks

385

60.9

0.0051

5.83

56.2

335

2055

1538

With

Locks

432

71.

0.

5.

53.

330

2425

1710

4

0045

04

4

Reduction

for SBL

Ab

sol

-47

-10

0

0

2

5

-370

-172

ute

.5

.0006

.79

.8

Relative

(%)

-12

-17

12

14

5

1

-18

-11

Significance Test

for Matched Pair

Comparison**

t

-0.71

-2.16

0.73

1.26

0.81

0.12

-1.07

-0.42

Significant?

no

no

no

no

no

no

no

no

*Values shown are the antilogs of the average of the logs of these criteria.

**t test with two-sided alpha = .05

44
of an effect of SBL. In particular, all the differences in Table 2-6 will

be shown nonsignificant (two-sided alpha - .05) in the next section.

Similarly, Table 2-7 displays the results for dummies in the

front seat that had a dummy behind them and Table 2-8 shows the average

scores for back seat dummies. The results of the comparisons are discussed in the next section.

2.3.2 Significance of the results; t tests of matched pairs

The sled tests were peformed over a range of speeds and impact

modes, with dummies in a variety of seat positions. These factors, which

were intentionally part of the experimental design, cause much variation

from test to test. It would not be appropriate to test statistical

significance in the ordinary way - i.e., to compute the average and

standard deviation for the tests with SBL and the same for the tests

without SBL and to do a t test for the difference between two samples -

because these standard deviations would be inflated and the significance

of differences understated. A better approach, consistent with the

experimental design, is to recognize that each dummy in a test with SBL

has a matching partner in a test without SBL - at the same seat position,

speed and impact mode. Table 2-2 shows which test without SBL is matched

to which test with SBL. Within tests, dummies are matched by seat

position, e.g., the driver dummy in test 2921 with the driver dummy in

test 2924. The differences in HIC, chest g's, etc. between two such

matching dummies are only attributable to SBL and/or sources of variation

that are beyond the experimenter's control, i.e., sampling error. The

45
TABLE 2-7

SLED TEST RESULTS - DUMMIES IN THE FRONT SEAT

WITH A DUMMY POSITIONED BEHIND THEM

(15 dummies in tests with SBL vs.

15 dummies in identical tests without SBL)

Average

Severity

Without

Locks

With

Locks

Reduction

for SBL

Ab- Relative

solute (%)

Significance Test

for Matched Pair

Comparison**

Signit ficant?

HIC* 491 394 97 20 0.75 no

Head g's from:

any contact

1st contact

later contacts

Mean Strain Crit.*

Tot. Lacerat. Index

Chest g's from

any contact

1st contact

later contacts

Chest Sev. Index*

Left femur load

Right femur load

68.7

68.1

40.5

0.0062

4.03

50.3

47.3

31.6

313

2457

1496

67.3

67.3

29.1

0.0050

4.13

56.4

46.2

39.6

413

2202

1243

1.4

0.8

11.4

0.0012

-0.10

-6.1

1.1

-8.0

-100

255

253

2

1

28

19

-2

-12

2

-25

-32

10

17

0.17

0.17

1.19

1.15

-0.31

-1.49

0.37

-1.03

-1.72

1.03

1.16

no

no

no

no

no

no

no

no

no

no

no

*Values shown are the antilogs of the average of the logs of these criteria.

**t test with two-sided alpha « .05

46
TABLE 2-8

SLED TEST RESULTS - DUMMIES IN THE BACK SEAT

(15 dummies in tests with SBL vs.

15 dummies in identical tests without SBL)

HIC*

Head g's

Mean Strain

Chest g's

Chest Sev.

Left femur

Right femur

Crit.*

Index*

load

load

Average

Severity

Without

Locks

507

83.0

0.0060

30.9

122

553

542

With

Locks

403

72.8

0.0060

32.4

98

508

673

Reduction

for SBL

Absolute

104

10.2

none

-1.5

24 -

45

-131

Relative

(%)

21

12

none

-5

20

8

-24

Significance Test

for Matched Pair

Comparison**

t

0.78

0.97

0

-0.38

0.85

0.57

-2.22

Significant?

no

no

no

no

no

no

yes

*Values shown are the antilogs of the average of the logs of these criteria.

**t test with two-sided alpha = .05

47
differences are computed for the 42 matched pairs of dummies, and a t test

is performed to check if the average difference is significantly different

from zero. The' results of the t tests are shown in the next to last

column of Table 2-5. The last column indicates whether the observed value

of t is significant (two-sided alpha - .05) with 41 degrees of freedom.

(Chest g's and left femur load were missing on one dummy with SBL - see

Section 2.1.4 - and Chest Severity Index could not be calculated for two

dummies. For these injury measures, the dummy with the missing data and

its matching partner without SBL were not used in the statistical analysis. Significance was tested with the remaining matched pairs, with 1

or 2 fewer df.)

Table 2-5 shows that none of the differences between SBL and

non-SBL injury rates even come close to statistical significance. The

largest value of t is 1.13, for Mean Strain Criterion, whereas t would

have to be over 2.02 for statistical significance (with two-sided alpha =

.05 and 41 df). Table 2-5 also illustrates that the value of t depends on

the repeatability of the injury criterion as well as the magnitude of the

observed difference. For example, the t value for chest g's is -0.85 even

though the difference between SBL and non-SBL is just 4 percent, whereas

the t value for HIC is 0.84 although there was a 12 percent reduction in

HIC. That is because chest g's are a far more repeatable criterion than

HIC, having only 1/3 as large a coefficient of variation.

Table 2-6 analyzes the 12 pairs of dummies that were in the

front seat and did not have another dummy sitting behind them. Here there

48
are 8 injury criteria, since the Total Laceration Index was measured on

all front seat dummies. There were no significant differences between SBL

and non-SBL on any of the criteria, although the 17 percent increase in

peak head g's in the SBL group comes "close" to significance: it has a t

value of -2.16, whereas -2.20 would be the critical value with 11 df. The

fact that another head injury measure (Mean Strain Criterion) decreased

even while head g's increased is also evidence that the observed increase

in head g's is probably not due to SBL. The observed increases in femur

loads are likewise not meaningful, as they do not come close to statistical significance.

Table 2-7 considers the 15 pairs of dummies that were sitting in

the front seat and had another dummy directly behind them. As explained

in Sections 2.1.2 and 2.2, they were subjected to two major impacts during

the crash: the initial contact with the front of the passenger compartment

and the subsequent interaction with the back seat dummy. Peak head and

chest g's were measured for each contact separately as well as for both

combined. The other criteria, such as HIC, MSC and CSI, are calculated

over both impacts. Table 2-7 does not show significant differences on any

of the criteria. Consistent with the discussion in Section 2.2, however,

the dummies in the crashes with SBL had 25 percent higher peak chest g's

on the impact caused by interaction with the back seat dummy, possibly

accounting for the 32 percent overall increase in Chest Severity Index and

the 12 percent increase in overall peak chest g's. None of those increases, on the other hand, was statistically significant although the one

for Chest Severity Index came fairly close (t - -1.72, where -2.15 is the

49
critical value with two-sided alpha - .05 and df - 14 and -1.76 is the

critical value with one-sided alpha - .05).

Finally, Table 2-8 analyzes the 15 pairs of dummies located in

the back seat. None of the observed differences are statistically

significant, except that the dummies with SBL had "significantly" higher

right femur loads. That result may be spurious: by now, 34 significance

tests have been performed in Tables 2-5 - 2-8 and, with alpha - .05, at

least one of the tests could be expected to have significant results even

if there were no real differences. At any rate, the femur loads for the

back seat dummies (542 without SBL and 673 with SBL) are always far below

the levels associated with serious injuries. Consistent with the discussion in Section 2.2, the back seat dummies with SBL had 21 percent

lower HIC than those without SBL and 12 percent lower peak head g's.

Neither of these reductions, however, even approaches statistical significance. Moreover, the other measure of head injury (Mean Strain Criterion) was unchanged.

2.3.3 Nonparametric significance tests

There is another, simpler way to analyze the data and determine

if SBL reduced injuries. For each matched pair of dummies, score one

point for SBL if the SBL dummy did better on a particular injury criterion

and score one point for non-SBL if that dummy did better. Tally the

scores for all the matched pairs. If the SBL dummies score significantly

more than 50 percent of the points, it is concluded that SBL is effective

in reducing that injury criterion. This nonparametric approach has the

50
advantage that a consistent trend in favor of one side would not be

obscured by a few outliers in the opposite direction. It has the disadvantage of being even less likely to detect significant differences than

the preceding matched pair t tests.

Table 2-9 shows the nonparametric test results for all 42

matched pairs. None of the observed differences were statistically

significant. The SBL dummies had lower HIC than their non-SBL counterparts in 26 out of 42 cases. Nevertheless, that is not significantly

different from a 50-50 split (one-sided alpha - .05, using an exact

binomial test). The SBL dummies likewise performed slightly better, but

not significantly better, on the other two head injury measures. The

non-SBL dummies had slightly better results on the two chest scores, with

lower chest g's in 24 out of 41 tries and lower CSI on 22 out of 40

tries. Again, those are not significantly different from 50-50. The

results on femur load were virtually even.

Table 2-10 subdivides the analysis by seating position. There

was no significant difference between SBL and non-SBL on any injury

criterion. (Even the 11-4 splits observed in two cases are not in the

critical range of a binomial distribution with p •= .5.) In all seat

positions, the SBL dummy did slightly better than the non-SBL dummy on the

head injury measures. The chest injury scores were marginally in favor of

the non-SBL dummies, while the femur load results were inconsistent (but

within the noise range of a binomial distribution with p » .5). Among the

front seat dummies with a dummy sitting behind them, the non-SBL group did
TABLE 2-9

NONPARAMETRIC ANALYSIS OF SLED TEST RESULTS - ALL DUMMIES

(42 dummies in tests with SBL vs.

42 dummies in identical tests without SBL)

Number of Matched Pairs with:

Injury Criterion

HIC

Head g's

Mean Strain Crit.

Chest g's

Chest Sev. Index

Left femur load

Right femur load

Non-SBL

Dummy

Better

16

18

18

24

22

20

22

SBL

Dummy

Better

26

23 ,

24

17

18

21

20

Tie

Score

0

1

0

0

0

0

0

Missing

Data

0

0

0

1

2

1

0

Significant

Difference*

between SBL

and Non-SBL?

no

no

no

no

no

no

no

*Exact binomial test with one-sided alpha » .05

52
TABLE 2-10

NONPARAMETRIC ANALYSIS OF SLED TEST RESULTS - BY SEAT POSITION

Number of Matched Pairs with:

Injury Criterion

FRONT

HIC

Head g's

Mean Strain Crit.

Tot. Lacerat. Index

Chest g's

Chest Sev. Index

Left femur load

Right femur load

FRONT

HIC

Head g's from:

any contact

1st contact

later contacts

Mean Strain Crit.

Tot. Lacerat. Index

Chest g's from:

any contact

1st contact

Uter contacts

Chest Sev. Index

Left femur load

Right femur load

HIC

Head g's

Mean Strain Crit.

Chest g's

Chest Sev. Index

Left femur load

Right femur load

Non-SBL

Dummy

Better

SBL

Dummy

Better

SEAT DUMMIES WITH

5

4

6

4

7

6

8

5

7

7

6

7

5

6

4

7

SEAT DUMMIES WITH

6

6

6

9

5

5

9

6

8

10

4

6

9

9

9

6

10

8

6

9

7

5

11

9

BACK SEAT

5

5

7

8

6

8

11

10

10

8

6

7

6

4

Tie

Score

NO DUMMY

0

1

0

1

0

0

0

0

A DUMMY

0

0

0

0 •

0

2

0

0

0

0

0

0

DUMMIES

0

0

0

0

0

0

0

Missing

Data

BEHIND THEM

0

0

0

0

0

0

0

0

BEHIND THEM

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

1

2

1

0

Significant

Differjence*

between SBL

and Non-SBL?

no

no

no

no

no

no

no

no

no

no

no

no

no

no

no

no

no

no

no

no

no

no

no

no

no

no

no

*Exact binomial test with one-sided alpha - .05

53
better on both "later contacts" measures, but not significantly better.

It would be tempting, but not valid, to combine the results on

several injury measures - say HIC, head g's, Mean Strain Criterion and

Total Laceration Index - to obtain a single large sample where, in fact,

the SBL dummies are better off "significantly" over 50 percent of the

time. But the significance test for a binomial distribution is only valid

if the elements of the sample are independent trials. In this case, HIC,

head g's, MSC and TLI are measured from the same pair of sled tests. If

HIC is high, it is almost always the case that peak head g's and MSC are

also high. Thus, HIC, head g's and MSC from the same sled tests are

hardly independent in a statistical sense.

54
CHAPTER 3

ANALYSES OF WASHINGTON, TEXAS AND NEW YORK STATE ACCIDENT DATA

Accident data from 3 States, comprising well over a million

persons involved in crashes, were statistically analyzed. They did not

show seat back locks (SBL) to be effective in reducing injury risk at any

severity level. They did not show effectiveness at any seat position or

specific injury type. SBL were not effective when occupants were restrained and likewise when they were unrestrained. Worse than that, the

risk of occupant ejection was always higher in 2 door cars than in 4 door

cars, but the Washington data hint at a possibility that the discrepancy

between the two types of cars became even wider after the introduction of

SBL.

3.1 Analysis method

The principal difficulty in analyzing State data files is that

the effect of SBL is expected to be small in comparison to the other

safety features that were introduced at about the same time, not to

mention the other trends and year to year changes of injury rates that are

typically found in State files. Moreover, the discussion of Section 1.6

and the sled tests of Chapter 2 do not suggest any specific injury type or

crash situation where SBL could be intuitively expected to have a high

effect (unlike, say, High Penetration Resistant windshields, which can

specifically be expected to reduce serious facial lacerations). So it is

not possible to focus the analysis on one specific injury type. In the

sled tests of Chapter 2 it was possible to control the crash conditions
and vehicle configurations and to obtain pairs of crashes that were

identical except that one of them was with SBL and the other without.

Such a priori control is impossible with highway accident data. Thus it

will be necessary to analyze State data for all types of crashes and to

eliminate the biases caused by other safety devices, etc., by the use of a

control group and by limiting the data to as few model years as possible

before and after the introduction of SBL.

•i

3.1-1 The control group: 4 door cars

In all analyses, the control group is the 4 door passenger car,

which was unaffected by the introduction of SBL. Ball originally developed this approach in his study of SBL [2] (see Section 1.8). The injury

reduction for 2 door cars with SBL versus those without SBL is measured

relative to the corresponding injury reduction in 4 door cars of the same

model years.

All of the analyses of accident data (Chapters 3-5) are limited

to the big 3 domestic manufacturers, since they appear to have been the

only companies that met the following conditions during the years before

and after the introduction of SBL (circa 1963-71): (1) The company

produced both 2 and 4 door cars; (2) The VIN can be decoded to reveal if a

car had 2 or 4 doors; (3) It is known exactly when SBL were introduced

(see Section 1.4).

The analysis method is illustrated, for example, by the first 4

lines of data in Table 3-1, which is based on 1973-77 Washington State

56
TABLE 3-1

WASHINGTON STATE 1973-77: K + A (FATAL OR SERIOUS) INJURY RATES,

ALL OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/

After SBL NET RED.

N of Install, in n of N of Injuries per Gross FOR SBL**

Doors 2 Door Cars Injuries Persons 100 Persons Red. (%)

Z Test

4 last 1 before 605 22,913 2.64

first 1 after 522 23,301 2.24 15

2 last 1 before 859 28,541 3.01

first 1 after 843 32,533 2.59 14 -1%

Z-0.19

4 last 2 before 1305 46,787 2.79

first 2 after 1102 47,855 2.30 17

2 last 2 before 1752 55,031 3.18

first 2 after 1842 67,435 2.73 14 -4%

Z=0.74

4 last 3 before 1980 68,407 2.89

first 3 after 1527 68,411 2.23 23

2 last 3 before 2496 77,660 3.21

first 3 after 2845 103,545 2.75 15 -11%

Z=2.38*

4 last 4 before 2568 85,055 3.02

first 4 after 1912 87,130 2.19 27

2 last 4 before 3010 90,724 3.32

first 4 after 3572 132,175 2.70 19 -12%

Z=2.95*

*Statistically significant effect (two-sided alpha = .05 - i.e. Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

57
crashes. The third line shows that in 2 door cars of the last model year

before SBL (i.e. 1966 GM, 1967 Ford and 1967 Chrysler products - see

Section 1.4), 859 of 28,541 occupants had serious or fatal injuries. (A

and K are the respective police codes for serious and fatal injuries.)

That is an injury rate of 3.01 percent. The fourth line shows that in 2

door cars of the first model year with SBL (1967 GM, 1968 Ford and 1968

Chrysler products) the serious injury rate was 2.59 percent. That is a 14

percent gross reduction in the injury rates in 2 door cars. Much if not

all of that reduction, however, may be attributed to energy absorbing

steering columns and other safety devices introduced in 1967-68 as well as

the typical downward trend of injury rates in State data as cars get

newer. The first two lines of data show the corresponding results for the

control group, 4 door cars. The first line shows that 1967 GM, 1968 Ford

and 1968 Chrysler products - i.e., cars that would have been "of the last

model year before SBL" if they had been 2 door cars - had a serious injury

rate of 2.64 percent. The second line shows that 1967 GM, 1968 Ford and

1968 Chrysler products with 4 doors (cars that would have been "of the 1st

model year with SBL" if they had been 2 door models) had a 2.24 percent

serious injury rate. That is a 15 percent injury reduction for the

control group. The 2 door cars, which were influenced by SBL, had only a

14 percent reduction. The crucial assumption with this method is that the

2 door cars and the control group were subjected to the same influences

except for the introduction of SBL in the 2 door cars. The gross reduction of the injury rates for the first MY after SBL were introduced

relative to the last MY before SBL should be the same for 2 and 4 door

cars, except to the extent that SBL affected injury risk. If so, the

58
relative difference in the gross injury reductions measures the net effect

of SBL. With a 14 percent gross reduction in the 2 door cars and 15

percent in the control group, the net effect is

1 _ (1-.14)/(1-.15) . -1 percent

i.e., the SBL are associated with an estimated 1 percent increase in

serious injuries.

3.1.2 Limiting the data set to reduce bias

Unfortunately, the crucial assumption that 2 door and 4 door

cars are subject to the same influences cannot be accepted without

reservations. In fact, there are several reasons besides SBL that 2 door

and 4 door cars may have different injury reductions from one model year

to another. The most obvious is that 2 and 4 door cars appeal to different segments of the market, with 4 door cars typically appealing to

older purchasers or for use as the "family" car. An initial remedy, which

was used with every data file, was to eliminate convertibles, which have

quite distinct driving patterns and injury risks and which are almost

always 2 door cars. But that remedy only scratches the surface of the

problem. Specifically, the 1960-75 period surrounding the introduction of

SBL witnessed a major increase in the demand for 2 door cars at the

expense of 4 door cars, as described in Section 1.7. The influx of

smaller 2 door cars, many with inexperienced drivers, could have resulted

in a steady widening of the gap between the injury rates of the 2 and 4

door cars. The analysis would misinterpret the widening gap as a negative

net "effect" for SBL. That was the principal critique of Ball's approach

(see Section 1.8).

59
The remedy is to limit the data set so as to minimize the drift

in the characteristics of the 2 and 4 door car populations. One way is to

use as few model' years as possible, preferably only the single model years

before and after the introduction of SBL. How much can the market shift

in a single year? The 2 door cars of the first year with SBL will account

for nearly the same market segment as the 2 door cars of the last year

before SBL; the 4 door cars of the SBL introduction year appealed to more

or less the same group as the 4 door cars of the year before SBL introduction. Thus the changes in the 2 door cars' injury rates from one year to

the next would only be affected by market shifts to a small extent;

likewise for the 4 door cars. The "net effect for SBL" - i.e., the

difference between the change in the 2 door cars' injury rates and the

change in the 4 door cars' injury rates - would not be biased to any large

extent by market shifts.

The drawback of using just one model year before/after SBL

implementation, of course, is that sample sizes are limited. A statistically more precise, although potentially more biased estimate is obtained

by using data of the two model years before/after SBL introduction. Even

more precise, but possibly more biased estimates are achieved by using 3

or 4 model years before/after SBL. The strategy in Table 3-1 and in all

other accident data analyses of this report is to obtain all 4 estimates:

the ones based on ±1, +2, ±3 and ±4 model years, respectively. For

example, in Table 3-1, the estimated "net effect of SBL" is negative 1

percent in the estimate based on the single model year before/after SBL,

-4 percent in the +2 MY estimate, -11 percent in the ±3 year sample and

-12 percent in the ±4 year sample.

60
The advantage of calculating all 4 estimates is that the

sequence of the 4 numbers gives an excellent intuitive feel for what might

be the best, unbiased value for net effectiveness and how statistically

reliable that value is. The sequence in Table 3-1 (-1, -4 , -11 and -12

percent) suggests the presence of a secular bias that works against SBL,

producing increasingly unfavorable results as the age span of the cars in

the sample is widened. The +4 year estimate of -12 percent is not valid

because of the bias. The most reasonable conclusion is that SBL had

little or no effect on the injuries described in Table 3-1. By contrast,

the sequence of estimates in Table 3-2 is far more consistent: -6, -6, -8

and -9 percent. It is possible, of course, that the apparent consistency

occurred by chance alone. Nevertheless, the most reasonable conclusion is

that there is little secular drift in the estimates - i.e., about a 1

percent worsening for each year of additional data. The sequence appears

to support a "best" estimate that SBL increased the likelihood of injuries

described in Table 3-2 by about 5 percent. This year-by-year approach was

used extensively in NHTSA's evaluations of side door beams [26], side

marker lamps [25] and windshields [27]. It proved useful for isolating

the effect of a safety device when there are many other factors influencing injury risk.

Another strategy for reducing bias, used in some of the

Washington analyses in this chapter as well as with the FARS data in

Chapter 4, is to make the 2 door and 4 door cars as similar as possible by

eliminating vehicles of the one type that have no counterpart in the other

type. Makes and models that were produced only with 2 doors, such as Ford

61
Mustang, Chevrolet Camaro and Plymouth Barracuda are excluded from the

analysis. They are mostly small, sporty cars and they had an increasing

market share in the later 1960's. Station wagons, which almost always

have 4 doors and are generally "family" cars were likewise excluded. So

were the subcompact cars, which were produced only in the post-SBL era and

generally have 2 doors - they had no counterpart in the pre-SBL era.

Finally, when data problems such as indecipherable VINs forced the

exclusion of certain pre-SBL vehicles (e.g., Chrysler products more than 2

years before the introduction of SBL), the corresponding post-SBL vehicles

were also excluded (viz., Chrysler products more than 2 years after SBL

introduction).

*

The resulting data set consists exclusively of 2 and 4 door

sedans and hardtops of the same makes and models - i.e., for any 2 door

SBL. equipped car on the file, there are 2 door pre-SBL, 4 door post-SBL

and 4 door pre-SBL cars of the same make, model and n of model years

before/after SBL introduction. That does not mean, of course, that the 2

and 4 door cars on this file have the same accident exposure. But it does

cut down the possibility for bias due to secular changes in market

patterns, introduction of new models with 2 doors but not with 4 doors,

etc.

3.1.3 Statistical significance testing

Since the analyses can be expected to show small effects, if

any, it 1s important to test the statistical significance of any observed

effect. Specifically, it is necessary to test if the "net effect for SBL"

62
is significantly different from zero - i.e., if the gross reduction for 2

door cars is significantly different from the gross reduction for 4 door

cars. The following data table is a generalized version of the entries in

Table 3-1 and all the other data analyses of this chapter:

N of

Doors

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 3 before

first j after

last j before

first j after

n of

Injuries

"40

"41

"20

"21

N of

Persons

N40

N41

N20

N21

Under the hypothesis that the net effect of SBL is zero, the actual

observed n's of injuries would have to be replaced by expected values

e20* e 2 T e40 ancl e41 sat ^s f y ^n 9 the equations:

(e2 1/N2 1)/(e2 0/N2 0) - <e4 1 /

N 4 1)/(e 4 0/ N 4 0}

e20 + e21 = n20 + n21

e40 + e41 = n40 + n41

e 2 Q + e 4 Q - n2 Q + n4 Q

The equations are solved for the e's, using the values of the n's and N's

actually observed in the tabulated data. The solutions are used to

calculate a chi square statistic:

X = (G2 O -

n 2 O)2/e 20 + "• + (e

4 r

n 4 1 )2/e 4 1

The observed net effect of SBL will be termed "statistically significant

with two-sided alpha - .05" whenever

Z - (X/fpc)'5

is greater than 1.96, where

63
fpc = 1 - min(n../N..)

/ i j ij

is a finite population correction. Two-sided alpha = .05 is the criterion

for statistical significance throughout Chapters 3-5. It should be noted,

though, that an effect is significant at the .1 level if Z exceeds 1.645

and at the .01 level if Z exceeds 2.58.

3.2 Washington State data

NHTSA has acquired numerous State accident tapes to compile its

Crash Avoidance Research data file. The tapes can be equally useful for

crashworthiness analyses. Washington, however, was the only State whose

tapes were suitable for evaluating SBL: NHTSA has tapes dating back to

1973 - and old data are needed to evaluate SBL, which were installed in

1967-68. Moreover, Washington data permit differentiation between 2 and 4

door cars - the VIN 1s recorded on the file and, additionally, the data

contain codes to identify the vehicle type. Finally, police are instructed to identify every occupant Involved in a crash, not just injured

occupants. That makes it possible to compute injury rates. Injury

severity is defined according to the ABC scale used by police in most

States, as follows: A » disabling injury, B « non-disabling (evident)

injury and C - possible injury. The data also indicate if an occupant was

ejected or restrained.

Washington data for calendar years 1973-77 were analyzed. By

1977, the newest pre-SBL vehicles were 10 years old. Since those 5 years

of data provided an ample number of accident cases, there was no need to

use later years of data where the pre-SBL cars would have been even

64
older. The first step of the data reduction was to identify those cars

which were GM, Ford or Chrysler products, built within 4 years of the

company's SBL implementation date. The VINs were analyzed and compared to

the police report's assessment of model, model year, number of doors and

body style. If there was disagreement, the VIN-based assessments were

used. Drivers, right-front, left-rear and right-rear passengers were

identified and other occupant records discarded. Initially a vehicle

oriented file was created, with occupant data on all 4 of those seat

positions. That made it possible to identify if front seat occupants had

somebody sitting behind them. Subsequently, an occupant oriented file was

produced for the analyses. The 5 years of Washington data contained

records of 395,624 front or rear outboard occupants of GM, Ford or

Chrysler cars produced within 4 years of the SBL implementation date.

3.2.1 Analysis of injuries - full data set

Table 3-1, which has already been discussed to some extent in

Sections 3.1.1 and 3.1.2, does not show any benefits for SBL in reducing

fatal (K) or serious (A) injuries. Two door cars of the first year with

SBL had an injury rate of 2.59 percent, which is 14 percent lower than the

rate for 2 door cars of the last year before SBL (3.01%). But the control

group of 4 door cars experienced a 15 percent reduction between the same 2

model years. (The energy-absorbing steering column, which was introduced

in the same model year as SBL in GM and Ford cars, is partly responsible

for the large injury reduction in both 2 and 4 door cars.) The net

reduction for SBL - based on a comparison of the gross reductions for 2

door cars and the control group - is negative 1 percent and is not

65
statistically significant, since Z is only 0.19, whereas 1.96 would be

needed for significance (see Section 3.1.3).

When the data are extended to include 2 model years before and

after the SBL implementation date, the net reduction for SBL is -4

percent, which is also nonsignificant. In the ±3 year comparison, the net

effect is minus 11 percent, which is significantly different from zero,

since Z = 2.38, which is greater than 1.96. The ±4 year result is

negative 12 percent, again significant. The sequence of effectiveness

estimates (-1, -4 , -11 and -12 percent) shows a drift toward steadily

worsening results. As stated in Section 3.1.2, this is probably due to an

increasing divergence in the severities of crashes experienced by 2 and 4

door cars. If the sequence could be extrapolated back to a "+0 year

estimate" - not subject to that kind of bias - it would probably show

little effect for SBL. Note, also, that the gross injury rates in Table

3-1 never show an actual increase for 2 door cars; they just don't

decrease as rapidly as for the 4 door cars. All this seems to support the

conclusion that SBL had little or no effect.

Table 3-2 shows the corresponding results for moderate to

serious (K, A or B) injuries. Here, the results are even less favorable

for SBL. The sequence of net effectiveness results is -6, -6, -8 and -9

percent. The last three are significantly different from zero. The

highly consistent sequence with only a slight negative drift is quite

compatible with a conclusion that SBL had a negative 5 percent effect

(although it is possible that the initial -6 percent result is just due to

bad luck and the subsequent ones, bias plus bad luck).

66
TABLE 3-2

WASHINGTON STATE 1973-77: K + A + B (MODERATE/SERIOUS) INJURY RATES,

ALL OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/

After SBL NET RED.

N of Install, in n of N of Injuries per Gross FOR SBL**

Doors 2 Door Cars Injuries Persons 100 Persons Red. (%)

Z Test

4 last 1 before 2,522 22,913 11.01

first 1 after 2,206 23,301 9.47 14

2 last 1 before 3,632 28,541 12.73

first 1 after 3,804 32,533 11.69 8 -6%

Z-1.86

4 last 2 before 5,429 46,787 11.60

first 2 after 4,573 47,855 9.56 18

2 last 2 before 7,379 55,031 13.41

first 2 after 7,928 67,435 11.76 12 -6%

Z-2.56*

4 last 3 before 8,134 68,407 11.89

first 3 after 6,449 68,411 9.43 21

2 last 3 before 10,579 77,660 13.62

first 3 after 12,047 103,545 11.63 15 -87.

Z=3.67*

4 last 4 before 10,403 85,055 12.23

first 4 after 8,160 87,130 9.36 23

2 last 4 before 12,564 90,724 13.85

first 4 after 15,285 132,175 11.56 16 -9%

Z»4.78*

*Statistically significant effect (two-sided alpha • .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

67
Table 3-3 shows that SBL had little or no net effect on overall

(K + A + B + C) injury rates. The sequence of net reductions is 0, -2 , -2

and -4 percent (only the last of them is statistically significant). It

suggests a modest downward drift of about 1 percent for each additional

model year and a starting point close to zero.

3.2.2 Analysis of elections - full data set

The most disquieting results from Washington are contained in

Table 3-4. They show a consistently large negative "net effect" of SBL on

the risk of occupant ejection in a crash - at least, based on the method

defined in Section 3.1.1 for measuring net effect. The sequence of

estimates is -41, -30, -25 and -35 percent. All of the estimates are

statistically significant, even the one based on +1 model year.

Major parts of this report are devoted to checking the effect

on ejections in other data files and assessing how "real" the observed

effects might be. Section 1.6 offered an intuitive explanation why

ejection risk might have increased slightly for front seat occupants as a

result of SBL. But that could hardly explain "increases" on the order of

30 percent as shown in Table 3-4. One possibility is that something

besides SBL was done to 4 door cars but not to 2 door cars (or vice versa)

at about the same time as SBL were implemented - i.e., that 4 door cars

are not a true control group for the analysis of ejections; it will be

discussed in Section 5.3.

Another explanation is that the "model" for estimating "net"

68
TABLE 3-3

WASHINGTON STATE 1973-77: OVERALL INJURY RATES,

ALL OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Injuries

5,132

4,910

6,990

7,551

10,784

10,022

13,797

15,639

15,889

14,221

19,575

23,885

20,118

17,997

23,022

30,564

N of

Persons

22,913

23,301

28,541

32,533

46,787

47,855

55,031

67,435

68,407

68,411

77,660

103,545

85,055

87,130

90,724

132,175

Injuries per

100 Persons

22.40

21.07

24.49

23.20

23.05

20.94

25.07

23.19

23.23

20.79

25.21

23.07

23.65

20.66

25.38

23.12

Gross

Red. (%)

6

5

9

7

11

8

13

9

NET RED.

FOR SBL**

Z Test

none

Z=0.32

-TL

Z-l.ll

-TL

Z-1.67

- 4%

Z-3.55*

*Statistically significant effect (two-sided alpha » .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

69
TABLE 3-4

WASHINGTON STATE 1973-77: EJECTEES PER 1OOO OCCUPANTS,

ALL SEATING POSITIONS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N Of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install. in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Ejectees

118

82

204

225

260

172

457

471

406

238

695

678

553

297

818

841

N of

Occ.

22,913

23,301

28,541

32,533

46,787

47,855

55,031

67,435

68,407

68,411

77,660

103,545

85,055

87,130

90,724

132,175

Ejectees per

1000 Occ.

5.15

3.52

7.15

6.92

5.56

3.59

8.30

6.98

5.94

3.48

8.95

6.55

6.50

3.41

9.02

6.36

Gross

Red. (%)

32

3

35

16

41

27

48

29

NET RED.

FOR SBL**

Z Test

-41%

Z-2.01*

-30%

Z=2.23*

-257.

Z=2.27*

-35%

Z=3.42*

*Statistically significant effect (two-sided alpha « .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

70
effectiveness, as defined in Section 3.1.2, simply does not function well

in the present context. Table 3-4 reveals that the ejection risk in 2

door cars of the early post-SBL years (circa 1970) was nearly twice as

high as in 4 door cars. (Note that convertibles have already been removed

from the data and are not accounting for any of the difference.) Some of

that difference is unquestionably due to the smaller size and more severe

crashes of the 2 door cars - but certainly not all: Table B-4 in Appendix

B shows that close to a 2 to 1 ratio persists even when the data are

limited to cars of the same makes and models. Conversely, Tables 3-1 -

3-3 show that the injury rates in 2 door cars are only 10 to 25 percent

higher than in 4 door cars - reflecting the fact that 2 door cars are in

more severe crashes, but nowhere near a 2 to 1 ratio.

If 2 door cars have an intrinsically much higher ejection rate

than 4 door cars, it could upset the model, which compares the relative

risk reductions in 2 and 4 door cars. It would take a much larger

absolute risk reduction in the 2 door cars to obtain the same relative

reduction as in the 4 door cars - i.e., a zero "net effect." Table 3-4

shows that the negative "net effects" are not in any way due to an actual

gross increase in the 2 door cars. In fact, the ejection risk always

decreased in the 2 door cars. In absolute terms, the decrease was about

the same as in the 4 door cars. But in relative terms, the decrease was

much less in the 2 door cars since the base was greater.

Table 3-5 summarizes the net effects computed in Tables 3-1 -

3-4 and their statistical significance. Henceforth, only summary tables

71
TABLE 3-5

WASHINGTON STATE 1973-77: NET EFFECT OF SBL ON

INJURY RISK (BY SEVERITY LEVEL) AND EJECTION RISK

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Summary of Tables 3-1 - 3-4)

Effect on

Risk of

Net Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

±1 ±2 ±3 ±4

K + A (serious) injuries

K + A + B (moderate/serious) inj

Any injuries

Ejections

z=

z=

z=

z=

- 1

.19

- 6

1.86

none

.32

-41

2.01*

-4

.74

-6

2.56*

-2

1.11

-30

2.23*

-11

2.38*

-8

3.67*

-2

1.67

-25

2.27*

-12

2.95*

-9

4.68*

-4

3.55*

-35

3.42*

*Statistica11y significant effect (two-sided alpha = .05 - i.e., Z

greater than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

72
will accompany the text. The detailed tables showing Injury rates are

contained 1n Appendix B.

3.2.3 Analysis of Injuries - matching makes and models

Section 3.1.2 suggested that biases could be further reduced by

limiting the data to 2 and 4 door cars of the same makes and models. The

results in Table 3-5, based on the full data set, tend to show a bias

against 2 door cars which gets worse as more model years are included. So

it is worthwhile to explore techniques that might reduce bias. With

Washington data, the first step was to eliminate all vehicles whose VIN

could not be deciphered to determine the number of doors - i.e., Chrysler

products of 1965 or earlier and Buicks, Oldsmobiles and Pontiacs of 1964

or earlier. Matching post-SBL vehicles were likewise eliminated: Chryslers of 1970 or later and Buicks, Oldsmobiles and Pontiacs of 1969 or

later. An additional precaution was to drop any car with unknown VIN or

whose police-reported model, model year or body style did not match the

VIN decode. All station wagons were dropped (convertibles had already

been eliminated even from the "full" data set). Finally, all models

produced only with 2 doors or only with 4 doors during 1963-71 were

excluded: Challenger, Barracuda, Mustang, Pinto, Lincoln Mark, Cougar,

Riviera, Cadillac Limousine, Eldorado, Corvette, Camaro, Monte Carlo,

Vega, Toronado, Firebird and Grand Prix. The exclusions had the effect of

cutting the sample size from 395,624 to 154,491.

Table 3-6 presents the net effectiveness estimates for the

reduced data set, just as Table 3-5 showed them for the full data set.

73
TABLE 3-6

WASHINGTON STATE 1973-77: NET EFFECT OF SBL ON

INJURY RISK (BY SEVERITY LEVEL) AND EJECTION RISK

2 DOOR VS. 4 DOOR SEDANS AND HARDTOPS OF THE SAME MAKES AND MODELS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Excludes station wagons, convertibles, and any make/models

that were produced only with 2 doors or only with 4 doors)

• (Summary of Tables B-l - B-4)

Effect on

Risk of

Net Effect of SBL (%)*

Number of Model Years before/after

SBL Installation in 2 Door Cars

±1 ±2 ±3 ±4

K + A (serious) injuries

K + A + B (moderate/serious) inj,

Any injuries

Ejections

z=

zz=

z=

+5

.49

-2

.48

none

.00

-12

.49

-2

.32

-3

.99

-1

.55

-17

1.00

-7

1.14

-4

1.15

-1

.65

-10

.68

-9

1.55

-4

1.41

_2

1.31

-20

1.42

kNote: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group. None of the

observed net effects were statistically significant (two-sided alpha =

.05 - i.e., Z greater than 1.96).

74
(Tables B-l - B-4 in Appendix B show the actual injury rates on which

Table 3-6 is based.) Table 3-6 suggests that the exclusions were generally helpful in reducing biases, because the negative results are not as

negative as in Table 3-5. In fact, none of the net effects with the

reduced data set are statistically significant. That may, however, be due

to the smaller sample size as well as a reduction in bias.

Specifically, the sequence of net reductions for SBL on serious

(K + A) injuries is 5, -2 , -7 and -9 percent. That could be indicative of

an initial positive effect for SBL with an ever increasing bias that masks

the effect. A likelier explanation, however, is that the effect of SBL is

close to zero, the bias against SBL is relatively weak and the initial,

nonsignificant positive 5 percent effect for SBL is a coincidence. The

evidence for the latter explanation is that the other results in Table 3-6

(moderate and minor injuries and ejections) show little bias or drift

against SBL and that the injury rate 1n 2 door cars of the first year with

SBL (see Table B-l) is lower than in subsequent years, contrary to the

usual age trend and probably due to chance alone.

The sequence for moderate to serious (K + A + B) injuries is

- 2 , -3, -4 and -4 percent. It suggests that SBL had little or no effect

and that the method of limiting the data set to matching makes and models

helped to eliminate most of the biases in the data. The sequence for all

types of injuries, 0, -1 , -1 , and -2 percent, is even more supportive of

both of those conclusions.

75
3.2.4 Analysis of ejections -matching.makes and models

The limited data set's results on ejections are not too

different from those of the full data set (Section 3.2.2). Table 3-6

shows that the sequence of estimates is -12, -17, -10 and -20 percent.

Although none of these estimates is significantly different from zero and

all of them are about half as large as for the full data set, the sequence

nevertheless shows a consistent negative effect for SBL. Since the

preceding section showed that the limited data set was successful in

removing most of the year to year biases from the injury analyses, it is

unlikely that the negative effect for ejections is due to those kinds of

biases. Instead, it is necessary to consider the other possibilities

mentioned in Section 3.2.2: (1) The negative effect is at least partly

"real"; (2) Something was done to 4 door cars in 1967-68 that was not done

to 2 door cars; (3) The model for computing effectiveness, based on the

relative differences in the gross improvements for 2 and 4 door cars, is

not suitable for studying ejection rates.

The limited data set shows clearly that the risk of ejection is

intrinsically higher in 2 door cars than in 4 door cars, at least in cars

of the 1963-71 era. Tables B-l, B-2 and B-3 show that injury rates in 2

door cars are only 10-15 percent higher than in 4 door cars of the same

makes and models (presumably reflecting the fact that the 2 door cars are

driven a little harder and have slightly more severe crashes, even for the

same makes and models). But Table B-4 shows that 2 door cars have 60-90

percent higher ejection rates than 4 door cars of the same makes and

models, a disproportionate increase in view of the small differences in

76
the injury rates. Table B-4 also shows that the 2 door cars with SBL had

consistently lower ejection rates than the 2 door cars without SBL and

that the absolute risk reductions for the 2 door cars was as large or

larger than the reductions in the 4 door cars. But in relative terms, the

risk reductions in the 4 door cars were always larger than in the 2 door

cars.

The Washington results on ejections will be compared to those

in fatal accidents (Sections 4.3, 4.4 and 4.8.2) and in NHTSA's detailed

data files (Sections 5.2 and 5.3). The report's conclusions on ejection

are presented in Section 6.3.

3.2.5 Effect of SBL on injuries by seat position - full data set

The discussion of the sled tests (Section 2.2) identified three

groups of persons with fundamentally different collision kinematics and

potential SBL effects: front seat occupants with nobody sitting behind

them, front seat occupants with somebody sitting behind them and back seat

passengers. The Washington data set contains enough cases for separate

analyses of all three groups. It is necessary, however, to use the full

data set; the limited set of matching makes and models would be too sparse

for the latter two groups of persons. The full data set, as found in

Section 3.2.1, can cause biases and it is necessary to inspect the

sequence of estimates for ±1, +2, +3 and +4 model years to distinguish

between possible real effects and biases.

The results for front seat occupants with nobody sitting behind

77
TABLE 3-7

WASHINGTON STATE 1973-77: NET EFFECT OF SBl ON

INJURY RISK (BY SEVERITY LEVEL)

- BY SEAT POSITION -

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Summary of Tables B-5 - B-13)

Net Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

Effect on

Risk of ±1 ±2 ±3 ±4

FRONT SEAT OCCUPANTS WITH NOBODY SITTING BEHIND THEM

K +

K +

Any

A

A

i

(serious) injuries

+ B (moderate/serious) inj.

njuries

Zz*

none

.00

-5

1.34

+1

.35

-6

.96

-9

3.11*

-2

1.26

-11

2.26*

_9

4.09*

-3

1.95

-13

2.95*

-11

5.50*

-5

3.94*

FRONT SEAT OCCUPANTS WITH SOMEBODY SITTING BEHIND THEM

K +

K +

Any

A

A

i

(serious) injuries

+ B (moderate/serious) inj.

njuries

Z=

U

Z=

-23

.79

-7

.48

-10

1.03

-10

.55

+6

.63

-4

.62

-15

.94

+2

.29

-2

.33

-13

.88

+2

.35

-2

.50

*Statistically significant effect (two-sided alpha « .05 - i.e., Z

greater than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

78
TABLE 3-7 (concluded)

Net Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

Effect on

Risk of ±1 ±2 ±3 ±4

BACK SEAT OCCUPANTS

K + A (serious) injuries

K + A + B (moderate/serious) inj.

Any injuries

*Statistically significant effect (two-sided alpha - .05 - i.e., Z

greater than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

z=

zu

-7

.24

-24

1.57

-12

1.28

+11

.60

-3

.33

+1

.14

-16

.93

—9

1.09

none

.07

-14

.91

-4

.62

-2

.40

79
them (explored in detail in Tables B-5, B-6 and B-7) ought to be similar

to those for all occupants, because 1n the Washington accident files, 80

percent of all occupants are in the front seat and have nobody behind

them. The top third of Table 3-7 shows that the observed "effects" for

SBL are almost all unfavorable. The sequence of estimates for serious

injuries is 0, -6, -11 and -13 percent. The last two numbers are statistically significant. The sequence appears to indicate that the true

effect of SBL is close to zero and that the data contain a bias against

SBL that grows by about 3 percent per year. The estimates for moderate to

serious injuries are -5, -9, -9 and -11 percent; the last 3 numbers are

statistically significant. This sequence suggests a slightly smaller bias

against SBL and a zero or slightly negative "true" effect for SBL. The

sequence for overall Injury rates, 1, -2, -3, and -5 percent, indicates

that SBL had little or no effect.

The data set of front seat occupants with somebody sitting

behind them (documented in detail in tables B-8, B-9 and B-10) contains

only a tenth as many cases as the preceding ones and is subject to more

sampling error. In particular, none of the observed effects is significantly different from zero. Nevertheless, the sequences shown in the

middle portion of Table 3-7 are rather consistent. The sequence for

serious injuries is consistently negative for SBL with little apparent

year to year bias: -23, -10, -15 and -13 percent. The sequences for

moderate injuries (-7, 6, 2 and 2 percent) and overall injury risk (-10,

- 4, -2 and -2 percent) appear to converge to a zero effect for SBL. The

Washington data are consistent with the sled test results (Table 2-7),

80
which showed no SBL benefits for dummies in the front seat with a dummy

positioned behind them. It is noteworthy that the reported injury rates

for front seat occupants with a person behind them (Tables B-8 through

B-10) are 25 to 100 percent higher than those for front seat occupants

with nobody behind them. That may be largely due to underreporting of

uninjured back seat occupants in minor crashes in Washington. But it may

also partly reflect the risk of injuries to front seat occupants as a

result of direct or indirect collisions with unrestrained back seat

occupants - phenomena often seen in the sled tests.

The results for back seat passengers (see the lower section of

Table 3-7 as well as Tables B-ll, B-12 and B-13) are based on about the

same number of cases as the preceding ones. The sequence of observed

effects on serious injuries was -7, 11, -16 and -14 percent. For moderate

to serious injuries it was -24, -3, -9 and -4 percent. The sequence for

overall injury risk was -12, 1, 0 and -2 percent. Those trends suggest

that the effect of SBL was zero, at best. They diminish the faint hopes,

based on the sled tests, that SBL may have been of some utility, at least,

for the back seat occupant.

3.2.6 Effect on injuries of restrained occupants - full data set

Belt usage has always been high in Washington [18]. There are

enough (63,640) restrained occupants in the full data set for an analysis

of the effects of SBL. The analysis is needed especially because the sled

test study was limited to unrestrained occupants.

81
Table 3-8 (which, in turn,; is based on the detailed Tables

B-14, B-15 and B-16) is not favorable for SBL. The sequence of effects on

serious (K + A) injuries is -16, -6, -25 and -21 percent. None of the

effects was statistically significant: since belted occupants have lower

injury rates than others, it takes a larger sample and/or effect to

achieve significance. The sequence does not indicate any obvious year to

year biases and a zero or negative effect for SBL. The sequence for

moderate to serious injuries is -19, -11, -11 and -8 percent. If there is

any bias here, it would seem to be in favor of SBL. Although the effects

are not significant at the .05 level, they come "close" to significance (Z

as high as 1.91) and the consistent trend seems to be negative for SBL.

The four estimates for overall injury risk, 2, 0, -1 and -1 percent,

indicate that SBL had little or no effect.

3.2.7 When children ride in the back seat

Intuitively, one situation where SBL might be especially

effective is when the back seat is occupied by a child, e.g., up to 12

years old. In a frontal crash at moderate speed, the SBL may withstand

the relatively light impact load of a child (see Table 5-4) and the

seatback may retain the child within the safer rear half of the passenger

compartment, whereas in a car without SBL, the child might be propelled

over the folded seatback and contact the windshield, header, instrument

panel or a front seat occupant. Table 3-9 presents Washington results on

children up to age 12 who rode in the back seat (top half) as well as the

persons who were sitting in front of those children (lower half). It is

not limited to frontal crashes but includes all crash modes, since impact

82
TABLE 3-8

WASHINGTON STATE 1973-77: NET EFFECT OF SBL ON

INJURY RISK (BY SEVERITY LEVEL)

- BELTED OCCUPANTS -

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Summary of Tables B-14 - B-16)

Effect on

Risk of

Net Effect of SBL (%)*

Number of Model Years before/after

SBL Installation 1n 2 Door Cars

±1 ±2 ±3 ±4

K + A (serious) injuries

K + A + B (moderate/serious) 1nj. -19

Any injuries

z=

z»

z -

-16

.60

-19

1.74

+2

.29

-6

.31

-11

1.47

none

.16

-25

1.59

-11

1.91

-1

.41

-21

1.30

-8

1.57

-1

.18

*Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group. None of the

observed net effects were statistically significant (two-sided alpha =

.05 - i.e., Z greater than 1.96).

83
TABLE 3-9

WASHINGTON STATE 1973-77: NET EFFECT OF SBL ON

NONEJECTED OCCUPANTS' INJURY RISK (BY SEVERITY LEVEL)

WHEN UNRESTRAINED CHILDREN* ARE IN THE BACK SEAT

- BY SEAT POSITION -

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

Net Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

Effect on

Risk of ±1 ±2 ±3 ±4

NONEOECTED UNRESTRAINED CHILDREN* IN THE BACK SEAT

K +

K +

Any

A

A

i

(serious) injuries

+ B (moderate/serious) inj.

njuries

Z=

U

U

+2

.03

+38

1.57

.42

-145

1.64

-3

.12

-17

1.12

-108

1.62

none

.00

-11

.90

-71

1.28

+7

.44

-17

1.43

NONEOECTED FRONT SEAT OCCUPANTS WITH AN

UNRESTRAINED CHILD* SITTING BEHIND THEM

K +

K +

Any

A (serious) injuries

A + B (moderate/serious) inj.

injuries

Z»

Z~

Z-l

-65

.79

-6

.20

-38

1.60

-234

2.74*

-23

.97

-32

1.95

-234

3.19*

-10

.59

-24

1.83

-162

2.76*

-11

.66

-27

2.28*

•Statistically significant effect (two-sided alpha = .05 - i.e., Z

greater than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

Children up to (and including) 12 years old

84
location is not coded in Washington, but ejectees are excluded so as to

eliminate a situation where results are known to be unfavorable for SBL.

The sample sizes for Table 3-9 are about 1/4 as large as those for back

seat occupants of all ages (Table 3-7), almost precluding any hope for

statistically meaningful results.

The numbers in Table 3-9 do not indicate effectiveness for

SBL. Eight of the 12 results for children riding in the back seat are

negative, although none are statistically significant even at the .1

level. All 12 of the effectiveness estimates for the persons sitting in

front of those children are negative (lower half of Table 3-9) and 4 of

them are statistically ,significant. Such results are unrealistic and

typically are obtained when data are cut too thin. At any rate, though,

there is nothing in Table 3-9 to support the hypothesis that SBL are

especially useful when children ride in the back seat.

3.3 Texas data on drivers' Injury risk

Automated Texas accident files for the years 1972-74 are

available at NHTSA and were used in the evaluations of head restraints

[24], side door beams [26], braking improvements [29], side marker lamps

[25], and windshield improvements [27]. (Texas data were also available

for 1977 and later calendar years but were not used because pre-1966 cars

did not have their model years identified.) Texas files do not list

VINs. On the other hand, they identify "body styles," such as 2 door

sedan, coupe or hardtop; 4 door sedan or hardtop. While these classifications cannot be checked against VINs as in Washington, their distributions

85
by make and model appeared to be correct. A major drawback of Texas data

is that uninjured passengers are usually not listed on the police report -

only drivers. Injury rates cannot be computed for passengers, so the

analyses had to be limited to drivers. For the same reason, it is

impossible to ascertain if there was anybody sitting behind the driver -

thus, drivers with a person sitting behind them could not be separately

analyzed. The data do not routinely indicate if a driver was ejected or

restrained. A compensating advantage of the Texas data is that vehicle

damage is described by the TAD system [44], making it possible to

distinguish frontal from other crashes with reasonable confidence. Texas

uses basically the same ABC scale for coding injuries as Washington.

The first step of the data reduction was to identify GM, Ford

and Chrysler cars built within 4 years of the company's SBL implementation

date. Since there are plenty of Texas data, station wagons as well as

convertibles were excluded and the analysis limited to sedans, hardtops

and coupes. A record was created for each driver, indicating the vehicle's model, model year, number of doors and damage location (frontal or

other) and the driver's injury rating. The 3 years of Texas data produced

a file containing 830,656 drivers, 346,540 of whom were known to have been

in frontal crashes. The sample, then, appears at first glance to be twice

as large as the 1973-77 Washington file. Texas, however, has a lower

reporting threshold and, as a result, a lower injury rate. The number of

injuries on the file (a more appropriate measure of effective sample size)

is about the same for Texas (3 years - drivers only) and Washington (5

years - including passengers).

86
3.3.1 AJJ__cjra_s.he_i

Table 3-10 (which, in turn, is based on the detailed tabulations B-17 - B-22) indicates a perplexing trend at all severity levels.

The results for the first model year after SBL installation, relative to

the last year before SBL installation, are always favorable to SBL and, in

the case of serious injuries, the 12 percent net reduction for SBL is

statistically significant, although "just barely" so, at the .05 level (Z

- 2.02). It is not significant at the .01 level. When one more model

year is added to the analysis, the net effect drops to nothing and in the

+3 and +4 year comparisons, the results are negative. Specifically, for

serious injuries, the sequence of effectiveness estimates is 12, 0, -4 and

-5 percent. For moderate to serious injuries, it is 6, -1 , -6 and -7

percent (the last two estimates are significantly different from zero).

The net effects on overall injury risk are 3, 0, -3 and a significant ~4

percent. These sequences are ambiguous, especially considering the higher

sampling error of the estimates based on fewer model years. One possibility is that the effect of SBL is positive, strongly so for serious

injuries, and that there is a strong year to year bias working against

SBL. A much likelier explanation is that the results for ±1 model year

are distorted by sampling error (even to the point of spurious significance in the case of serious injuries), the net effect of SBL is close to

zero and there is a slight year to year bias against SBL, resulting in

modestly negative estimates in the +3 and ±4 model year comparisons. The

best evidence for the second explanation is that the sequences become

stable after the ±2 model year comparison and especially after the ±3

model year comparison, showing little year to year drift. Most of the

87
TABLE 3-10

TEXAS 1972-74: NET EFFECT OF SBL ON

DRIVERS' INJURY RISK (BY SEVERITY LEVEL)

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Summary of Tables B-17 - B-22)

Net Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

Effect on

Risk of ±1 ±2 ±3 ±4

DRIVERS IN ALL CRASHES

K +

K +

Any

A

A

1

(serious) injuries

+ B (moderate/serious) inj.

njuries

Z=

Z=

Z=

+12

2.02*

+6

1.83

+3

1.29

none

.08

-1

.43

none

.01

-4

1.11

-6

3.08*

-3

1.91

-5

1.35

-7

4.15*

-4

2.86*

DRIVERS IN FRONTAL CRASHES

K +

K +

Any

A

A

i

(serious) injuries

+ B (moderate/serious) inj.

njuries

ZZu

+16

2.03*

+8

1.83

+8

2.16*

+6

1.00

+3

.98

+3

1.32

+2

.48

-2

.33

+1

.59

+1

.51

-2

1.08

none

.18

*Statistically significant effect (two-sided alpha - .05 - i.e., Z

greater than 1.96)

^Note: positive numbers indicate that the SBL group (2 door.cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

88
movement is between the ±1 and +2 year results, indicating that the ±1

year result is "off" due to sampling error and the additional model year

"evened things up."

3.3.2 Frontal crashes

The pattern for frontal crashes, shown in the lower half of

Table 3-10, is nearly Identical to the one for all crashes, although

slightly more favorable to SBL. The sequence for serious injuries is

uniformly positive: 16, 6, 2 and 1 percent. The first of those estimates

is "barely" significant at the .05 level (Z = 2.03) but not at the .01

level; the remainder are not significant even at the .1 level. For

moderate injuries, the sequence is 8, 3, -2 and -2 percent (none of them

significant). The observed effects of SBL on overall injury risk in

frontal crashes are 8, 3, 1 and 0 percent, only the first of which is

significant at the .05 level, although not at the .01 level. As above,

the best explanation would appear to be that the first year's results are

off due to sampling error, the effect of SBL is small or zero and there is

a slight year to year bias against SBL. The most optimistic note is that

the results for frontal crashes are uniformly a bit better than for all

crashes. That might be construed as a slight positive effect in frontal

crashes, although, to be sure, there is no statistically significant basis

for saying so.

3.4 New York State data - effect of SBL bv iniurv type

Automated New York State accident files have the exceptional

advantage of using the New York State Injury Coding System (NYSICS), which

89
describes the body region, type and severity of a victim's principal

injury. The data were used to great advantage in NHTSA's evaluation of

windshield improvements [27], where they appropriately identified what

types of injuries were mitigated by High Penetration Resistant windshields. Only the 1974 and 1977 files were available for access at

NHTSA. The latter was not used because it contains too few pre-SBL cars

(the lifespan of the average auto is short because of the New York

climate). For the evaluation of SBL, injury rates are computed for each

specific injury type - e.g., the percentage of all occupants who have a

fractured leg. Each of these specific injury rates is then subjected to

the same analyses as were applied to the K + A, K + A + B, and overall

injury rates in Washington and Texas.

The data reduction for New York was almost the same as for

Washington. New York codes the vehicle make and body style (2 or 4 doors)

but not the model; however, the VIN is coded as well. As in Washington,

GM, Ford and Chrysler cars built within 4 years of the SBL implementation

date were selected. The number of doors was based on the VIN, when known

and decodable, and on the police report, otherwise. Station wagons were

included but convertibles excluded. An occupant oriented file was

created, containing a record for each driver, right-front, left-rear or

right-rear passenger. The file, based on a single year of New York data,

contained 142,862 records.

Table 3-11 shows the net effect of SBL on each of the 19 most

common specific injury types in New York. The detailed data on the

90
numbers of Injuries of each type and the injury rates is presented in

Table B-23 for the ±1 model year comparison, Table B-24 for +2 model

years, Table B-25 for +3 model years and Table B-26 for +4 model years.

The injuries are listed by frequency of occurrence, with the most common -

minor head or facial bleeding - listed first. Five more severe injury

types, which would usually be rated 2 or higher on the Abbreviated Injury

Scale (AIS) [1] are denoted by capital letters.

Table 3-11 suggests that SBL might have increased the risk of

injuries with minor head or facial bleeding. The sequence of

effectiveness estimates, -9, -5, -9, and -9 percent shows no drift or

bias in either direction. The last two numbers in the sequence are

statistically significant. The results for injuries involving severe head

or facial bleeding are also unfavorable, with a sequence of -10, -11, -12

and -17 percent. None of those estimates is statistically significant,

however, and a slight year to year drift against SBL may have made the

last number worse than it really is. The results for concussions are

likewise bad: -53, -20, -14 and -16 percent. In fact, for every type of

head injury, the numbers are consistently negative. The sled test results

(Table 2-5) showed a slight, although nonsignificant tendency toward lower

head injury with SBL. The New York data essentially end that hope.

Table 3-11 shows little net effect on injuries to other body

regions. The sequence for neck or back pain is uniformly negative: -18,

-13, -6 and -7 percent. The ±2 year estimate is "barely" significant at

the .05 level (Z « 2.01). The smaller, nonsignificant ±3 and +4 year

91
TABLE 3-11

NEW YORK STATE 1974: NET EFFECT OF SBL ON INJURY RISK

- BY INJURY TYPE -

(Ordered by frequency of occurrence - most common first.

Capital letters denote the more serious injury types.)

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Summary of Tables B-23 - B-26)

Net Effect of SBL (7.)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

Body

Region

Head

Neck/back

Head

Leg

Head

Arm

Torso

HEAD

Injury

Type

minor bleed.

pain

pain

pain

contusion

pain

pain

SEVERE BLEED.

ZZ=

Z=

Z=

ZZ=

Z=

Z=

±1

-9

1.23

-18

1.90

-3

.25

+1

.18

+10

.74

+9

.60

+1

.05

-10

.60

±2

-5

.89

-13

2.01*

+2

.24

+2

.26

-12

1.16

+11

1.04

+2

.13

-11

.92

±3

-9

1.98*

-6

1.03

-2

.37

+2

.28

-6

.70

+2

.25

+3

,33

-12

1.14

±4

-9

2.21*

-7

1.43

-7

1.20

+5

.64

-10

1.23

+4

.51

+4

.50

-17

1.82

""Statistically significant effect (two-sided alpha = .05 - i.e., Z greater

than 1.96)

••Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

92
TABLE 3-11 (concluded)

Body

Region

Leg

Arm

Injury

Type

contusion

minor bleed

Head abrasion

HEAD CONCUSSION

Leg minor bleed.

All over pain

Arm

Leg

LEG

ARM

TORSO

contusion

abrasion

FRACTURE

FRACTURE

INTERNAL

Net Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

+ 1 ±2 ±3 ±4

zzu

zzzzzzzz-

-12

.52

-24

.90

-3

.12

-53

1.63

+ 1

.03

+52

2.49*

-13

.36

-19

.52

+ 14

.45

+27

.78

-24

.52

-16

.91

-22

1.18

-7

.41

-20

1.01

none

.02

+12

.63

-24

.92

-16

.63

-8

.31

+ 18

.73

+4

.14

-9

.62

-24

1.52

-20

1.24

-14

.85

-2

.11

-3

.16

-31

1.37

-21

.94

+11

.56

+20

.93

+6

.22

-6

.52

-17

1.21

-13

.90

-16

1.03

-10

.62

none

.01

-24

1.23

-10

.51

+1

.01

+ 10

.45

+ 14

.69

*Statistically significant effect (two-sided alpha « .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

93
TABLE 3-12

NEW YORK STATE 1974: NET EFFECT OF SBL ON INJURY RISK,

NONEJECTED OCCUPANTS IN FRONTAL IMPACTS

- BY INJURY TYPE -

(Ordered by frequency of occurrence - most common first.

Capital letters denote the more serious injury types.)

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

Net Effect of SBL (X)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

Body

Region

Head

Head

Neck/back

Leg

Head

HEAD

Torso

Arm

Injury

Type

minor bleed.

pain

pain

pain

contusion

SEVERE BLEED.

pain

pain

ZZ«

Z=

Zzzz»

z-

±1

-8

.96

-18

1.35

-21

1.19

-2

.14

_1

.05

+5

.29

+10

.51

-5

.26

+2

none

.01

-3

.33

• -20

1.66

+6

.57

-13

1.03

+ 1

.07

+13

.98

+9

.69

±3

-4

.85

-6

.81

-8

.79

+8

.87

-9

.88

-3

.23

+12

1.06

-1

.07

+4

-6

1.26

-9

1.22

-11

1.24

+ 11

1.33

-15

1.64

-9

.86

+13

1.29

+2

.15

*Statistically significant effect (two-sided alpha

than 1.96)

.05 - i.e., Z greater

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

94
TABLE 3-12 (concluded)

Body

Region

Injury

Type

Net Effect of SBL (*)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

±1 ±2 ±3 ±4

Leg contusion

Head abrasion

HEAD CONCUSSION

Arm minor bleed,

Leg minor bleed.

All over pain

Leg

Arm

LEG

HEAD

TORSO

ARM

abrasion

contusion

FRACTURE

FRACTURE

INTERNAL

FRACTURE

z=

u

z«

z=

z*

zu

z.

zzz«

z-

-56

1.70

-10

.33

-16

.46

none

.01

+17

.56

+65

2.82*

-60

1.19

+22

.60

+25

.74

+10

.23

-10

.19

-6

.12

-38

1.71

-13

.62

-15

.64

-14

.57

+9

.43

-7

.25

-27

.88

+17

.60

-6

.22

+7

.21

+ 10

.32

+ 1

.02

-31

1.73

-22

1.24

.13

-24

1.14

+12

.67

-23

.87

-36

1.29

-1

.02

-7

.30

+ 16

.62

+14

.52

+3

.09

-22

1.38

-12

.75

-6

.32

-20

1.09

-2

.13

-22

.89

-19

.83

-4

.16

-20

.81

+16

.65

+24

.99

-5

.18

*Statistically significant effect (two-sided alpha = .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater Improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

95
estimates, however, suggest that the significance of the ±2 year number

may be spurious. The only injury type with consistently good results is

arm fractures, with 27, 18, 20 and 10 percent reductions. None of them

are even close to statistical significance (Z is always less than 1). The

mildly positive results on arm pain (a less severe form of blunt arm

trauma) support the result on the fractures, but the results on arm

contusions are consistently negative. The New York data show a slightly

positive, although certainly nonsignificant effect on internal torso

injuries, but little or no effect on torso "pain." Effects on leg

Injuries are close to zero.

Table 3-12 is identical to Table 3-11 except that it is limited

to frontal impacts and occupants who were not ejected - i.e., the crash

mode where SBL might be expected to have the most favorable results. The

effectiveness estimates differ little, however, from those in Table 3-11.

For example, 1n the +1 year comparison column of Table 3-11, there are 8

positive and 11 negative results, whereas there are 7 positives and 12

negatives in the corresponding column of Table 3-12. Similarly, there are

6 positive results in the +4 year column of Table 3-11 and 5 in the same

column of Table 3-12. The only finding that offers any encouragement is

that 7 of the 8 results for torso injuries are positive; none of them,

however, are statistically significant even at the .1 level. Moreover,

the positive results on torso injuries might be a consequence of the

negative findings on almost all other injury types: since New York police

record only one Injury per person, an increase of more visible wounds in

other body regions means fewer opportunities to record torso injuries.

96
CHAPTER 4

ANALYSES OF THE FATAL ACCIDENT REPORTING SYSTEM

The Fatal Accident Reporting System (FARS) is a census of the

nation's fatal accidents since 1975. By the end of 1985, FARS contained

records of 36,993 fatalities who were occupants of passenger cars built

within 4 years before or 4 years after the seat back lock (SBL) implementation date. Analyses of the FARS data did not show SBL effective in

reducing fatality risk. Just as in nonfatal accident data from Washington

State, the risk of ejection was always higher in 2 door cars than in 4

door cars, but significantly more so in the years after the implementation

of SBL.

4.1 Analysis method

The analysis of State accident data (Section 3.1) consisted of

calculating the change in injury rates in 2 door cars, before vs. after

SBL, and comparing it to the corresponding change in the control group of

4 door cars (see Section 3.1.1). The analysis was initially limited to

cars of the last model year before SBL was implemented vs. the first year

after SBL implementation. Subsequently, it was extended to 2 model years

before/after SBL, then ±3 model years, then ±4 (see Section 3.1.2). The

same procedure is used for analyzing FARS, with one important difference.

The State data contained records of all crash involved occupants and it

was possible to compute injury rates directly - i.e., the number of

injured persons divided by the number of crash involved persons (injured

plus uninjured). FARS does not contain records of all crash involved

97
persons, only those who were in fatal crashes. Fatality rates cannot be

computed per 100 crash involved persons. Instead, the rates must be

formulated as the number of fatalities per some unit of exposure. In this

chapter, fatality rates are computed per million vehicle years, as

described in Section 4.1.2.

4.1.1 Fatality data

FARS includes VINs for crash involved vehicles. For those

make, model and model year combinations where the VIN contains information

on the number of doors, this information was used in the analysis. FARS

also contains a separate variable explicitly indicating the number of

doors. Unfortunately, this variable is useless because cars whose VIN

does not contain information about the number of doors are almost always

coded as "2 door cars." Thus, the analysis had to be limited to cars with

known and decodable VIN's. That limited the file to GM, Ford and Chrysler

products. Furthermore, all 1963-64 Buicks, Oldsmobiles and Pontiacs and

all 1964-65 Chrysler products were excluded, because the VINs could not be

decoded to indicate the number of doors. It was decided to exclude all

1969-70 Buicks, Oldsmobiles and Pontiacs and 1970-71 Chrysler products,

since these cars produced 3-4 years after the SBL implementation date

would have had no counterparts on the file, produced 3-4 years before

SBL. All convertibles were excluded to avoid biases, as explained in

Section 3.1.2. So were subcompacts (Pinto and Vega), which had no

counterpart in the pre-SBL years. The file was limited to drivers and to

passengers in the right front, left rear and right rear seat positions.

This was the "full" FARS data set used for most of the analyses. It

98
contained records of 36,993 fatalities as well as 11,068 occupants who

were ejected from their vehicle (but not necessarily killed).

Some analyses were performed with a "reduced" data set from

which station wagons and all make/models which were produced only with 2

doors or only with 4 doors had been eliminated. The detailed definitions

for the reduced data set are identical to those used with Washington

data. The reduced data set contained 28,618 fatalities. The data losses

between the full and the reduced sets are less severe than in Washington

since, as described above, many cases were already excluded from the

"full" FARS data set.

4.1.2 Exposure data

The unit of exposure used as the denominator in the calculation

of fatality rates is a million vehicle years. The number of vehicle years

exposure accumulated by a particular make/model/model year/n of doors

combination is the number of vehicles of that type registered in 1975,

plus the number registered in 1976, etc., up to and including the number

registered in 1985. Ideally, this number could be obtained directly from

R. L. Polk registration files, which show the number of cars of a given

type and model year that are registered in a given calendar year [13].

The Polk files, unfortunately, do not provide detailed make/model data for

model years earlier than 1966. In any given calendar year, they do not

provide detailed data for cars more than 10 years old. For this study,

which requires exposure data for cars as old as model year 1963 during

calendar years as recent as 1985, an imputation procedure comprising 5

steps was devised.

99
££e&.J: Extraction of the actual Polk registration data for

calendar year 1975, for cars of the last model year before SBL (1966 for

GM and 1967 for Ford and Chrysler) and the first model year after SBL

implementation (1967 GM and 1968 Ford and Chrysler), using automated files

available at NHTSA. Convertibles, limousines and pickup-based passenger

cars were dropped. All others were classified as 2 door cars (hardtops,

coupes and sedans), 4 door cars (hardtops and sedans) or 4 door station

wagons. These constituted the "full" data set. Make/models produced only

with 2 doors or only with 4 doors, such as Mustang and Camaro, as well as

station wagons were identified and excluded from a "reduced" data set.

The full and reduced data sets correspond to those defined for FARS data

in Section 4.1.1 and indicate the exposure accumulated during 1975 by cars

produced within 1 year of the SBL implementation date.

Step 2: Estimation of 1975-85 exposure of cars built within 1

year of the SBL implementation date. Step 1 indicated exactly how many

cars of each make/model/body style were on the road in 1975. This step

estimates how many remained on the road in each subsequent year. "MVMA

Motor Vehicle Facts & Figures '82" specifies how many cars of each model

year were still on the road in any calendar year up to 15 years later

[37]. The proportions of cars still in service were averaged across model

years to obtain survival rates up to age 15. Between ages 10 and 15,

nearly 20 percent of the cars still remaining on the road are retired each

year. Survival rates for ages 16-22 are estimated on the assumption that

the 20 percent annual decay rate persists beyond age 15. The survival

rates for cars aged 2-22 are:

100
Age 1 Surviving

2

3

4

5

6

7

8

99

98

95

92

89

83

75

ge

9

10

11

12

13

14

15

1 Surviving

65

55

44

35

28

22

17

Age

16

17

18

19

20

21

22

t Surviving

13

10.5

8

6.5

5

4

3

During 1975, for example, 1966 GM cars were 9 years old and an estimated

65 percent of the original supply were still on the road. During 1976,

only 55 percent of the original supply were on the road. Thus, the 1966

GM cars accumulated 55/65 as many exposure years during 1976 as they did

in 1975. They accumulated (65+55+44+35+28+22+17+13+10.5+8+6.5)/65 - 4.68

times as many exposure years during 1975-85 as they did during 1975

alone. A similar multiplication factor is developed for each of the other

model years:

Exposure Years Accumulated 1n 1975-85

Model Year Exposure Years Accumulated in 1975

1963 4.34

1964 4.39

1965 4.44

1966 4.68

1967 4.97

1968 5.39

1969 5.91

1970 6.58

1971 7.19

The Polk registration figures for calendar year 1975 are multiplied by the

appropriate multiplication factor to estimate the registration years

accumulated during 1975-85. For example, if 100 vehicles of a particular

1968 car were still on the road in 1975, these 100 vehicles would accumulate approximately 539 exposure years during 1975-85. Finally, the Polk

101
data, weighted by the appropriate multiplication factors, are tabulated to

obtain the following exposure estimates for both the full and reduced data

sets:

N of

Doors

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

N of Exposure Years

Accumulated in 1975-85

N l 4 0

N l4 1

N12O

N19T

Step 3: Polk data are not generally available for cars produced

more than one model year before the SBL implementation date. Exposure

data for those model years, as well as the corresponding model years more

than one year after SBL, are imputed from 1973-77 Washington State

accident involvements, adjusted in Steps 4 and 5 to estimate 1975-85

nationwide vehicle years. The starting point is the "full" file of front

and rear outboard occupants of GM, Ford and Chrysler cars, produced within

4 years of the SBL implementation date, involved in crashes during 1973-77

in Washington State (see Section 3.2). The underlying premise is that the

number of accident involved occupants (including nonfatal and noninjury

crashes), for a particular type of vehicle, is a measure of exposure of

that type of vehicle and, with appropriate adjustment factors, "accident

involvements" can be used as a surrogate for "vehicle years."

The first task is to subset the "full" Washington file so that

102
it corresponds exactly to the definition of the "full" FARS and Polk

files. Any case with unknown VIN or for which the VIN was inconsistent

with the police report's codes for make, model or model year was dropped

from the Washington data. So were Buicks, Oldsmobiles, Pontiacs and

Chrysler Corp. products produced more than 2 years after the SBL implementation date as well as all subcompacts. The "reduced" Washington file

already corresponds exactly to the "reduced" FARS and Polk files.

S_teji_4: The Washington accident involvements for calendar years

1973-77 (median year 1975) are weighted by the multiplication factors

defined in Step 2 (e.g., 4.34 for MY 1963 cars) to estimate the number of

accident involvements these cars would have accumulated during 1975-85.

For example, if 100 occupants of a particular 1968 make/model were in

accidents during 1975, approximately 539 would have been in crashes during

1975-85. The data, weighted by the appropriate multiplication factors,

are tabulated to obtain the following Washington State "exposure" estimates for both the full and reduced data sets:

N of

Doors

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last j before

first j after

last j before

first j after

Washington

Exposure Units

Accumulated in 1975-85

WJ40

NJ41

WJ20

H j 2 1

103
Ste.P__5: The Washington "exposure" estimates defined in Step 4

are not good national estimates, primarily because Washington had a higher

proportion of 2 door cars than the national average, year after year. The

Washington accident counts Wj k i for the jth year before/after SBL

implementation, however, can be converted into national estimates of

vehicle exposure years N'j^. as follows:

where Nlk- is the Polk estimate defined in Step 2. Note that

N'lk. -Nl kikikik i

In other words, use the Polk estimates for the +1 model year exposure

data, where they are known. For the +2, ±3 and +4 year estimates,

multiply the ±1 year Polk number by the relative growth in the Washington

accident counts ^j^i/WK,) .

The exposure estimates as defined in Steps 1-5 will undoubtedly

be inaccurate for some makes and models. For example, models that have

greater than average longevity will have their exposure underestimated,

the more so the older they get, because the procedure assumes the same

survival rate for all models. Nevertheless, biases of this source would

not tend to be large between one model year and the next one, but rather

to increase gradually as a wider range of model years is added to the

analysis. For example, if 4 door cars have higher survival rates than 2

door cars, the fatality rate for 4 door cars, based on the exposure

estimates defined here, would be slightly higher in the last year without

SBL than in the first year with SBL; the rate for 2 door cars would be

slightly lower; the net result would be a slight bias against SBL. But if

104
the analysis is expanded to include cars up to 4 years before and after

the SBL implementation date, the bias would be much larger. Thus, the

technique of obtaining a sequence of 4 estimates - for the ±1, +2, ±3, and

+4 model year analyses - is especially important in this chapter, since

any biases in the exposure data should manifest themselves as a gradual

drift in the sequence of effectiveness estimates.

4.1.3 Statistical significance testing

The procedure for testing the statistical significance of FARS

results is nearly the same as was defined for State data in Section

3.1.3. The only differences are that the first equation in Section 3.1.3

is replaced by:

(e2 1/N'J2 1)/(e2 0/N'J2 0) - (e^/N'j^me^/N'j^ )

where N'j^, are the estimates of exposure defined in Section 4.1.2 and

that the finite population correction (fpc) is always 1 - i.e., there is

no fpc since the fatality count is a Poisson, not a binomial variable.

The observed net effect of SBL will be termed "statistically significant

with two-sided alpha - .05" whenever

Z - X'5

is greater than 1.96. Two-sided alpha - .05 is the criterion for statistical significance throughout Chapters 3-5. It should be noted, though,

that an effect is significant at the .1 level if Z exceeds 1.645 and at

the .01 level if Z exceeds 2.58.

4.2 Effect on fatalities

Table 4-1 shows that SBL did not have a beneficial effect on

105
TABLE 4-1

FARS 1975-85: FATALITY RATES PER MILLION VEHICLE YEARS,

ALL OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N Of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install. In

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Deaths

' 2463

2586

2863

3827

4232

. 5738

4963

8679

4845

7436

5824

12550

5286

9548

6200

15959

Millions

of Car

Years

12.632

14.688

10.858

13.942

24.934

32.552

20.198

31.280

30.866

40.356

25.300

41.782

36.013

49.249

28.141

50.068

Deaths per

Million Yrs.

195.0

176.1

263.7

274.5

169.7

176.3

245.7

277.5

157.0

184.3

230.2

300.4

146.8

193.2

220.3

318.7

Gross

Red. (%)

10

-4

-4

-13

-17

-30

-32

-45

NET RED.

FOR SBL**

Z Test

-15%

Z-3.80*

-9%

Z-3.09*

-11%

Z»4.34*

-10°/.

Z=4.16*

*Stat1stically significant effect (two-sided alpha

than 1.96)

.05 - i.e., Z greater

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

106
overall fatality risk. During 1975-85 there were 2463 occupant fatalities

in 4 door cars of the last model year before SBL; those cars accumulated

12.632 million vehicle years in that time. The fatality rate was 195 per

million vehicle years. The fatality rate in 4 door cars of the first year

after the SBL implementation date was 176.1; thus, the fatality rate

decreased by 10 percent in the control group. But in the 2 door cars, the

rate increased by 4 percent in the first year with SBL. The net effect -

the relative difference of the gross reductions - is a statistically

significant 15 percent increase of fatality risk associated with SBL. In

fact, the increase is significant at the .01 level because Z - 3.80.

When the data are extended to include 2 model years before and

after the SBL implementation date, the net reduction for SBL is -9

percent, which is statistically significant. In the +3 year comparison,

the net effect is -11 percent and in the ±4 year comparison, it is -10

percent, both significant. The sequence of effectiveness estimates (-15,

-9, -11, and -10 percent) shows great stability after the first year and

little or no drift in either direction.

One trend that is obvious in Table 4-1 is that fatality rates

per million vehicle years get steadily lower as the cars get older. The

median age of cars in these PARS analyses is approximately 10 years. At

those ages, cars are driven fewer miles each additional year and the

decrease in absolute mileage is no longer offset by increases in the

"riskiness" of the miles, as it would be at an earlier stage in a car's

lifespan. Thus, the fatality rate per year decreases. (NHTSA's evaluation of side marker lamps [25], pp. 109-115, provides additional detail.)

107
Specifically, the sequences of gross reductions in the fatality rate

worsen by almost exactly 14 percent for each additional model year of

data, both for the control group (10, -4 , -17, and -32) and the 2 door

cars (-4, -13, -30, and -45). Since both sequences of gross reductions

worsen at the same rate, there is little or no drift in the sequence of

net reductions.

The exception to the 14 percent trend is the model year in

which SBL were implemented. It coincided with the implementation of other

important safety devices, such as energy absorbing steering assemblies in

GM and Ford cars. The 4 door cars experienced a 10 percent decrease

rather than a 14 percent increase in the fatality rate per million years,

reflecting a large reduction of "intrinsic" fatality risk. The 2 door

cars experienced a 4 percent increase rather than a 14 percent increase -

not as good as the 4 door cars but still a reduction of "intrinsic" risk.

As will be shown in Section 4.3, most of the added benefits in the 4 door

cars are attributable to a reduction of occupant ejections.

Table 4-2 is identical to Table 4-1, except that the analysis

is performed on the "reduced" data set, which excludes station wagons and

models that were produced only with 2 doors or only with 4 doors. The

exclusion of station wagons, which are typically owned by "safe" drivers,

increased the fatality rates for 4 door cars (e.g., 216.4 for cars of the

first 4 years after the SBL implementation date, vs. 193.2 in Table 4-1).

The exclusion of "sporty" models produced only with 2 doors decreased the

fatality rates for 2 door cars (e.g., 302.7 for cars of the first 4 years

108
TABLE 4-2

FARS 1975-85: FATALITY RATES PER MILLION VEHICLE YEARS,

ALL OCCUPANTS

2 DOOR VS. 4 DOOR SEDANS AND HARDTOPS OF THE SAME MAKES AND MODELS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Excludes station wagons, convertibles, and any make/models

that were produced only with 2 doors or only with 4 doors)

MY Before/

After SBL Millions NET RED.

N of Install, in n of of Car Deaths per Gross FOR SBL**

Doors 2 Door Cars Deaths Years Million Yrs. Red. (%)

Z Test

4 last 1 before 2089 10.121 206.4

first 1 after 2186 11.402 191.7 7

2 last 1 before 2079 8.367 248.5

first 1 after 2724 10.317 264.0 -6 -14%

Z=3.18*

4 last 2 before 3615 19.870 181.9

first 2 after 4869 25.096 194.0 -7

2 last 2 before 3578 15.928 224.6

first 2 after 6181 23.607 . 261.8 -17 -9%

Z=2.92*

4 last 3 before 4151 24.381 170.3

first 3 after 6263 30.586 204.8 -20

2 last 3 before 4118 19.309 213.3

first 3 after 9056 31.865 284.2 -33 -11%

Z=3.73*

4 last 4 before 4522 28.379 159.3

first 4 after 8038 37.147 216.4 -36

2 last 4 before 4481 22.362 200.4

first 4 after 11577 38.241 302.7 -51 -11°/.

Z=4.16*

•"Statistically significant effect (two-sided alpha - .05 - i.e., Z greater

than 1.96)

''Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

109
with SBL, vs. 318.7 in Table 4-1). Thus, the reduced data set made the

fatality rates in 2 door cars closer to those in 4 door cars. But it did

so 1n equal measure for the pre-SBL and post-SBL eras. As a result, the

net effect of SBL was virtually unchanged. The sequence of effectiveness

estimates for the reduced data set (-14, -9, -11, and -11 percent, all

statistically significant) is almost identical to the sequence in Table

4-1 (-15, -9, -11, and -10).

4.3 Effect on ejections

All crashes on FARS must have at least one fatality in them,

but FARS also provides data on the survivors in those crashes. In order

to maximize sample sizes, the analyses of ejections include persons who

were ejected and killed plus persons who were ejected and survived a crash

that was fatal to someone else.

Table 4-3 shows a consistently large negative net effect for

SBL on the number of ejectees per million vehicle years. The sequence of

effectiveness estimates, -42, -26, -24 and -23 percent (all statistically

significant) is remarkably similar to what was observed in the Washington

State nonfatal accident data (-41, -30, -25, and -35 percent, according to

Table 3-4). The FARS sequence is quite stable after the first model year

and, if anything, shows a slight drift in favor of SBL, suggesting a

"true" effectiveness value of perhaps -27 percent. Since, during the SBL

implementation era, about 25-30 percent of all passenger car fatalities

were ejectees, the 10 percent net increase in overall fatality risk (see

Table 4-1 and Section 4.2) is to a large extent attributable to the 27

percent net increase in ejections.

no
TABLE 4-3

FARS 1975-85: EJECTEES# PER MILLION VEHICLE YEARS,

ALL SEATING POSITIONS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/

After SBL Millions

N of Install, in n of of Car

Doors 2 Door Cars Ejectees Years

last 1 before 597 12.632

first 1 after 516 14.688

last 1 before 1004 10.858

first 1 after 1360 13.942

last 2 before 1019 24.934

first 2 after 1190 32.552

last 2 before 1727 20.198

first 2 after 3026 31.280

last 3 before 1191 30.866

first 3 after 1595 40.356

last 3 before 2084 25.300

first 3 after 4361 41.782

last 4 before 1341 36.013

first 4 after 2048 49.249

last 4 before 2234 28.141

first 4 after 5445 50.068

Ejectees per

Million Yrs.

47.3

35.1

92.5

97.6

40.9

36.6

85.5

96.7

38.6

39.5

82.3

104.4

37.2

41.4

79.4

108.8

Gross

Red. (7.)

26

-6

11

•13

-2

-27

-11

NET RED.

FOR SBL**

Z Test

-42°/o

Z=4.80*

-26%

Z=4.49*

-24%

Z-4.56*

*Statistically significant effect (two-sided alpha

than 1.96)

-37

.05 - i.e., Z greater

-23%

Z=4.81*

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

^Persons who were ejected in a crash that was fatal to somebody, but who,

themselves, were not necessarily killed.

Ill
Table 4-4, which is based on the "reduced" FARS data set, shows

nearly the same sequence of effectiveness estimates: -47, -25, -20 and -21

percent, all statistically significant. Although the exclusion of 4 door

station wagon and 2 door "sporty" models slightly reduced the difference

between 2 door and 4 door ejection rates, it did so equally for pre and

post-SBL. As a result, net effects were the same as for the "full" data

set. Since, with FARS, there are few differences between the full and

reduced data sets, the analytic results in the remainder of the chapter

will mostly be based''on the full data set.

Statistically, what is causing the alarming "net" increases in

ejection risk? In the Washington data (Tables 3-4 and B-4), gross

ejection rates decreased substantially for both 2 door and 4 door cars.

In fact, the absolute risk decreases were equally large for both types of

cars. Since 2 door cars, however, had a much higher baseline ejection

rate than 4 door cars, the relative decrease was larger in the 4 door

cars; hence, the negative net effect. In FARS, the picture is even less

attractive. Table 4-3 shows that the sequence of "gross" reductions for 4

door cars is 26, 11, -2, and -11 percent. For 2 door cars it is -6, -13,

-27, and -37 percent. Both sequences have a drift of about -11 percent

per added model year. If the sequences could be carried backwards to a

"+0 model year" comparison (indicating the "true" effects without age

related bias), they would indicate about a 33 percent reduction of

ejection risk in 4 door cars in the SBL implementation year and only a 7

percent reduction in the 2 door cars.

112
TABLE 4-4

FARS 1975-85: EJECTEES# PER MILLION VEHICLE YEARS,

ALL SEATING POSITIONS

2 DOOR VS. 4 DOOR SEDANS AND HARDTOPS OF THE SAME MAKES AND MODELS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Excludes station wagons, convertibles, and any make/models

that were produced only with 2 doors or only with 4 doors)

MY Before/

After SBL Millions

N of Install, in n of of Car

Doors 2 Door Cars Electees Years

last 1 before 509 10.121

first 1 after 421 11.402

last 1 before 684 8.367

first 1 after 909 10.317

last 2 before 852 19.870

first 2 after 978 25.096

last 2 before 1204 15.928

first 2 after 2035 23.607

last 3 before 991 24.381

first 3 after 1301 30.586

last 3 before 1423 19.309

first 3 after 2956 31.865

last 4 before 1119 28.379

first 4 after 1676 37.147

last 4 before 1568 22.362

first 4 after 3725 38.241

Ejectees per

Million Yrs.

50.3

36.9

81.7

88.1

42.9

39.0

75.6

86.2

40.6

42.5

73.7

92.8

39.4

45.1

70.1

97.4

Gross

Red. (7.)

27

-8

-14

-5

-26

-14

-39

NET RED.

FOR SBL**

Z Test

-47%

Z=4.63*

-25X

Z=3.83*

-20%

Z=3.48*

-21%

Z=3.96*

*Statistically significant effect (two-sided alpha » .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

^Persons who were ejected in a crash that was fatal to somebody, but who,

themselves, were not necessarily killed.

113
In other words, something was done to 4 door cars during the

SBL implementation year to reduce ejections dramatically (by 33 percent).

Obviously, that' "something" had nothing to do with SBL, since they were

not installed in 4 door cars. In 2 door cars, ejection risk decreased

only marginally (by 7 percent) - much less than in 4 door cars in relative

and even in absolute terms. Four factors might possibly account for the

difference:

o Whatever was done to 4 door cars, reducing ejections, was not

done to 2 door cars. This factor has nothing to do with SBL.

o Whatever was done to 4 door cars was also done to 2 door cars,

but it did not work nearly as well. This factor also has

nothing to do with SBL.

o SBL have a genuine negative effect on ejections, which, in the

case of the 2 door cars, masked the highly beneficial effect of

whatever was done to decrease ejections in all cars.

o Something other than SBL was done to 2 door cars (but not to 4

door cars) and masked the highly beneficial effect of whatever

was done to decrease ejections in all cars.

Section 5.3 is devoted to exploring which of the factor(s) may have been

at work. Sections 4.4, 4.6 and 4.8 present more disaggregate FARS data on

ejections.

4.4 Fatalities and elections, bv seat position

The discussion of the sled tests (Section 2.2) identified three

groups of persons with fundamentally different collision kinematics and

potential SBL effects: front seat occupants with nobody sitting behind

them, front seat occupants with somebody sitting behind them and back seat

passengers. FARS identifies the seat position of each occupant and makes

it possible to determine if someone was sitting behind a front seat

occupant. Exposure data corresponding to the fatality counts are obtained

114
by multiplying the numbers of vehicle years in Table 4-1 by the proportion

of all occupants in Washington accidents whb were in a particular seating

position. For example, Table 4-1 shows that 2 door cars of the first 2

years with SBL accumulated 31,280,000 exposure years in 1975-85. Since

61,500 (see Table B-5) of 67,435 (see Table 3-1) occupants of 2 door cars

of the first 2 years with SBL in Washington accidents were front seat

occupants with nobody sitting behind them, the appropriate "measure of

exposure" for that seating position is

frl.SOP 31,280,000 - 28,138,000 years

67,435

It should be noted that these measures of exposure are intended only to

make comparable the fatality rates, across model years and body styles,

for a given seating position. But the fatality rates for different

seating positions should not be compared to one another, because car

occupancy in Washington might not be representative of the rest of the

nation anc|, above all, because occupancy of. some positions may have been

underreported in Washington (see Section 3.2.5).

Table 4-5 summarizes the observed net effects of SBL on

fatality rates, by seat position. It is based on the detailed fatality

rates of Tables C-l, C-2 and C-3 in Appendix C of this report. The

sequence of effectiveness estimates for front seat occupants with nobody

sitting behind them was -17, -11, -13 and -11 percent, all significantly

worse than zero. That sequence is just slightly worse than the one for

all occupants, as derived from Table 4-1 (-15, -9, -11 and -10 percent).

Like the Washington data, FARS does not show SBL to be beneficial for this

type of occupant.

115
TABLE 4-5

FARS 1975-85: NET EFFECT OF SBL ON FATALITY RISK

-BY SEAT POSITION2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Summary of Tables C-l - C-3 and 4-1)

Effect on

Fatality

Risk of

Net Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

±1 ±2 ±3 ±4

Front seat occupants with

nobody sitting behind them

Front seat occupants with

somebody sitting behind them

-17

Z= 3.80*

-20

1.47

-11

3.46*

.55

-13

4.76*

-8

.93

-11

4.23*

-11

1.39

Back seat occupants +2

.16

+7

.64

+5

.50

-7

.74

ALL OCCUPANTS -15

Z= 3.80*

-9

3.09*

-11

4.34*

-10

4.16*

*Statistically significant effect (two-sided alpha - .05 - i.e., Z

greater than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

116
The data set of front seat occupants with somebody sitting

behind them (documented in detail in Table C-2) is only a tenth as large

as the preceding one and is subject to more sampling error. In particular, none of the observed effects is significantly different from zero.

Nevertheless, the sequence of observed effects, -20, -5, -8 and -11

percent, is almost as consistent as the preceding one and shows much the

same pattern. As in Washington, the most reasonable conclusion is that

SBL are not beneficial for front seat occupants, even when someone is

sitting behind them.

The sequence of estimates for back seat occupants is 2, 7, 5

and -7 percent. Due to the small sample sizes and the small magnitude of

the estimates, there is no clear pattern. The numbers do not indicate a

significant benefit for SBL, although some might draw encouragement from

the fact that they are not consistently negative as at the other seat

positions.

Table 4-6 computes the observed net effect of SBL on occupant

ejection rates, by seat position. (It is based on the detailed data in

Tables C-4, C-5 and C-6.) The effect for front seat occupants with nobody

sitting behind them was consistently negative: -38, -30, -28 and -25

percent, all statistically significant. The effect for front seat

occupants with somebody sitting behind them was, in each case, even more

negative: -77, -37, -30 and -35 percent. Even though sample sizes are

smaller, these observed effects are so large that three of them are

statistically significant. If SBL did indeed have any "true" negative

117
TABLE 4-6

FARS 1975-85: NET EFFECT OF SBL ON OCCUPANT EJECTION* RATES

-BY SEAT POSITION2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Summary of Tables C-4 - C-6 and 4-3)

Effect on

Ejection*

Rates of

Net Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

±1 +2 ±3 +4

Front seat occupants with

nobody sitting behind them

Front seat occupants with

somebody sitting behind them

Back seat occupants

ALL OCCUPANTS

z=

z«

z=

z=

-38

3.94*

-77

2.66*

-43

1.55

-42

4.80*

-30

4.48*

-37

2.01*

+4

.25

-26

4.49*

-28

4.69*

-30

1.87

+1

.08

-24

4.56*

-25

4.65*

-35

2.27*

-13

.89

-23

4.81*

*Statistically significant effect (two-sided alpha = .05 - i.e., Z

greater than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

^Persons who were ejected in a crash that was fatal to somebody, but

who, themselves, were not necessarily killed.

118
effect on ejection, these results are consistent with the intuitive

explanation suggested in Section 1.6 - viz., that in pre-SBL cars, the

seatbacks could fold down and block front seat occupants' avenues of

ejection, especially if there are back seat occupants who help force the

seatbacks to fold down.

Table 4-6 shows little or no effect of SBL on the ejection of

back seat occupants. The sequence of estimates is -43, 4, 1, and -13

percent, none of them statistically significant. The more favorable

results for back seat occupants are again consistent with intuition: the

folding over of the front seat in a pre-SBL car could give the back seat

occupant a wider avenue to the front door, even while it blocks front seat

occupants' ejection through that door.

4.5 Effect for restrained occupants

Unlike Washington State nonfatal accident data, belt usage is

very low on FARS. Only 409 of the 36,993 fatalities in these aging,

1960's era cars were belted. That is barely enough for an analysis of

fatality rates. Table 4-7 indicates that the effect of SBL was probably

close to zero. The sequence of estimates is 26, 20, 3 and 1 percent.

None of those estimates is statistically significant and the first two

have much sampling error. Since the FARS effectiveness sequences, so far,

have shown little drift in either direction, the last two estimates in the

sequence, which are both close to zero, are probably unbiased. Since

ejection is infrequent for belted occupants and accounted for most of the

negative effects in the analyses of unbelted occupants, it is appropriate

that net effectiveness for belted persons is close to zero.

119
TABLE 4-7

FARS 1975-85: FATALITY RATES PER MILLION VEHICLE YEARS,

ALL RESTRAINED OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N Of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Deaths

22

34

30

38

39

77

45

84

49

101

52

131

52

130

54

173

Millions

of Car

Years

12.632

14.688

10.858

13.942

24.934

32.552

20.198

31.280

30.866

40.356

25.300

41.782

36.013

49.249

28.141

50.068

Deaths per

Million Yrs.

1.74

2.31

2.76

2.73

1.56

2.37

2.29

2.69

1.59

2.50

2.06

3.14

1.44

2.63

1.92

3.46

Gross

Red. (7«)

-33

1

-51

-21

-58

-53

-82

-80

NET RED.

FOR SBL*

Z Test

+26%

Z-0.81

+20%

Z=0.84

+3%

Z-0.14

+1%

Z=0.05

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha « .05 - i.e., Z greater than

1.96).

120
4.6 Effectiveness in frontal crashes

FARS has a variable called "principal impact point" and values

1, 11 and 12 c6rrespond to frontal impacts. There are two reasons to

analyze frontal crashes separately. Intuitively, the seatback in a

pre-SBL car may be more likely to fold over in a frontal crash than in

other crash modes; the effect of SBL, which are designed to inhibit the

folding over of the seatback, may be more prominent in frontals. Furthermore, the Texas analysis of nonfatal frontals had ambiguous results (see

Section 3.3.2). Table 3-10 showed "barely" significant injury reductions

in the ±1 year comparison, dwindling to zero in later comparisons. Was

that a true positive, subsequently masked by biases or a spurious positive, subsequently corrected by a larger sample?

Table 4-8 shows that SBL are not effective in reducing fatalities in frontal crashes. The sequence of effectiveness estimates, -9, -3,

-8 and -8 percent, is stable and the last two numbers are statistically

significant. The results are nearly the same as for all crash modes (-15,

-9, -11, and -10 percent), at least in the ±3 and ±4 year comparisons.

Table 4-9 presents consistently negative results for ejections

in frontal crashes. The sequence of estimates is -40, -30, -25 and -24

percent, all statistically significant. It is virtually identical to the

sequence for ejections in all crash modes (-42, -26, -24 and -23 percent).

Finally, Table 4-10 represents the best possible situation:

frontal fatalities who were not ejectees. Yet, there are no benefits for

121
TABLE 4-8

FARS 1975-85: FATALITY RATES IN FRONTAL CRASHES.

PER MILLION VEHICLE YEARS, ALL OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Frontal

Crash

Deaths

1326

1387

1395

1759

2299

3012

2464

3960

2640

3853

2885

5751

2863

4903

3076

7392

Millions

of Car

Years

12.632

14.688

10.858

13.942

24.934

32.552

20.198

31.280

30.866

40.356

25.300

41.782

36.013

49.249

28.141

50.068

Deaths per

Million Yrs.

105.0

94.4

128.5

126.2

92.2

92.6

122.0

126.6

85.5

95.5

114.0

137.6

79.5

99.2

109.3

147.6

Gross

Red. (%)

10

2

none

-4

-12

-21

-25

-35

NET RED.

FOR SBL**

Z Test

- 9%

Z-1.67

- 3%

Z=0.88

- 8%

Z=2.29*

-8%

Z=2.49*

*Statistically significant effect (two-sided alpha

than 1.96)

.05 - i.e., Z greater

''Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

122
TABLE 4-9

FARS 1975-85: EJECTEES* PER MILLION VEHICLE YEARS,

• IN FRONTAL CRASHES, ALL SEATING POSITIONS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Frontal

Crash

Ejectees

233

193

335

428

403

432

589

975

463

583

702

959

506

750

750

1788

Millions

of Car

Years

12.632

14.688

10.858

13.942

24.934

32.552

20.198

31.280

30.866

40.356

25.300

41.782

36.013

49.249

28.141

50.068

Ejectees per

Million Yrs.

18.4

13.1

30.9

30.7

16.2

13.3

29.2

31.2

15.0

14.4

27.7

30.1

14.1

15.2

26.7

35.7

Gross

Red . (°/o)

29

none

18

_-j

4

-21

—8

-34

NET RED.

FOR SBL**

Z Test

-40%

Z=2.75*

-30%

Z=3.04*

-25%

Z=2.93*

-24%

Z-2.99*

*Statistically significant effect (two-sided alpha

than 1.96)

.05 - i.e., Z greater

**Note: positive numbers Indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

^Persons who were ejected in a crash that was fatal to somebody, but who,

themselves, were not necessarily killed.

123
TABLE 4-10

FARS 1975-85: N0NEJECTI0N FATALITY RATES IN FRONTAL CRASHES,

PER MILLION VEHICLE YEARS, ALL OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

frontal

Nonejection

" Deaths

1159

1244

1145

1432

1992

2687

2028

3233

2291

3423

2370

4695

2476

4347

2529

6033

Millions

of Car

Years

12.632

14.688

10.858

13.942

24.934

32.552

20.198

31.280

30.866

40.356

25.300

41.782

36.013

49.249

28.141

50.068

Deaths per

Million Yrs.

91.8

84.7

105.4

102.7

79.9

82.5

100.4

103.3

74.2

84.8

93.7

112.4

68.8

87.9

89.9

120.5

Gross

Red. (%)

8

3

-3

-3

-14

-20

-28

-34

NET RED.

FOR SBL*

Z Test

- 6%

Z=0.95

none

Z=0.09

-5°/.

Z-1.31

-57.

Z=1.36

*Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group. None of the

observed net effects were statistically significant (2-sided alpha = .05 -

i.e., Z greater than 1.96).

124
SBL here, either. The sequence of effectiveness estimates is -6, 0, -5

and -5 percent, none of which are significantly different from zero.

Probably the only reason that the overall fatality results for frontal

crashes (Table 4-8) are a bit less negative than for all crash modes

(Table 4-1) is that ejectees account for a slightly smaller proportion of

frontal fatalities than in the other crash modes.

Tables 4-8, 4-9 and 4-10 show that the results for SBL in

frontal crashes are nearly the same as in other crash modes. The most

optimistic conclusion that could be drawn from this is that all these

negative effects are due to causes other than SBL, for if SBL are designed

to work primarily in frontal crashes, their effect should be seen primarily in frontals - i.e., any effects seen equally in frontals and other

crashes must be due to reasons other than SBL. But even this "most

optimistic" conclusion does not go so far as to imply positive benefits

for SBL.

4.7 Effect on fatal accidents involving fire

Section 1.6 raised a concern that the old style of manually

operated SBL could impede the egress of back seat passengers from a 2 door

car in emergency situations such as vehicle fires. Intuitively, the

problem is the reverse of ejection: with fires, the objective is to

expedite rather than block exits from the car.

FARS records whether fire occurred in an accident and which

occupants were fatally injured. It does not document whether the fatal

125
injury was caused by the fire, itself or by crash impacts. In the FARS

file of cars built within 4 years of the SBL implementation date, 1277 of

the 36,993 fatalities were in accidents involving fire - enough for a

statistical analysis.

Table 4-11 shows that SBL has little or no overall net effect

on fire-involved fatalities but may have strong effects at certain seat

positions, consistent with intuition. For all seat positions combined

(detailed data: Table C-7), the sequence of effectiveness estimates is

-21, 0, 4 and 6 percent, none of them statistically significant. The

sequence for front seat occupants with nobody sitting behind them (detailed data: Table C-8) likewise suggests little or no effect: -28, -3, 2,

4 percent (none significant).

The interesting results, however, are those for front seat

occupants with somebody sitting behind them and for back seat occupants,

as documented in detail in Tables C-9 and C-10. Table 4-11 shows a

consistent beneficial effect for the front seat occupants with somebody

behind them: 40, 68, 50 and 57 percent. The results for back seat

occupants, -23, -64, -34 and -32 percent, are just as consistently

negative. The numbers are certainly in the direction suggested by

intuition; however, none of the 8 estimates is statistically significant,

even at the .1 level. Tables C-9 and C-10 suggest, though, that the

estimates fail to achieve significance primarily because the sample of

fires was small in the firg-SBL cars. The following table, confined to

cars of the first 4 post-SBL years, does show a significant difference

126
TABLE 4-11

FARS 1975-85: NET EFFECT OF SBL ON FATALITY RISK

IN CRASHES INVOLVING FIRES*

-BY SEAT POSITION2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Summary of Tables C-7 - C-1O)

Effect on

Fire Crash

Fatality*

Risk of

Net Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

±1 ±2 ±3

Front seat occupants with

nobody sitting behind them

Front seat occupants with

somebody sitting behind them

-28

Z« 1.06

+40

Z- .54

-3

.16

+68

1.57

+2

.18

+50

1.06

+4

.32

+57

1.38

Back seat occupants -23

Z- .29

-64

.85

-34

.56

-32

.55

ALL OCCUPANTS 21

.42

none

.02

+4

.27

+6

.47

*Persons who were-killed in a vehichle that caught fire. FARS, however,

does not specify whether the person's fatal injury was due to the fire

or to other causes.

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group. None of the

observed net effects were statistically significant (2-sided alpha -

.05 - i.e., Z greater than 1.96).

127
between 2 and 4. door cars 1n the seat positions of fire related fatalities:

Front Seat

(Someone Behind) Back Seat

4 doors 38 24

2 doors 46 65 Chi Square=6.27

In the 4 door cars, back seat occupants were less likely to die in fires

than the persons sitting in front of them; in 2 door cars with SBL, more

so. It is unlikely, though, that the difference is purely due to SBL

(FARS does not have enough pre-SBL cases to provide a meaningful answer):

even without SBL it would seem harder for a back seat passenger to get out

of a 2 door car than a 4 door car, especially if the front seat passenger

has to get out first.

The most reasonable conclusion, based on Table 4-11, would

appear to be that SBL have had little net effect on fire fatalities and

that a possible negative effect for back seat occupants has been offset by

slightly easier egress for the far more numerous front seat occupants. Of

course, the conclusion applies only to the old type of manual SBL. For

current automatic SBL, the effect should be zero since the seatback moves

freely, like in a pre-SBL car, except during the moment of impact.

4.8 Effectiveness bv car size and manufacturer

The data on overall fatality rates and ejections were analyzed

separately for two car size groups (compact and intermediate cars; full

sized and luxury cars) and for the three manufacturers (GM, Ford and

Chrysler). The purpose was to find if some particular group is primarily

128
responsible for the overall negative effects. For theso analyses, the

FARS, Polk and Washington data were subdivided into car size and manufacturer groups by make and model, using the same definitions for each file.

4.8.1 Effect on fatalities

Table 4-12 (based on detailed tables C-ll - C-15) shows that,

indeed, the negative estimates of fatality reduction are confined to the

smaller than average cars (compacts and intermediates). For the smaller

cars, the sequence of effectiveness estimates is -18, -10, -13, and -9

percent, all of which are statistically significant. For full sized and

luxury cars, the sequence is -4, 1, 4 and 5 percent, none of them significant. The sequence for the larger cars shows a slight drift in favor of

SBL and suggests that the true effect is close to zero. The observed

negative effect for the smaller cars as well as the near zero effect for

the larger cars are explained by what happened to ejections, as will be

discussed below.

The lower half of Table 4-12 provides effectiveness estimates

for each manufacturer. The GM and Ford results are based on the "reduced"

rather than the "full" FARS data set: Camaro and Firebird were introduced

in 1967, simultaneous with the implementation of SBL by GM. These sporty

2 door cars raised the GM fatality rates for the full data set, resulting

in spuriously large negative effects for SBL. These new models in turn

reduced the market for Mustangs; with relatively fewer Mustangs, the

fatality rates for 2 door Ford products declined, resulting in spurious

positive effects for SBL. It made more sense to use the reduced data

129
TABLE 4-12

•FARS 1975-85: NET EFFECT OF SBL ON FATALITY RISK

-BY CAR SIZE AND MANUFACTURER2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Summary of Tables C-ll - C-15)

Effect on

Fatality

Risk in

Net Effect of SBL <%)* *

Number of Model Years before/after

SBL Installation in 2 Door Cars

±1 ±2 ±3 ±4

Compacts and intermediates

Full sized and luxury cars

General Motors*

Ford*

Chrysler

ALL CARS

BY CAR SIZE

-18

Z- 2.80*

-4

Z« .73

BY MANUFACTURER

-13

Z« 2.15*

-11

Z= 1.17

-10

Z- 1.10

-15

Z= 3.80*

-10

2.25*

+1

.32

-4

.82

-4

.57

-12

1.90

-9

3.09*

-13

3.21*

+4

.94

-6

1.60

-5

.97

n. a.

-11

4.34*

-9

2.49*

+5

1.54

-5

1.43

-9

1.85

n.a.

-10

4.16*

*Statistically significant effect (two-sided alpha = .05 - i.e., Z

greater than 1,96)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

^Excludes station wagons, convertibles and any make/models that were

produced only with 2 doors or only with 4 doors.

130
sets, which exclude Camaro, Firebird and Mustang, among others. (But when

the data are not subdivided by manufacturer, the Camaro-Firebird and

Mustang effects' cancel each other out and there is little difference

between the full and reduced data sets, as noted in Sections 4.2 and

4.3). As for Chrysler, effectiveness estimates are available only for the

±1 and +2 year comparisons, since the VIN before 1966 did not identify the

number of doors.

Table 4-12 shows remarkable similarity in the results of the

three manufacturers. The sequence of estimates is -13, -4, -6 and -5

percent for GM, -11, -4, -5 and -9 percent for Ford and -10 and -12

percent for Chrysler. These numbers are a few percent closer to zero than

the ones for all manufacturers combined (-15, -9, -11 and -10), suggesting

that the aggregate data may have a slight bias against SBL that made the

results more negative than they should have been.

4.8.2 Ejections

Table 4-13 (which summarizes Tables C-16 - C-20) emphatically

indicates that the negative results on ejection are confined to smaller

cars. The sequence of estimates for compacts and intermediates is -70,

-41, -41 and -40 percent - all statistically significant and completely

stable after the first year. (Further subdivision of the cars into

compacts and intermediates showed approximately 40 percent increases for

each of those groups.) For full sized and luxury cars, the estimates are

-14, 0, 10 and 11 percent - none of them significant. Since the numbers

are drifting slightly in favor of SBL, the actual effect would appear to

131
TABLE 4-13

FARS 1975-85: NET EFFECT OF SBL ON OCCUPANT EJECTION** RATES

-BY CAR SIZE AND MANUFACTURER2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Summary of Tables C-16 - C-2O)

Effect on

Ejection**

Rates in

Net Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

±1 ±2 ±3 ±4

Compacts and intermediates

Full sized and luxury cars

General Motors*

Ford*

Chrysler

ALL CARS

BY CAR SIZE

-70

Z» 4.44* '

-14

Z- 1.20

BY MANUFACTURER

-30

Z* 2.31*

-57

Z- 2.59*

-41

Z- 2.19*

-42

Z- 4.80*

-41

4.05*

none

.03

-13

1.45

-14

1.11

-42

3.14*

-26

4.49*

-41

4.55*

+10

1.46

-16

1.89

none

.04

n.a.

-24

4.56*

-40

4.87*

+11

1.78

-14

1.84

-11

1.28

n.a.

-23

4.81*

*Statistically significant effect (two-sided alpha - .05 - i.e., Z

greater than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

*Excludes station wagons, convertibles and any make/models that were

produced only with 2 doors or only with 4 doors.

**Persons who were ejected in a crash that was fatal to somebody, but

who, themselves, were not necessarily killed.

132
be close to zero here. Table C-16 shows that, among smaller cars, 2 door

cars had double the ejection rate of 4 door cars before SBL. Four door

cars experienced a dramatic reduction of ejections in the year that SBL

were introduced in 2 door cars, while the 2 door cars experienced little

or no reduction. Once again, it is a reduction in the control group

rather than an increase in the 2 door cars that causes the large negative

effect for SBL. Table C-17 shows that, by contrast, there was only a

moderate difference in the ejection rates of large 2 door and 4 door

cars. Both groups experienced moderately large reductions of ejection

risk in the years following the introduction of SBL (after correcting for

the downward drift in the "gross" rates). Large 4 door cars have only a

slightly lower ejection risk than small 4 door cars, whereas large 2 door

cars have less than half the ejection risk of small 2 door cars, especially in the years after SBL. Many of these phenomena would seem to have

more to do with door structures than with SBL, as will be discussed

further in Section 5.3.

Table 4-13 shows that the observed effect of SBL on ejections

was more negative at Chrysler than at GM or Ford. The sequences of

estimates were -30, -13, -16, and -14 percent at GM; -57, -14, 0 and -11

percent at Ford and -41 and -42 percent at Chrysler. The ±1 year estimates from GM and Ford and both Chrysler numbers are statistically

significant. Table C-20 shows that the very negative results from

Chrysler are due to an especially large reduction of ejections in the 4

door cars, rather than any increase in the 2 door cars.

133
4.9 When children ride in the back seat

As explained in Section 3.2.7, one situation where SBL might be

especially effective is when the back seat is occupied only by children,

e.g., up to 12 years old, especially in a frontal crash where nobody is

ejected. Table 4-14 presents FARS results on children up to age 12 who

rode in the back seat (top half) as well as the persons who were sitting

in front of those children (lower half). This occupancy mode is analyzed

first for all fatalities, then all nonejection fatalities and, finally,

for frontal crashes in which nobody is ejected. The sample sizes for

Table 4-14 are at most 1/6 as large as those for back seat fatalities of

all ages (Table C-3), almost precluding any hope for statistically

meaningful results.

All 12 results for children riding in the back seat (top half

of Table 4-14) are negative, although none are statistically significant

even at the .1 level. Nine of the 12 of fhe effectiveness estimates for

the persons sitting in front of those children are negative (lower half of

Table 4-14), although none of these, either, are statistically significant

at the .1 level. There is nothing in,Table 4-14 to support the hypothesis

that SBL save lives when children ride in the back seat.

134
TABLE 4-14

FARS 1975-85: NET EFFECT OF SBL ON FATALITY RISK

WHEN CHILDREN* ARE IN THE BACK SEAT

-BY SEAT POSITION2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

Net Effect of SBL (%)**

Number of Model Years before/after

Effect on

Fatality

Risk of

CHILDREN* IN THE

All fatalities

Nonejection fatal

Frontal noneject.

BACK SEAT

ities

fatals.

SBL

±1

-21

Z- .47

-13

Z- .28

-1

Z= .02

Installation

±2

-25

.77

-16

.45

-99

1.36

in 2 Door

±3

-34

1.12

-16

.52

-70

1.17

Cars

±4

-39

1.34

-32

.99

-93

1.52

FRONT SEAT OCCUPANTS WITH A CHILD* SITTING BEHIND THEM

All fatalities

Nonejection fatal

Frontal noneject.

ities

fatals.

-51

Z- 1.24

-53

U 1.16

-126

Z- 1.54

-9

.35

-3

.11

-11

.30

-4

.17

none

.01

+10

.32

-4

.18

-3

.12

+5

.17

*Children up to (and including) 12 years old.

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group. None of the

observed net effects were statistically significant (2-sided alpha »

.05 - i.e., Z greater than 1.96).

135

CHAPTER 5

ANALYSES OF THE PERFORMANCE OF CAR DOORS AND SEATS

IN NHTSA ACCIDENT FILES

Detailed NHTSA accident files such as Multidisciplinary

Accident Investigation (MDAI) and the National Crash Severity Study (NCSS)

do not have large enough samples for statistically meaningful analyses of

the overall effectiveness of seat back, locks (SBL). But they provide

invaluable data on the performance of doors and seats in crashes. The

data show that the integrity of doors of 4 door cars improved greatly at

the same time that SBL were introduced in 2 door cars. Door integrity did

not improve in 2 door cars. This is a difference between 2 and 4 door

cars that is unrelated to SBL and may largely explain the significant

negative "effects" on ejection associated with the implementation of SBL

in Chapters 3 and 4.

The MDAI data show that SBL or other seat components in 2 door

cars often give way in crashes of moderate severity, especially if there

is a back seat occupant loading the front seatback. They- confirm the sled

test results, where seats gave way in all crashes at 26.5 mph when there

were back seat occupants. The inability of SBL to hold seats in place is

consistent with the principal statistical findings of Chapters 3 and 4 -

viz., no reduction of injuries and fatalities.

5.1 Description of the data files

The Multidisciplinary Accident Investigation (MDAI) file

137
contains detailed records of 9623 cars Involved in crashes [8]. Many of

the data were collected 1n 1970-74 and the study emphasized new cars;

thus, much of the file consists of cars of the early 1970's, although

there are also many cars of the I960's. The unsurpassed level of detail

Includes complete, reliable Information on the performance of door and

seat components 1n the crash. The principal drawback of MDAI 1s that it

1s not a probability sample of crashes and, 1n particular, 1s skewed

toward more severe crashes and Injuries. But it can reasonably be assumed

that the bias 1s no stronger for 2 door cars than for 4 door cars of the

same model year - I.e., the analysis technique developed 1n Sections 3.1

and 4.1 (comparing the rates in pre and post-SBL 2 door cars to those in 4

door cars of the same model years) should not be strongly biased.

Likewise, there 1s no evidence that MDAI Investigators went out of their

way to find crashes 1n which the seats separated - I.e., for crashes of a

given severity level, the seat separation rates 1n MDAI (Section 5.4) may

be representative. Another disadvantage Is that the MDAI file does not

contain estimates of Delta V. Damage data are used to obtain a rough

approximation of Delta V.

The National Crash Severity Study (NCSS) is a probability

sample of 12,050 towaway crashes that occurred during 1977-79 at 7 areas

that, in combination, were heuristically representative of the nation

[41]. Cars of all ages were sampled equally; as a result, NCSS contains

numerous cars of the early post-SBL years and a moderate number of

relatively old pre-SBL cars. NCSS Includes complete data on occupant

ejection and on doors that opened during the crash. More detailed vehicle

138
performance data such as the separation of seat components was collected

only after March 1978 and was not as complete as In MDAI. NCSS U a

weighted stratified sample - i.e., the crashes resulting in fatalities or

injuries requiring emergency treatment were oversampled and the other

crashes must be given a higher weight to obtain unbiased estimates.

Interestingly, unweighted NCSS data have about the same bias toward higher

injury as MDAI. In Sections 5.2 and 5.3, the unweighted NCSS and the MDAI

files are combined to produce a single, reasonably homogeneous data set

with the largest possible sample size.

5.2 Analysis of ejections

The combined MDAI and NCSS files contain 114 cases of ejection

from cars of the last 4 model years before SBL implementation: far fewer

than the 1371 in the Washington State data (Table 3-4) or the 3575 in FARS

(Table 4-3). It is unreasonable to expect a statistically reliable

analysis of ejections, let alone injury rates. Table 5-1 shows that,

indeed, the sample is too small to produce consistent results. The

sequence of "observed effectiveness estimates for SBL" (derived exactly as

in Section 3.1) is 57, 25, 13 and -12 percent; only the first is statistically significant at the .05 level, and "barely" so (Z-2.09). The

sequence could be indicative of an extremely high "true" effectiveness for

SBL with a large year to year bias against SBL. Far more likely, it

indicates anomalous data in the small sample of cars built within 1 year

before and after implementation and a correction of the anomaly as the

sample is broadened. Indeed, Table 5-1 shows a large increase in the

ejection rate for 4 door cars of the first year after the SBL implementation date - contrary to all the other data sets,, which showed decreases.

139
TABLE 5-1

NCSS AND MDAI FILES: EJECTEES PER 1000 OCCUPANTS,

ALL SEATING POSITIONS

(Unweighted Data)

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install. 1n

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Ejectees

11

29

36

43

22

55

52

109

29

77

67

219

40

94

74

305

N Of

Occ.

381

605

551

925

718

1466

990

2265

912

2516

1158

4510

1044

3637

1292

7045

Ejectees per

1000 Occ.

28.87

47.93

65.34

46.49

30.64

37.52

52.52

48.12

31.80

30.60

57.86

48.56

38.31

25.85

57.28

43.29

Gross

Red. (%)

-66

29

-22

8

4

16

33

24

NET RED.

FOR SBL**

Z Test

+57%

Z-2.09*

+25%

Z=0.98

+13%

Z-0.54

-12%

Z=0.50

*Stat1st1cally significant effect (two-sided alpha - .05 - i.e., Z greater

than 1.96)

**Note: positive numbers Indicate that the SBL group (2 door cars) had greater

Improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

140
The ejection rates 1n the ±4 year comparison are much closer to what was

seen In other data files: a 33 percent decrease of ejection risk in the 4

door cars and a 24 percent decrease 1n the 2 door cars (smaller in

relative terms than in the 4 door cars, thus resulting in a negative "net

effectiveness" estimate, but about equally large in absolute terms). In

short, the sample is far too small to shed new light on ejection, but at

least the ±4 year comparison is consistent with the Washington and FARS

data.

5,3 Analysis of door integrity in crashes

The NCSS data show that 64 percent of ejections in cars of the

last 5 years before SBL were through the side doors. Obviously, any

safety Improvement that would reduce the likelihood of doors opening in

crashes would significantly reduce the risk of ejection. If something was

done to doors that reduced door opening in 4 door cars more than in 2 door

cars, ejection rates in 4 door cars would decline more swiftly than in 2

door cars. If that "something" happened to coincide with the introduction

of SBL, 1t would result in all the analyses of Chapters 3 and 4 attributing a negative "net effect" on ejections to SBL, when, in fact, it may

have been a result of changes to doors.

The combined NCSS and MDAI files Include records of 226 cars of

the last 4 years before SBL Implementation in which at least one door came

open during the crash. In other words, door opening occurs about twice as

often as occupant ejection, even when all ejection portals are included -

thus, NCSS and MDAI are far more likely to produce a statistically

141
meaningful analysis of door opening rates than for ejections. Table 5-2

shows that door opening rates decreased by 23 percent in 4 door cars in

the year that SBL was implemented 1n 2 door cars and continued to decrease

by even greater amounts in subsequent years. In 2 door cars, door opening

decreased by only 4 percent in the SBL implementation year, with no

additional decrease in the next year. Only in the third and fourth years

after SBL did fewer doors open and, even then, the reduction lags behind

the one for 4 door cars. As a result, Table 5-2 "attributes" a consistent

sequence of negative effects on door Integrity to SBL: -25, -31, -25 and

-37 percent, the last.of which is statistically significant.

Of course, there is no intuitive reason to believe that SBL had

any effect on doors opening in crashes. Undoubtedly, the effects are due

to some other factor that happened to coincide with the implementation of

SBL. What is most interesting about Table 5-2 1s how closely the door

opening rates 1n MDAI and NCSS parallel the ejection rates in Washington

(Tables 3-4 and B-4) and FARS (Tables 4-3 and 4-4, except to the extent

that the exposure measure used with FARS causes the gross ejection rates

to increase as cars.get newer). In all cases, the negative "net effect

for SBL" is not due to an actual worsening in 2 door cars, but rather to

an immediate improvement 1n the 4 door cars which is only belatedly

achieved in the 2 door cars. Moreover, the magnitude of the effects is

similar: FARS and Washington suggest about a 20-25 percent negative net

effect on ejections. NCSS and MDAI show a 25-30 percent negative effect

on door opening. Since about 64 percent of ejectees in pre-SBL cars were

through the doors, the 25-30 percent increase in door opening more or less

142
TABLE 5-2

NCSS AND MDAI FILES: INCIDENCE OF DOORS OPENING DURING IMPACTS

1 (Unweighted Data)

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/

After SBL

N of Install. 1n

Doors 2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

4 last 2 before

first 2 after

2 last 2 before

first 2 after

4 last 3 before

first 3 after

2 last 3 before

first 3 after

4 last 4 before

first 4 after

2 last 4 before

first 4 after

Cars where

at Least 1

Door

Opened

38

44

54

75

109

96

213

87

157

114

373

100

187

126

526

N Of

Cars

215

322

293

495

382

757

517

1194

461

1300

604

2465

528

1895

674

3951

Door

Opening

Rate (%)

17.67

13.66

18.43

17.78

19.63

14.40

18.57

17.84

18.87

12.08

18.87

15.13

18.94

9.87

18.69

13.31

NET RED.

Gross FOR SBL**

Red. (%)

Z Test

23

4

27

4

36

20

48

29

-257,

Z=0.85

-31%

Z-1.51

-257.

Z-1.41

-37%

Z-2.08*

*Stat1stically significant effect (two-sided alpha - .05 - I.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

143
explains the 20-25 percent increase in ejections and leaves little room

for a "true" SBL effect.

Table 5-2 begs the question as to why door opening rates

decreased substantially in 4 door cars in 1967-68 but only slightly in 2

door cars. One possibility that must be examined is that major vehicle

redesigns or introductions coincided with SBL, biasing the results. In

••••••• '

fact, the introduction of the Chevrolet Camaro and Pontiac Firebird and

the restyling of the Dodge Charger were the only major model changes that

coincided with SBL [26], pp. 122-124. GM restyled their intermediates and

compacts one year later; Ford and Chrysler restyled them one or two years

earlier. The introduction of Camaro and Firebird could be responsible for

some of the relatively poorer door performance of 2 door cars, but not too

much since those models only accounted for 8 percent of the 2 door cars

sold that year [49]. Most of the effect in Table 5-2, then, would appear

due to genuine vehicle modifications rather than biases in the data.

Here 2 door cars modified in a different way from 4 door cars

or were they similarly modified, but with different effects? Garrett

performed a detailed analysis of door modifications and opening rates in

cars of the I960's [14]. He found that GM, Ford and Chrysler all made

significant Improvements in door latch designs in 1967-68 (the exact year

depending on the make and model), resulting in a 29 percent reduction in

door openings, relative to 1965-66 cars, on the Automotive Crash Injury

Research (ACIR) file. There were no modifications in door latches between

1964 and 1967 and the door opening rate remained about constant for model

144
years 1964-66. Garrett did not subdivide the door opening rates in his

study by 2 door vs. 4 door cars. Recently, however, Garrett stated that

Identical, simultaneous improvements were made to the door latches of 2

and 4 door cars in 1967-68 and that no substantial further improvements

were made for some years after that [15]. Humphrey of GM [21] and Grush

of Ford [19] agreed with Garrett1 s views. It must be concluded that the

same modifications were made to the door latches of 2 and 4 door cars, but

that they brought about a relatively much greater performance improvement

in the 4 door cars. The Intuitive support for the conclusion is that the

doors of 4 door cars are significantly narrower than in 2 door cars.

Nider doors have a greater tendency to bow at the time of impact and

separate the latches from the frame. The 1967-68 improvement may have

made the latches on the 4 door cars strong enough to resist all but the

most severe impacts, while only beginning to solve the problem for 2 door

cars. If, in addition, the styling of the late I960's resulted in even

longer doors on 2 door cars (e.g., Camaro), 1t 1s plausible that the

performance benefit of the 1967-68 improvements was realized primarily on

the 4 door cars.

The door opening rates in Table 5-2 are consistent with these

ideas. Door opening rates remained constant during the 4 pre-SBL model

years, just as in Garrett1 s ACIR data. At the time of SBL implementation

(1967-68), which coincided with the door latch Improvements, the rate

dropped by about 25 percent 1n the 4 door cars but much less 1n the 2 door

cars. In the third and fourth year after SBL implementation, the rates

drop in both types of cars, partly due, perhaps, to age related biases in

145
the data but also to the side door beams that were Introduced in full

sized and sporty GM and Ford cars as well as Intermediate GM cars at that

time [263, p. 108. Side door beams significantly reduce the Incidence of

doors opening 1n crashes [26], pp. 318-327.

5.4 Seat performance 1n crashes of 2 door cars with SBL

One of the noteworthy findings 1n the sled tests 1s that seat

separation occurred in all three frontal tests at 26.5 mph with dummies in

the back seat (see Section 2.2). In one of the three tests, both SBL were

sheared off; 1n the other two, the seat tore loose from the track. In all

cases, the net result was that the seat acted like one that did not have

SBL. (Seat separation did not occur even at 30 mph with the back seat

empty.) The Immediate question 1s whether the sled buck fashioned from a

1981 Chevrolet Citation 1s representative of the performance of the range

of real vehicles 1n highway crashes.

The MDAI file 1s unique in that 1t contains detailed seat

performance data on nearly all vehicles (most of which are cars of the

late 1960's and early 1970's equipped with the old style of manual SBL).

The file contained 3259 2 door cars with SBL that had frontal impacts.

The following types of seat separation were documented on MDAI:

108 cases: the left SBL broke

115 cases: the right SBL broke

67 cases: the seat tore loose from one of the tracks

52 cases: the seat and track tore loose from the floor

16 cases: the front seatback tore loose from the seat

Note that breakage of the SBL 1s the most common cause of seat separation

in frontal crashes.

146
For comparison with the sled tests, the MDAI data need to be

tabulated by Delta V range and back seat occupancy. MDAI does not contain

estimates of Delta V. It has to be approximated from the Collision

Deformation Classification (CDC) [63 of the damaged vehicle. As 1n

NHTSA's evaluation of child safety seats, the approximation consists of

two steps [22], pp. 221-222. First, the CRASH program is executed, using

the CDC of the case vehicle and assuming the damage is the result of

impacting a rigid immobile fixed object [33], pp. 5, 20-22. This first

step, however, usually overestimates Delta V in comparison with a reconstruction based on the full CRASH program [22], p. 222, perhaps because

CRASH overestimates the extent of damage that occurs in the "average" car

with a given CDC. The estimate based on this first step alone will be

referred to as the "higher estimate of Delta V" or DVH. The best

estimate (based on regression of Delta V from the full CRASH program with

DVH) is

DVL - 4.645 + .7082 DVH

henceforth referred to as the "lower estimate of Delta V." As for back

seat occupancy, MDAI contains a variable that indicates if any back seat

passenger made contact with the front seatback.

Table 5-3 is based on the lower estimate of Delta V, showing

the percentage of cars that experienced any of the modes of seat separation documented in MDAI (broken SBL, seat torn loose from tracks or floor,

or seatback separated from seat). The results are remarkably parallel to

what was found in the sled tests.

147
TABLE 5-3

MDAI DATA: FRONT SEAT SEPARATION* RATES,

2 DOOR CARS EQUIPPED WITH SBL,

AS A FUNCTION OF DELTA V, IN FRONTAL CRASHES

-LOWER ESTIMATE OF DELTA V**-

Delta V**

Range (mph)

Less than 10

10-14.9

15-19.9

20-24.9

25-29.9

30-39.9

40 or more

N of .

Cars

BACK SEAT

609

1097

662

286

114

71

38

n of Cars with

Seat Separation*

UNOCCUPIED

3

29'

42

33

30

22

17

Separation*

Rate (%)

0.5

2.6

6.3

11.5

26.3

31.0

44.7

Less than 10

10-14.9

15-19.9

20-29.9

30 or more

BACK SEAT OCCUPANT CONTACTED FRONT SEATBACK

55 3

90 19

80 28

60 28

17 15

5.5

21.1

35.0

46.7

88.2

*Note: The front seat "separated" if any of the following occurred: (1) An

SBL broke. (2) The seat tore loose from one of the tracks. (3) The

seat tore loose from the floor. (4) The front seatback tore loose from

the seat.

**The "lower" estimate of Delta V 1s obtained by running the CDC-only

CRASH program, assuming the damage is due to striking a rigid barrier.

This number is then multiplied by 0.7082 and added to 4.645.

148
When the back seat is unoccupied (top half of Table 5-3), seat

separation usually does not occur, although 1t 1s not uncommon at Delta V

above 25 mph. At least one form of seat separation occurred in 6 percent

of the cars with frontal Delta V 15-19 mph, 11 percent with Delta V 20-24

mph, 26 percent with Delta V 25-29 mph and in 31 percent of the cars with

Delta V in the 30's. Thus, in the 20-30 mph range which is critical for

protecting the unrestrained occupant, a moderate percentage (17) of the

potential benefit of SBL 1s Tost because the SBL or other components did

not hold in place.

The potential for SBL to Influence occupant kinematics,

however, is far greater when there is somebody In the back seat (see

Section 2.2). Here, the lower half of Table 5-3 shows that much of the

potential 1s lost a priori because the SBL or other seat components did

not hold. Seat separation occurred in 35 percent of the crashes with

Delta V 15-19 mph, 47 percent of the crashes in the critical 20-30 mph

range and 88 percent of the crashes with Delta V over 30. Under these

circumstances, 1t 1s even less surprising that the Washington and FARS

accident data showed no casualty reductions for front seat occupants with

somebody sitting behind them or for back seat occupants.

The situation is slightly better, though, especially at the

lower speeds, when the back seat occupant weighs less than 100 pounds -

e.g., a child. In Table 5-4, the frontal crashes in which there are back

seat occupants are further subdivided. In the top half, there is only one

back seat occupant and that person weighed less than 100 pounds. In the

149
TABLE 5-4

MDAI DATA: FRONT SEAT SEPARATION* RATES,

BY SIZE OF BACK SEAT OCCUPANT,

2 DOOR CARS EQUIPPED WITH SBL,

AS A FUNCTION OF DELTA V, IN FRONTAL CRASHES

-LOWER ESTIMATE OF DELTA V**-

Delta

Range

Less

10-14

15-19

20-29

30 or

v**

(mph)

than 10

.9

.9

.9

more

N of

Cars

BACK SEAT OCCUPANT

12

11

13

8

2

n of Cars with

Seat Separation*

WEIGHING LESS THAN 100 POUNDS

0

0

3

3

2

Separation*

Rate (%)

0

0

23.

37.

100

1

,5

Less than 10

10-14.9

15-19.9

20-29.9

30 or more

BACK SEAT OCCUPANT WEIGHING 100 POUNDS OR MORE

34 3 8.8

64 19 29.7

59 22 37.2

45 23 51.1

13 11 84.6

*Note: The front seat "separated" if any of the following occurred: (1) An

SBL broke. (2) The seat tore loose from one of the tracks. (3) The

seat tore loose from the floor. (4) The front seatback tore loose from

the seat.

**The "lower" estimate of Delta V 1s obtained by running the CDC-only

CRASH program, assuming the damage is due to striking a rigid barrier.

This number 1s then multiplied by 0.7082 and added to 4.645.

150
lower half, there was a back, seat occupant weighing 100 pounds or more.

With the lightweight back seat occupants, seat separation did not occur

below 15 mph Delta V, whereas 30 percent of the seats were already

separating at 10-15 mph when an adult was sitting in the back. At speeds

above 15 mph, the separation rate with small back seat occupants is lower

than with adults, although higher than when the back seat is unoccupied

(top half of Table 5-3). Such results are, of course, consistent with

intuition.

Table 5-5 uses the higher estimate of Delta V to analyze seat

separation rates. Although the higher estimate was found to be a clear

overestimate in files such as NCSS and the National Accident Sampling

System it might be more accurate for MDAI if this file's crashes are more

severe than average, even for a given CDC. The results in Table 5-5,

however, are nearly the same as in Table 5-3, especially in the critical

20-30 mph range of Delta V.

One important caveat is that Tables 5-3, 5-4 and 5-5 are based

only on MDAI data, not NCSS. After March 1978, NCSS did collect information on seat performance similar to the MDAI variables. But the seat

separation rates in unweighted post-March 1978 data are only about 1/8 as

great as in MDAI. Yet unweighted NCSS and MDAI have about the same rates

for injury, ejection, and door opening and about the same Delta V distribution. The most plausible conclusion is that the seat separation rates

are so much lower in NCSS because MDAI had a more thorough, detailed

vehicle examination that identified seat separation with far greater

completeness than NCSS.

151
TABLE 5-5

MDAI DATA: FRONT SEAT SEPARATION* RATES,

2 DOOR CARS EQUIPPED WITH SBL,

AS A FUNCTION OF DELTA V, IN FRONTAL CRASHES

-HIGHER ESTIMATE OF DELTA V**-

Delta V**

Range (mph)

Less than 10

10-14.9

15-19.9

20-24.9

25-29.9

30-39.9

40 or more

N of

Cars

BACK

984

756

571

244

128

121

73

n of Cars with

Seat Separation*

SEAT UNOCCUPIED

10

22

31

32

16

37

28

Separation*

Rate (%)

1.0

2.9

5.4

13.1

12.5

30.6

38.4

Less than 10

10-14.9

15-19.9

20-29.9

30 or more

BACK SEAT OCCUPANT CONTACTED FRONT SEATBACK

82 10

70 17

65 18

47 25

38 23

12.2

24.3

27.7

53.2

60.5

*Note: The front seat "separated" if any of the following occurred: (1) An

SBL broke. (2) The seat tore loose from one of the tracks. (3) The

seat tore loose from the floor. (4) The front seatback tore loose from

the seat.

**The "higher" estimate of Delta V is obtained by running the CDC-only

CRASH program, assuming the damage is due to striking a rigid barrier.

152
CHAPTER 6

SUMMARY AND COMPARISON OF FINDINGS

The introduction to the evaluation (Section 1.6), stated

several hypotheses as to why seat back locks (SBL) might be beneficial in

crashes. It raised some concerns that the SBL actually installed in cars

might fall short of the performance of the ideal seat or that they might

have negative side effects in some types of crashes. With all the

analyses from Chapters 2-5 in hand, it is possible to examine the hypotheses one by one and compare what each of the data sources says about

them.

6-1 Do SBL hold seatbacks in place during crashes?

This is the question that has to be asked first. If the answer

is "yes," the next question is whether casualties are mitigated. If "no,"

this is already strong evidence that SBL have limited benefits. The

primary concern raised in Section 1.6 is that the force levels experienced

in severe crashes, or even in moderately severe crashes if there are back

seat occupants, exceed the component strength requirements of Standard 207

-i.e., that SBL might not remain intact. A second issue is whether SBL,

even when they remain intact, significantly reduce the impact forces

experienced by occupants.

There is a critical difference between crashes where the back

seat is unoccupied and those in which unrestrained back seat occupants

make contact with the front seatback. When there is nobody in the back

153
seat, SBL usually accomplish their mechanical function of keeping the

seatback away from the front seat occupants at the time they contact the

steering assembly, instrument panel, etc. Table 6-1 shows that in the

MDAI accident data, the SBL or other seating components separated in only

6 percent of frontal crashes with Delta V 15-20 mph and 12 percent with

Delta V 20-25 mph. In the 25-40 mph range, about 30 percent of the seats

had a component separation, most frequently the SBL not holding the

seatback. Table 6-1 also shows that seating components remained intact in

6 sled tests at 24-32 mph Delta V, with the back seat unoccupied. In

other words, for the overwhelming majority of serious injuries and a

substantial majority of the fatalities, seating components remain intact.

Even when seat components remain intact, excessive deflection of the

seatback may defeat the action of SBL. The sled tests, however, showed

that the SBL "caught" the seatbacks before they had deflected half the

distance from their original position to the steering wheel: this was

adequate to prevent any contact between the seatback and front seat

dummies at the time they were impacting frontal structures in the vehicle.

Although SBL successfully keep seatbacks in place when the back

seat is unoccupied, they are less successful in reducing impact forces on

front seat occupants. The sled test films, as described in Section 2.2,

showed that the seatback in a car without SBL does not make contact with

front seat occupants until fairly late into their impacts with the

steering assembly and instrument panel. The additional load of the

seatback may have added slightly to the loads experienced by the left

front dummy's thorax, but over an extended time period and without any

154
TABLE 6-1

FRONT SEAT SEPARATION* RATES, 2 DOOR CARS EQUIPPED WITH SBL,

AS A FUNCTION OF DELTA V, IN FRONTAL CRASHES

MDAI

(Table

5-3)

SLED TESTS

(Section

2.2)

MDAI

(Table

5-3)

SLED TESTS

(Section

2.2)

Percent of Cars with

Delta V (mph) Seat Separation*

BACK SEAT UNOCCUPIED

Less than 10

10-14.9

15-19.9

20-24.9

25-29.9

30-39.9

40 or more

24** (2 tests)

28.4** (3 tests)

32** (1 test)

1

3

6

12

26

31

45

0

0

0

BACK SEAT OCCUPANT CONTACTED FRONT SEATBACK

Less than 10

10-14.9

15-19.9

20-29.9

30 or more

16** (1 test)

24** (4 tests)

28.4** (3 tests)

5

21

35

47

88

0

0

100

*Note: The front seat "separated" if any of the following occurred: (1) An

SBL broke. (2) The seat tore loose from one of the tracks. (3) The

seat tore loose from the floor. (4) The front seatback tore loose from

the seat.

**In the sled tests, Delta V averages 7.2% more than the impact speed.

155
noticeable spike. The seatback just feebly scrapes against the right

front dummy.

When there are back seat occupants, SBL often do not remain

intact in the crash. Table 6-1 shows that SBL or another seat component

separated in 35 percent of frontal crashes with Delta V 15-20 mph, 47

percent with Delta V 20-30 and 88 percent of crashes with Delta V over

30. Likewise, seat separation occurred in all three sled tests at a Delta

V of 28.4 mph. Seat components do np_t remain intact in the overwhelming

majority of fatal crashes and in about half the crashes that would result

in serious injuries. What is worse, the sled tests showed that even when

seat components remain Intact, the back seat dummies deflect the seats

just as far forward as when there are no SBL and are just as likely to

ramp up the deflected seatback into the front part of the passenger

compartment. Although SBL somewhat modify occupant trajectories, they do

not reduce the severity of the impacts. (Performance was better, however,

especially at low speeds, when the back seat occupant was a child or

weighed less than 100 pounds - see Table 5-4).

Based on these considerations, SBL cannot be expected to have

much overall effect on occupant injuries, regardless of whether the back

seat is occupied.

6.2 Do SBL reduce fatalities and injuries in crashes?

6.2.1 All occupants - all crashes

Table 6-2 presents the overall casualty reduction that was

156
TABLE 6-2

OVERALL CASUALTY REDUCTION FOR SBL

(2 door vs. 4 door cars,

before vs. after installation of SBL in 2 door cars)

Effect on

Risk of

Net, Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

+1 ±2 ±3 ±4

FARS

(Table

4-1)

EARS

(matching

make/models;

Table 4-2)

WASHINGTON

(Table

3-5)

WASHINGTON

(matching

make/models;,

Table

3-6)

TEXAS

(Table

3-10)

Fatalities

Fatalities

K + A (serious) inj.

K+A+B (moderate) inj.

Any injuries

K + A injuries

K + A + B injuries

Any injuries

K + A injuries

K + A + B injuries

Any Injuries

-15*

-14*

-1.

-6

none

+5

_2

none

+12*

+6

+3

-9*

-9*

-4

-6*

-2

-2

-3

-1

none

-1

none

-11*

-11*

-11*

-8*

-2

-7

-4

-1

-4

-6*

-3

-10*

-11*

-12*

_g*

-4*

-9

-4

_2

-5

-7*

-4*

*Statistically significant effect (two-sided alpha - .05)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

157
measured for SBL in three accident data files: FARS, Washington State and

Texas. Since almost all the results in the table are negative, there is

certainly little evidence that SBL reduced casualties. Is there any

reason to believe that SBL increased them?

The FARS data provide the most consistently negative results.

The sequence of effectiveness estimates (fatality reduction in 2 door cars

relative to the control group of 4 door cars, as defined in Sections 3.1

and 4.1) was -15 percent for cars of the first model year with SBL vs.

cars of the last year before SBL, -9 percent for the +2 model year

comparison, -11 percent for ±3 model years and -10 percent for ±4 model

years. The sequence exhibits no drift in either direction and all results

are significantly different from zero (as defined in Sections 3.1.3 and

4.1.3, with two-sided alpha = .05). Even when the analysis 1s limited to

"matching makes and models" (second line of Table 6-2) - i.e., sporty 2

door cars and station wagons are removed from the data - the results are

virtually the same. Thus, FARS clearly points to a negative "effect" on

the order of 10 percent. The effect, however, can almost entirely be

explained by what happened to occupant ejections in the model years that

SBL were implemented. Table 6-9 shows that the effectiveness model used

in this evaluation attributes a 20-30 percent negative "effect" on

ejections. (The reasons for the effect are discussed in Section 6.3.)

Since close to 30 percent of fatalities in cars of the 1960's were

ejectees, the 20-30 percent relative increase in ejections explains most

if not all of the 10 percent net fatality increase. In other words, SBL

had little or no effect on the fatality risk of persons who remained

158
inside the car - there is certainly no evidence of a positive effect in

FARS.

The results for nonfatal injuries ought to differ from those on

fatalities, since only a small proportion of nonfatal injuries are

ejectees. Indeed, the sequences of effectiveness estimates for the State

data files in Table 6-2, unlike FARS, typically show a trend from little

or no effect in the +1 model year comparison to significant negative

results in the +4 year comparison. In Washington, the trend is stronger

when all cars are included in the comparison and weak when the data are

limited to matching makes and models. In both cases, however, the trend

line appears to start from approximately zero effectiveness. The Texas

data are a little more perplexing, since they start with a positive effect

in the ±1 year comparison, significant at the .05 but not the .01 level,

quickly drop to zero in the +2 year estimate, and finally display a more

gradual negative trend 1n the +3 and +4 year estimates. The most likely

conclusion, based on comparison with the other data files as well as the

discussion in Section 6.1, is that the positive effect in the ±1 year

comparison is spurious, due to the limited sample size, and is cancelled

out by the additional data in the ±2 year comparison. The remainder of

the Texas sequences looks just like the Washington results. The most

appropriate conclusion is that SBL had little or no effect on nonfatal

injuries.

6.2.2 Frontal crashes

Table 6-3 shows the effectiveness results from FARS and Texas

159
TABLE 6-3

CASUALTY REDUCTION FOR SBL IN FRONTAL CRASHES

Net Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

FARS

(Table 4-8,

4-10)

TEXAS

(Table

3-10)

NEW YORK

(Table

3-12)

SLED TESTS

(Table

2-5)

Effect on

Risk of

Fatalities

Nonejected fatalities

K + A injuries

K + A + B injuries

Any injuries

Head minor bleed.

Head pain

Neck/back pain

Leg pain

Head contusion

Head severe bleed.

Torso pain

Effect on

HIC

Head g's

Mean Strain Criterion

Chest g's

Chest Severity Index

Left femur load

Right femur load

±1

-9

-6

+16*

+8

+8*

-8

-18

-21

-2

_]

+5

+10

Reduction

for SBL (%)

+12

+2

+12

-4

-3

none

_]

±2

•'

-3

none

+6

+3

+3

none

-3

-20

+6

-13

+1

+13

±3

-8*

-5

+2

-2

+1

-4

-6

-8

+8

-9

_3

+12

±4

-8*

-5

+1

-2

none

-6

-9

-11

+11

-15

-9

+13

*Statistically significant effect (two-sided alpha = .05)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

160
when the data were limited to frontal crashes as woll as the sled test

results, all of which are based on simulated frontal crashes. The

Washington data could not be subdivided by crash mode. The FARS results

are almost identical to those for all crashes, except that they are

slightly less negative, because ejection accounts for a smaller (yet

substantial) percentage of the frontal fatals than in the other crash

modes. The effect for persons who are not ejected is close to zero.

The Texas results for frontal crashes are likewise less

negative than for all crashes. Here, however, the difference is not

explained by the ejectees, which only account for a small proportion of

the nonfatal injuries. Furthermore, unlike the consistently negative FARS

results, the Texas sequences start out fairly positive and do not work

their way down to zero until the ±3 or +4 year estimate. They are the

closest thing to a positive effect for SBL of all the accident data

reviewed in this evaluation. Now, if the true effect of SBL were zero,

the laws of probability suggest that approximately half of the estimates

from all data files should be positive and 1 in 40 should even be positive

and (spuriously) significant (two-sided alpha - .05). In that light, the

Texas results, none of which are significant at the .01 level, cannot be

viewed with great optimism.

The New York data did not show significant changes for any of

the more common injury types in frontal crashes.

The sled test data, for all seat positions combined, did not

161
show a statistically significant effect for SBL on any injury criterion -

i.e., the observed effects of SBL were small relative to natural test to

test variability. The experimental design for the sled tests is described

in Section 2.1 and the method for testing statistical significance, based

on matched pair comparisons of tests with and without SBL, is documented

in Section 2.3.2. Table 6-3 shows positive, but nonsignificant effects of

2-12 percent on the three head injury criteria. Importantly, though, the

effects were slightly negative on peak chest g's and sustained chest g's

as measured by the Chest Severity Index. Section 1.6 explained that one

of the primary potential benefits of SBL could be a reduction of chest

injury severity. The reduction was not achieved and the sled tests also

showed why (as described in Section 6.1). The sled tests showed virtually

no effect on femur loads.

6.2.3 By injury type

The New York State data indicate each person's most serious

specific injury and allow effectiveness estimates by injury type (see

Section 3.4). Table 6-4 compares the sled tests and the New York results. Whereas the sled tests had generally positive, although nonsignificant results for head injury criteria, the New York estimates on head

injuries were consistently negative, sometimes even significantly so,

especially concussions, severe bleeding and minor bleeding. Conversely,

while the sled tests had slight, nonsignificant negative numbers on chest

injuries, the New York data showed modest, nonsignificant positive

results. In both cases, the nonsignificant results from the two files

cancel each other out, suggesting little or no effect for SBL. Both data

162
TABLE 6-4

CASUALTY REDUCTION FOR SBL - BY INJURY TYPE

Net Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

NEW YORK

(Table

3-11)

SLED TESTS

(Table

2-5)

Body

Region

HEAD

TORSO

LEG

ARM

Neck/back

All over

Body

Region

HEAD

TORSO

LEG

Injury

Type

CONCUSSION

SEVERE BLEED,

minor bleed,

pain

contusion

abrasion

INTERNAL

pain

FRACTURE

pain

contusion

minor bleed,

abrasion

FRACTURE

pain

minor bleed,

contusion

pain

pain

Injury

Crit.

HIC

Head g's

MSC

Chest g's

CSI

L femur load

R femur load

±1

-53

-10

—9

-3

+10

-3

-24

+1

+14

+ 1

-12

+1

-19

+27

+9.

-24

-13

-18

+52*

Reduction

for SBL (%)

+12

+2

+12

-4

-3

none

-1

±2

-20

-11

-5

+2

-12

_7

+4

+2

-8

+2

-16

none

-16

+18

+ 11

-22

-24

-13*

+12

±3

-14

-12

-9*

_7

-6

-20

+6

+3

+ 11

+2

-9

-2

-21

+20

+2

-24

-31

-6

-3

±4

-16

-17*

-9*

-7

-10

-13

+14

+4

+ 1

+5

-6

-10

-10

+10

+4

-17

-24

-7

none

*Statistically significant effect (two-sided alpha - .05)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

1,63
sources show little or no effect on leg injuries. The New York results on

arm injuries are mixed, with positive numbers on fractures and pain

injuries but negative ones on minor bleeding and contusions; all effects

are nonsignificant and the best conclusion is that there was little or no

effect.

6.2.4 By seat position

FARS, Washington and the sled test data can be subdivided into

the three fundamental types of occupants: those sitting in the front seat

with nobody sitting behind them, front seat occupants with somebody behind

them, and back seat passengers. Table 6-5 shows that front seat occupants

with nobody behind them had a consistent, significant fatality increase of

about 11 percent in FARS. As described in Section 6.2.1, the fatality

increase is almost entirely attributable to the "effect" of SBL on

ejections. In Washington, the observed effects are close to zero in the

+1 model year comparison and become gradually worse thereafter, suggesting

a vehicle age bias and little or no true effect for SBL. The sled test

results are mixed. Nonsignificant improvements on two of the four head

injury criteria were offset by comparable aggravation of the other two.

Marginal improvement on the chest injury criteria was accompanied by

nonsignificant increases in femur loads, suggesting that SBL have little

effect on torso injuries. The results are consistent with the discussion

in Section 6.1, which suggested that SBL, at best, resulted in a marginal

amelioration of chest impacts with the steering assembly and had little or

no effect on chest impacts with the instrument panel.

164
TABLE 6-5

CASUALTY REDUCTION FOR SBL

-FRONT SEAT OCCUPANTS WITH NOBODY SITTING BEHIND THEMNet Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

FARS

(Table

4-5)

WASHINGTON

(Table

3-7)

SLED TESTS

(Table

2-6)

Effect on

Risk of

Fatalities

K + A injuries

K + A + B injuries

Any injuries

Effect on

HIC

Head g's

Mean Strain Criterion

Tot. Lacerat. Index

Chest g's

Chest Severity Index

Left femur load

Right femur load

±1

-17*

none

-5

+1

Reduction

for SBL (%)

-12

-17

+12

+14

+5

+ 1

-18

-11

±2

-11*

-6

-9*

_2

±3

-13*

-11*

-9*

-3

±4

-11*

-13*

-11*

-5*

*Statistically significant effect (two-sided alpha = .05)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

165
Table 6-6 presents results on front seat occupants with

somebody sitting behind them. The overall effects in the accident data

are basically similar to those in Table 6-5, except for the larger

variation due to the smaller sample size. In other words, the increase in

fatality risk is fairly consistent and close to 10 percent. The sequences

for moderate to serious (K + A + B) and overall injury risk appear to

converge on zero. The effects for serious injuries, however, are consistently negative, although nonsignificant. The statistical results from

the sled tests mirror what was observed in the films - viz., that SBL do

change the trajectories of occupants, but without much net effect on

injury severity. The head injury readings were generally improved with

SBL, although not significantly so, while the chest injury measures

worsened. In particular, the peak head g's for the interaction between

the rear and front seat occupants were reduced by 28 percent but the peak

chest g's for that interaction increased by 25 percent (see Section 2.2

for more discussion).

The sample sizes of back seat occupants were too small for

fine-tuned results; nevertheless Table 6-7 makes it clear that SBL had no

major effects. The sequences of effectiveness estimates for fatalities

and serious injuries shuttle between positive and negative. The results

for moderate and minor injuries, after a bad start in the ±1 year comparison, seem to converge on zero. The sled test findings are inconsistent,

with zero or negative effects on one of the criteria for each of the three

body regions and nonsignificant positive effects for the other criteria

for each region.

166
TABLE 6-6

CASUALTY REDUCTION FOR SBL

-FRONT SEAT OCCUPANTS WITH SOMEBODY SITTING BEHIND THEMNet Effect of SBL (%)*

Number of Model Years before/after

SBL Installation in 2 Door Cars

FARS

(Table

4-5)

WASHINGTON

(Table

3-7)

SLED TESTS

(Table

2-7)

Effect on

Risk of

Fatalities

K + A injuries

K + A + B injuries

Any injuries

Effect on

HIC

Head g's from:

any contact

1st contact

later contacts

Mean Strain Criterion

Tot. Lacerat. Index

Chest g's from:

any contact

1st contact

later contacts

Chest Severity Index

Left femur load

Right femur load

±1

-20

-23

-7

-10

Reduction

for SBL (%)*

+20

+2

+1

+28

+19

-2

-12

+2

-25

-32

+10

+17

±2

-5

-10

+6

-4

±3

-8

-15

+2

-2

±4

-11

-13

+2

-2

*Note: positive numbers indicate favorable results for SBL; negative

numbers indicate unfavorable results for SBL. None of the observed

effects was statistically significant (two-sided alpha - .05).

167
TABLE 6-7

CASUALTY REDUCTION FOR SBL

-BACK SEAT OCCUPANTS-.

Effect on

Risk of

Net Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

±1 ±2 ±3 ±4

FARS

(Table

4-5)

WASHINGTON

(Table

3-7)

SLED TESTS

(Table

2-8)

Fatalities

K + A injuries

K + A + B injuries

Any injuries

Effect on

HIC

Head g's

Mean Strain Criterion

Chest g's

Chest Severity Index

Left femur load

Right femur load

+2

-7

-24

-12

Reduction

for SBL (X)

+21

+ 12

none

+20

+8

-24*

+7

+11

-3

+1

+5

-16

-9

none

-14

-4

-2

*Statistically significant effect (two-sided alpha = .05)

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative

numbers indicate that SBL did worse than the control group.

168
In the special case where the back seat occupant is a child up

to 12 years old, impact forces on the front seatback are less severe due

to the child's smaller mass and seat separation is less likely to occur -

see Table 5-4. Nevertheless, Washington (Table 3-9) and FARS (Table 4-14)

data do not associate with SBL any casualty reductions for the children or

for the persons sitting in front of them. Both these analyses, however,

are based on samples too small to be statistically meaningful.

6.2.5 Restrained occupants

The sled tests showed that SBL allow a substantial amount of

forward deflection of the seatback in frontal crashes. Seatbacks deflected half the distance from their original position to the steering wheel

even when there was no back seat dummy adding its load to the momentum of

the seatback. It is reasonable to expect that SBL would have little

benefit for the belted front seat occupant, who remains within the area

where the seatback is displaced before it is caught by the SBL. Indeed,

Table 6-8 shows no benefits for SBL. Fatality reduction, after a positive

start, converges on zero: since none of the other FARS sequences showed a

year to year trend against SBL, it should be concluded that the sequence

in Table 6-8 reflects a zero effect and small sample sizes in the first

two estimates, rather than a positive effect and a trend against SBL. The

Washington results for serious and moderate injuries are consistently

negative, although nonsignificant. The results for overall injury

reduction are close to zero throughout the sequence.

169
TABLE 6-8

CASUALTY REDUCTION FOR SBL

-BELTED OCCUPANTS-

(2 door vs. 4 door cars,

before vs. after installation of SBL in 2 door cars)

Net Effect of SBL (%)*

Number of Model Years before/after

SBL Installation in 2 Door Cars

FARS.

(Table

4-7)

WASHINGTON

(Table

3-8)

Effect on

Risk of

. Fatalities

K + A injuries

K + A + B injuries

Any injuries

±1

+26

-16

-19

+2

±2

+20

-6

-11

none

±3

. +3

-25

-11

-1

±4

+1

-21

-8

-1

*Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative numbers indicate that SBL did worse than the control group. None of the observed net effects were statistically significant (two-sided alpha - .05).

170
6.2.6 Summary

This preliminary evaluation analyzes numerous sled tests and

over a million ' accident cases, but none of the data showed significant

benefits for SBL. Statistically, it is easy to prove definitively that a

safety device is effective but difficult to prove that it is not. That is

because statistical methods typically test a null hypothesis that a safety

device is not effective. If the statistically significant positive

effects are found, the null hypothesis is rejected - i.e., there is

convincing evidence that the safety device is effective. But the lack of

statistically significant positive results is not, by itself, convincing

evidence that the null hypothesis is true and the device is not effective. For that reason, this study, none of whose results show effectiveness, is called a "preliminary" evaluation. There might still be some

limited, narrowly defined portion of the crash environment where SBL are,

in fact, beneficial, but there are simply not enough data of that type of

crash for a statistically meaningful effectiveness analysis. Specifically, it seems logical that SBL might be beneficial for children riding in

the back seat in moderate speed frontal crashes: the SBL may withstand the

relatively light impact load of a child (see Table 5-4) and the seatback

may retain the child within the safer rear half of the passenger

compartment, whereas in a car without SBL, the child might be propelled

over the folded seatback and contact the windshield, header, instrument

panel or a front seat occupant. Even though the analyses of this evaluation do not show any fatality or injury reductions with SBL for child back

seat occupants, the sample size for this limited crash situation is too

small for the results to be convincing evidence that, in fact, SBL are not

171
effective there. Similarly, it seems logical that SBL might account for a

modest reduction of nonfatal injuries in low to moderate speed frontal

crashes (Delta V 15 mph or so), especially when there are adults in the

back seat; as above, the available data did not show a significant positive effect but do not preclude the possibility that there is, in fact, a

modest positive effect. These crash situations could be possible topics

for further study. Nevertheless, the data make a strong case that SBL

have little or no overall effect on fatalities and injuries.

6.3 Do SBL affect the risk of occupant ejection?

Without doubt, the most decisive finding in the accident data

was that the risk of occupant ejection fell dramatically in 4 door cars in

the model years that SBL were introduced in 2 door cars, but the risk did

not decrease in the 2 door cars until several model years later. Since

the effectiveness measure used with the accident data is based on the

change in 2 door cars relative to the change in 4 door cars, large,

significant negative "effects" were consistently generated for SBL.

Obviously, the big reduction of ejection in 4 door cars had nothing to do

with SBL. But the failure to achieve a similar reduction in 2 door cars

is conceivably due to a negative effect of SBL in 2 door cars which masks

the positive effect seen in the 4 door cars. Section 1.6 presented a

hypothesis as to why SBL could increase ejection risk for front seat

occupants: without a folded seatback blocking the way, the occupant has an

easier path to an open door. Intuitively, though, such an effect, if any,

would not be large.

172
How large are the observed "effects" for SBL? Table 6-9 shows

that the effect is always statistically significant In FARS and is

consistently near -25 percent, for all occupants. For front seat occupants alone, it approaches -30 percent and perhaps even -35 percent for

front seat occupants with somebody sitting behind them. When the data are

separately analyzed by car size or by company (as in Table 4-13), though,

there 1s a suggestion that the preceding results may have an additional

bias against SBL due to certain market shifts and that the effect for

front seat occupants may be in the -20 to -25 percent range. The Washington data show a consistent, significant -30 percent effect for SBL;

however, when the data set is limited to matching makes and models, the

effect is closer to -20 percent. The NCSS-MDAI data, after a completely

anomalous start, rapidly converge to a similar value. The net effect on

ejections seems to be in the -20 to -30 percent range.

A critically important finding in the NCSS and MDAI data,

however, is that the likelihood of doors opening in crashes decreased

sharply in 4 door cars in the model years that SBL were introduced in 2

door cars, but it did not drop in 2 door cars until several years later -

exactly what was seen in the ejection rates. The lower part of Table 6-9

shows that the change in 2 door cars relative to the change in 4 door cars

was consistently on the order of -30 percent. The likelihood of doors

opening in crashes presumably has nothing to do with SBL but 1t is closely

related to the risk of occupant ejection. Section 5.3 examined door

modifications of the I9601s and concluded that major door latch improvements in 1967-68 (nearly coinciding with the implementation of SBL) are

173
TABLE 6-9

OBSERVED "EFFECTS" OF SBL ON OCCUPANT EJECTION

AND ON DOORS OPENING DURING IMPACTS

(2 door vs. 4 door cars,

before vs. after installation of SBL in 2 door cars)

Net "Effect" of SBL (1)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

FARS

(Table

4-6)

WASHINGTON

(Table 3-4)

(Table 3-6)

NCSS-MDAI

(Table 5-1)

NCSS-MDAI

(Table 5-2)

Type of

Occupant ±1

EJECTIONS

All ejectees

Front seat,

nobody behind them

Front seat,

somebody behind them

Back seat

All ejectees

Matching make/models

All ejectees

-42*

-38*

-77*

-43

-41*

-12

+57

±2

-26*

-30*

-37*

+4

-30*

-17

+25

DOORS OPENING DURING IMPACTS

Vehicle Type

All vehicles -25 -31

±3

-24*

-28*

-30

+1

-25*

-10

+13

-25

±4

-23*

-25*

-35*

-13

-35*

-20

-12

-37*

*Statistically significant effect (two-sided alpha - .05)

**Note: positive numbers Indicate that the SBL group (2 door i

greater improvements than the control group (4 door cars); i

numbers indicate that SBL did worse than the control group.

174
responsible for the dramatic reduction of door opening in 4 door cars.

These same Improvements were made in 2 door cars but were apparently not

nearly as effective. The intuitive support for the conclusion 1s that the

doors of 4 door cars are significantly narrower than in 2 door cars.

Wider doors have a greater tendency to bow at the time of Impact and

separate the latches from the frame. The 1967-68 improvement may have

made the latches on the 4 door cars strong enough to resist all but the

most severe impacts, while only beginning to solve the problem for 2 door

cars.

If the effect of the latch Improvements on door opening in 2

door cars relative to 4 door cars 1s -30 percent, it would explain a 20

percent relative Increase 1n ejection risk, since two-thirds of the

ejections in cars of the I9601 s were through the doors. Thus, 1f the

observed "effect" of SBL on ejections is close to 20 percent, as many of

the data suggest, it could be attributed entirely to the door latch

modifications and actually have nothing to do with SBL. But 1f the

"effect" of SBL on ejections 1s higher than 20 percent - as it may be in

the FARS data on front seat occupants with somebody sitting behind them -

the possibility of a small but genuine negative effect for SBL cannot be

ruled out.

Table 6-9 shows that the "effect" of SBL on the ejection of

back seat occupants may be converging on -10 to -15 percent in FARS. If

this number is slightly less negative than the effect on ejection that

would be attributed to the door latch modifications, 1t could indicate a

175
small but genuine positive effect for SBL, possibly canceling out part of

the negative effect for the front seat occupants.

6.4 Do SBL affect fatality risk in car fires?

Section 1.6 raised a concern that the old style of manually

operated SBL could impede the egress of back seat passengers from a 2 door

car in emergency situations such as vehicle fires. But SBL might have

modest benefits for front seat occupants in post-crash fifes because it

would keep folded over seatbacks out of their way. FARS is the only data

file that contains a large number of cases involving vehicle fires and,

even in FARS, the sample is not large enough for statistically significant

results. Nevertheless, there are some consistent trends 1n Table 6-10.

Table 6-10 shows a consistently negative, although nonsignificant, effect for manual SBL on the likelihood of a back seat occupant

dying in a vehicle that caught fire, as was feared in Section 1.6. The

effect seems to be converging to about -30 percent, but it is important to

note that none of the estimates are statistically significant, even at the

.1 level. Intuitively, this effect would not be expected in domestic cars

of the 1980's with automatic inertial SBL, since the back seat occupant

would be able to fold the seatback without first having to disengage the

SBL.

Table 6-10, however, also shows a highly positive, although

nonsignificant effect of SBL for front seat occupants with somebody

sitting behind them: on the order of 50 percent. The estimate is based on

176
TABLE 6-10

EFFECT OF SBL ON FATALITY RISK

IN CRASHES INVOLVING FIRES*

(2 door vs. 4 door cars,

before vs. after installation of SBL in 2 door cars)

Effect on

Fire Crash

Fatality*

Risk of

Net Effect of SBL (%)**

Number of Model Years before/after

SBL Installation in 2 Door Cars

±2 ±3 ±4

FARS

(Table

4-11)

All occupants

Front seat occ.,

nobody behind them

Front seat occ ,

somebody behind them

Back seat occupants

-21

-28

+40

-23

none

_3

+68

-64

+4

+2

+50

-34

+6

+4

+57

-32

*Persons who were killed in a vehichle that caught fire. FARS, however,

does not specify whether the person's fatal injury was due to the fire

or to other causes.

**Note: positive numbers indicate that the SBL group (2 door cars) had

greater improvements than the control group (4 door cars); negative nunv

bers indicate that SBL did worse than the control group. None of the observed net effects were statisticaily significant (2-sided alpha - .05).

177
small samples, especially for pre-SBL cars, and far exceeds the benefit

that could intuitively be expected. The net effect for all seating

positions combined is close to zero. The most reasonable conclusion,

based on Table 6-10, would appear to be that SBL have had little net

effect on fire fatalities and that a possible negative effect for back

seat occupants may have been offset by slightly easier egress for the far

more numerous front seat occupants. Of course, the conclusion applies

only to the old type of manual SBL. For current automatic SBL, the effect

.1

should be zero since the seatback moves freely, like in a pre-SBL car,

except during the moment of impact.

178
APPENDIX A

SLED TEST RESULTS

179
S U G TEST RESULTS

CONTENT* OF S*S 0*TA SET WORK.NCR

10:56 THURSDAY, JULT 10, 1986

00

o

TRACKS USED-1 SUBEXTENTS«1 OSSERVATIONS«6* CREATED Sr OS JOB KHEUUR1 ON CPUtO 20-3090-170*03

AT 10I5B THURSDAY, JULY 10, 1986 tX SAS RELEASfc 82.4 0SNANE»SYS84191.T105833.RA000.KHE«UR1.RO0O0O01 BLKSIIE«23396

LRECL-172 OBSERVATIONS PER TRACK-272 GENERATED &Y DATA

ALPHABETIC LIST OF VARIABLES

1NF0RMT LABEL

PEAK CHEST «'S FROM ANY CONTACT

PEAK CHEST G*S FROM 1ST CONTACT

PEAK CHEST C'S FROM LATER CONTACTS

CHEST SEVERITY INOEX

DELTA V CNPh)

PEAK HEAD C'S FROM ANY CONTACT

PEAK HEAO C'S FR?M 1ST CONTACT

PEAK HEAO C'S FROM LATER CONTACTS

hEAO INJURY CRITERION

LEFT FEMUR LOAD CPOUNOS)

WITH OR WITHOUT SEAT BACK LOCKS

LEFT OR RIGHT SIDE

MEAN STRAIN CRITERION

PRINCIPAL DIRECTION OF FORCE

RIGHT FEMUR LOAD (POUNDS)

SEAT POSITION

SLED G'S

ACTUAL IMPACT SPEED (MPH)

TARGETED IMPACT SPEEO CMPH)

CONTRACTOR'S TEST NUMBER

TOTAL LACERATION INOEX

»

13

14

IS

9

3

10

11

12

8

16

6

7

19

5

17

21

4

2

20

1

18

VARIABLE

CHEST

CHESTE

CHESTL

CSI

OV

HEAO

HEAuE

HEAOL

HIC

LFEM

LOCKEO

LR

MSC

PDOF

RFEM

SEATPOS

SLECG

SPEEC

TAftbET

TESTNO

TLI

TYPE

NuH

NUM

NJM

NUH

nor

HUf

H U H

NUH

NUM

NUM

NUH

NUH

NUH

HUM

NUM

NU*

NUM

NUM

SUM

NUR

NUM'

LENGTH

*

S

a

a

8

a

b

s

o

ai

6

:

6

8

S

9

6

S

POSITION

iCO

108

116

;o76

a*

92

60

124

44

52

1*8 .

36

132

164

26

12

lie

140

FORMAT

OATAFS.

3ATAF$.

OATAFS.

0ATAF5.

0ATAF5.

OATAFS.

OATAFS.

LOCKEOFB.

LRFS.

PO0FF13.

SEATPCSF20

DATAF5.
SLED TEST RESULTS 10158 THURSDAY* JULY 10* 1986

00

BS

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

If

19

20

21

22

23

24

25

26

BS

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

TESTNO

2918

2918

2921

2921

2924

2924

2927

2927

2935

2935

2938

2938

29*1

2941

2944

2944

2947

2947

2950

2950

2957

2957

2457

2957

2962

2962

LR

LEFT

RIGHT

LEFT

LEFT

LEFT

LEFT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIuHT

LiFT

rflGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LOCKED

NO SSL

NO SbL

NO SSL

NO SbL

UITH S&L

UITH SSL

NO S6L

NO S8L

UlTh SSL

UITH SSL

UITH SSL

UITH SBL

NO S3L

NO SdL

NO SSL

NO SDL

NO SbL

NO SSL

UITH SDL

UITH SSL

UITH SOL

UITH SfcL

UITH SttL

UITH SBL

WITH SdL

UITH SsL

HIC

213

146

131

56

85

33

533

195E

477

218

351

298

321

459

433

345

543

42?

762

J7t

57o

327

237

H i

3S>7

4307

HEAD

43.P

50.3

44.1

io.O

32.3

20.7

65.8

76.1

6o.«

54.9

74.2

oO. 1

Sa.O

S7.6

78.4

56.1

66.11

59.7

95.J

!>•».*

<<4. 4

42.4

71.1

46.6

o4,7

30.;

TARGET

22.5

22.5

15.0

15.0

15.0

15.0

30.0

30.-'

26.5

26. S

26.5

26.5

26.5

26.5

2o.5

26.5

2b. 5

2o. 5

2 6. 5

26.5

26.5

26.5

i6. 5

26.5

26.5

26.5

MEACE

h.

H.

S.

32

H.

N.

N.

N.

N.

N.

N.

N .

ts.

N.

N.

N.

N .

N.

U.

0 *

42

N.

N.

64

JO

A.

A.

.1

A.

.3

A.

A.

A.

A.

A.

A.

i.

S.

A.

A.

M m

A.

i.

A.

•i *

.4

.*

A.

A .

.7

.5

SPEED

22.2

22.2

14.8

14.8

14.9

1*.9

«'>.6

29.o

25.7

25.7

26.9

26.9

^6.5

26.5

26.6

26.5

26.3

26.3

26.5

26.5

26.5

2 6. 5

26.5

26.5

io.5

26.5

HEAOL

N.

N.

N.

l».

M.

N.

N.

H.

N.

«,.

f«.

N .

H.

N.

N.

M.

•, .

N.

N.

A.

A.

30

a.

13

A.

A.

A.

A.

A.

A.

A.

A.

A .

A.

A.

A.

A.

A.

A.

S5

0

A.

A.

45

20

OV SLEDG

24.00 11.0

24.00 11*0

16.40 9.0

16.40 9.0

16.80 8.5

16.80 8.3

31.60 14.5

31.60 14.5

28.20 14.0

ZS.ZO 14.0

28.90 14.0

28.90 14.0

28.50 14.5

28.50 14.5

28.10 14.0

28.10 14.0

27.80 14.0

27.80 14.0

26.70 14.0

26.70 14.0

28.30 13.0

28.30 13.0

28.30 13.0

23.30 13.0

27.90 13.5

27.90 13.5

HSC

0.005384

0.003255

0.092247

0.001757

0.001458

0.001773

0.004218

0.019313

0.005010

0.002955

0.005365

0.002931

0.003657

0.008210

0.004447

0.003827

0.003477

0.0090*5

0.003367

0.003070

0.00*217

0.005*67

0.003598

0.002983

0.004355

0.024524

PDOF

12:00

12:00

12:00

12(00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

TLI

6.3

7.7

0

N . A.

0

N. A .

5

10.3

3.7

5.3

5.4

5.6

5.1

5.2

4.8

8.5

3.9

1.9

4.9

5.7

3.4

4.8

N. A.

N. A.

5.2

5.3

FRONTAL'

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

CHEST

41.2

28.8

34.5

22.3

23.3

12.8

54.8

79.2

53.9

50.9

51

75.8

47.7

70.1

67.7

67.S

67.7

73.5

54.7

55.6

72.6

74.1

45.9

12.4

73.8

78.1

SEATPOS

FRONT,

FRONT,

FRONT,

NOBODY BEHINO

NOBODY BEHIND

SOMEONE BEHINO

BACK SEAT PASSENCER

FRONT,SOMEONE BEHINO

BACK SEAT PASSENGER

FRONT,

FRONT,

FRONT,

FRONT,

FRONT,

FRONT,

FRONT,

FRONT,

FRONT,

FRONT,

FRONT,

FRONT,

FRONT,

FRONT,

NOBODY BEHINO

NOBODY BEHINO

NOBODY BEHINO

NOBGOY BEHIND

NOBODY BEHINO

NOBODY BEHIND

NOSCOY 8EHIN0

NOBODY BEHINO

NOBCCV BEHINO

NOSOCY BEHINO

NGS02Y BEHIND

N0B02Y BEHINO

NO&OOT BEHINO

NOBOJY BEHIND

FRONT,SCMEQNE BEHINO

FRONT,SOMEONE BEHINO

SACK SEAT PASSENGER

SACK SEAT PASSENGER

FRONT,SOMEONE BEHINO

FRONT,SOMEONE BEHIND

CHESTE

N . 1

N . 1

34.

N. 1

23.

N . 1

N . /

N . 1

H. 1

N . i

H. I

it. t

H. 1

N . 1

N . 1

M. I

H. 1

N . i

N. /

N.

m

5

3

.

I.

I.

72.6

74.1

N . A.

N. A.

50

78. 1

CHESTL CSI

N . A. 178

N. A. 148

IS 127

N. A. 40

3 64

N . A. UNK.

H. A. 367

H. A. 733

H. A. 305

•>. 1. 308

S. A. 355

N. A. 595

N . A. 288

s. A. 679

N. A. 498

N. t. 317

fi. A. 482

-». A. 578

S. A. 400

\. A. 410

42 735

0 552

N. A. 179

N. A. 21

73.8 862

65 980

LFEH

1152

497

904

335

530

223

865

2213

1745

1349

1965

2323

2062

1418

1803

2779

2622

3668

2831

2322

1624

2702

924

669

2564

3459

- .

RFEM

1178

513

503

272

719

487

1428

1745

1935

1276

998

2251

1536

2921

1079

1873

72 5

2983

530

2133

721

1535

1035

610

562

2901
SLEC TEST RESULTS 10158 THURSDAY, JULY 10,

CO

ro

DBS

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

OSS

27

28

29

30

31

32

33

34

35

36

37

3«

39

40

41

42

43

44

45

46

47

48

49

50

51

52

TESTNO

2962

2962

2967

2967

2967

2967

2972

2972

2972

2972

2977

2977

2977

29TT

2982

2982

2982

2982

2994

2994

2994

2994

2995

2995

2995

2995

LR

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

KIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIiiHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LOCKED TARGET

WITH SSL

KITH SbL

NO SBL

NO SBL

NC SBL

NC SoL

NO SbL

NO SBL

NO SbL

fcO SBL

NO SbL

NO SBL

NC SBL

HO SbL

WITH SoL

WITH S3L

WITH SBL

KITH SSL

WITH Sal

WITH S6L

WITH SBL

WITH 33L

WITH S3L

WITH S3L

WITM S;L

WITH S3L

MIC "EAO

1167 89.1

495 91.7

640 7s.7

934 64.4

4363 130.•>

1320 70.?

226 48.j

819 103.0

521 93.4

120 0 135 . ••>

450 77.3

2723 137.6

313 76.3

1317 164.5

326 4',. I

3?4 71.3

184 61.5

534 102.0

293 51.6

376 t<0.6

321 65.0

613 =.2.5

3b!> S3. t477 36.3

734 Jo.5

73* 73.5

26.5

26.5

26.5

26.5

26. 5

26.5

26.5

26.5

26.5

26.5

26. S

26.5

24. S

26.5

26. 5

26.5

26.5

26.5

22.5

2Z.S

22.5

22.5

22.5

22.5

22.3

22.5

H? ;js

N. A.

N. A.

73.7

64.•

N . A .

N . A .

48

103

N . A .

N. {.

77.3

100

N. A.

N. A.

49.2

7 1. 3

N. A.

N. A.

'1.8

80.6

N. 1.

n. t.

93.6

^*. 3

N. A.

•». A.

SPSEO

24

26

26

>

; *

2a

24

26

t6

; J,

is

26

2 *

2*

16

26

26

.-•s

22

22

.12

22

ck

.5

.5

.5

# 5

.5

.5

.8

.8

.8

.*

. *

.2

# 2

.5

.5

.5

. 5

, 2

, ?

. 7

.7

.7

N.

H.

N.

I ).

N.

N.

ICL

A.

A.

35

60

A.

A.

10

70

A.

A.

50

1C7.6

N.

N.

1.

N.

N.

N.

N.

N.

A.

A.

44

3*

A.

A.

'5

35

A.

A.

30

55

A.

A.

OV SLEOG

27.90 13.5

27.90 13.5

2S.20 13.5

.-•O.20 13.5

2 3.. 2 0 13.5

2o.20 13.5

30.40 13.5

30.40 13.5

3C.40 13.5

30.40 13.5

27. HO 14.0

27.90 14.0

27.10 14.0

27.30 14.0

28.50 14.0

2:1.50 14.0

2ft.SO 14.0

23.50 14.0

-: 3. * o u.o

; ••.»o u. o

. 3.40 U.O

23.4C U.O

24.03 U.O

24.03 U.O

24.33 U.O

2..03 U.O

MSC

0.010712

0.017079

0.006588

0.012768

0.019244

0.014075

0.005399

0.00.7115

0.005*43

0.006705

0.007514

0.016422

0.003851

0.016311

0.003696

0.'005660

0.004123

0.009472

0.003632

0.003286

0.003590

0.003186

0.006825

0.005032

0.007263

0.010549

N

N

H

H

H

N

H

N

N

N

N

N

N

N

POCF

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

12:00

TLI

. A .

. A .

4.8

5.4

. A .

. A .

4.5

6.1

. A .

. A .

4.3

6.3

. A .

. A .

6

6

. A .

. A .

1

5.1

. A .

. A .

3.8

5.2

. A.

. A.

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

CHEST

37.8

46.1

75.5

30

69.8

24.6

50.3

53.2

42.7

25.2

59.5

55.8

38.9

29.6

56.7

60.3

42.6

42

41.7

35.9

34.6

23.2

80.3

24.9

32.7

29.9

SCATPOS

SACK

BACK

SEAT PASSENGER

SEAT PASSENGER

FRONT,SOMEONE BEHIND

FRONT

SACK

SACK

.SCXECNE BEHIND

SEAT PASSENGER

SEAT PASSENGER

FRONT,SOMEONE BfHlNO

FRONT

BACK

BACK

.SQHEONE BEHI-JO

SEAT PASSENGER

SEAT PASSENGER

FRONT,SOMEONE BEHIND

FRONT

BACK

BACK

FRONT

FRONT

BACK

BACK

FRONT

FRONT

BACK

BACK

FRC»lT

FRONT

BACK

BACK

CHESTE

N . A .

N. A.

60

80

N. A.

N. A.

50.3

53.2

N. A.

N. A.

59,5

55.8

H. A.

N. A.

56.7

60.3

N. A.

N . A.

41.7

20

N . A.

H. A.

35

24.9

N . A .

N . A .

•SOMEONE BEHIND

SEAT PASSENGER

SEAT PASSENGER

•SOMEONE 8EHIN0

• SOMEONc 8EHI.M3

SEAT PASSENGER

SEAT PASSENGER

•SOMEONE 8EHIN0

•SOMEONE BE*INO

SEAT DASSEhStR

SEAT PASSENGER

,SOMEONE BEHIND

•SOMEONE BEHINO

SEAT PASSENGER

SEAT PASSENGER

CHESTL CSI

N. A.

N . A.

148

169

75.5 1113

12

N. A.

N . A.

22

35

N . A .

N . A .

52

40

N . A .

N . A .

55

0

N . A .

N. A.

27

3S.9

N . A.

H. A .

80.3

20

N . A.

N . A.

681

346

85

310

451

311

126

436

444

211

3 50

902

4 81

188

261

274

152

134

36

941

191

113

64

LFEM

535-

649

4039

3647

363

713

1166

2875

1164

674

2056

2459

1120

560

1450

2775

233

UNK.

872

1103

579

402

1313

1829

527

510

RFEM

551

632

650

3320

542

705

987

2036

597

539

767

2933

728

623

777

2279

1332

1022

642

901

621

598

581

485

506

491
SLED TEST RESULTS 10158 THURSOAY, JULY 10, 1486

OSS TESTNO LOCKED TARGET SPEED OV SLEDS PDOF SE*TPOS

CO

GO

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

DBS

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

69

70

71

72

73

74

75

76

77

78

2996

2996

2996

2996

3002

3002

3003

3003

3004

3004

3008

3008

3008

3008

3009

3009

3009

3009

3016

3016

3016

3016

3017

3017

3017

3017

LR

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LfcFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

NO SBL

NO SBL

NO S3L

NC SSL

WITH SBL

WITH SBL

WITH SBL

WITH 58L

NO SSL

NC SSL

NO SBL

NO S8L

NO SBL

NO SSL

WITH SSL

WITH 3BL

WITH S3L

WIT* SBL

WITH SSL

wITn SSL

WITH SSL

WITH SSL

NO S6L

NO SSL

NO SBL

NO SBL

HIC HEAD

457 53

474 65

392 78

136 43

234 52

5*6 93

235 55

394 87

277 56

234 61

227 52

337 91

436 42

455 77

185 52

413 65

663 84

357 67

736 111

253 72

701 104

1336 62

13S 39

1111 79

o27 9s

J4»t |j

.3

.3

.3

.1

.5

.6

.7

.6

.6

.0

.6

.6

.5

.8

.3

.4

.2

.1

.3

• e

.3

.a

.6

.3

.1

.6

22.5

22.5

22.5

22.5

22.5

22.5

22.5

22.5

22.J

22.5

22.5

22.5

22.5

22.5

22.5

22.5

22.5

22.5

*. i- m •

22.5

22.5

22.5

22.5

22.5

22.5

22.5

HEAOE

H

N

N

U

N

N

S

N

N

1

\

N

58.3

65.3

. A.

. A.

. A.

. %.

. A.

. A.

. A .

. A.

52.6

•il . 6

. A.

. A.

C 2 . 3

S3.*

. A.

. 4.

111.3

N

N

U

u

rz.i

. A.

. A.

?9.6

7>. 3

. A.

. A.

22.5

22.5

22.5

22.5

22.4

22.4

22.7

22.7

2 2 . i

22.5

23.3

23.3

23.3

2 3.?

23.1

2? .1

23.1

23.1

23.3

23.3

23.3

23.3

22.5

22.5

22.5

2 2 . *

H6ADL

45

25

N. 1

N. 1

H. 1

N . i

N.

N . /

N. 1

N. /

N. 1

M.

N.

\.

N.

N.

N.

N.

I.

i .

i .

t.

& .

1 a

I.

k .

.0

0

I.

1.

IZ

0

I.

1.

25

15

4.

17

50

1.

a.

23.5

23.5

23.5

23.5

24.2

2*.2

24.6

24.6

2».7

24.7

24.6

24.6

24.6

24.6

24.4

2«.4

24.4

24.4

24.7

24.7

24.7

2*.7

23.5

23.5

23.5

23.5

C

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

c

0

0

0

0

0

0

0

0

11.0

11.0

11.0

11.0

12.0

12.0

11.0

11.0

11.0

11.0

10.0

10.0

10.0

10.0

11.0

11.0

11.0

11.0

11.0

11.0

11.0

11.0

11.0

11.0

11.0

11.0

MSC

.003529

.005163

.004667

.002298

.006135

.004564

.003053

.005044

.004867

.002986

.004458

.006255

.003942

.006073

.003013

.006404

.008043

.003953

.003928

.004644

.007868

.014832

.002561

.013263

.006073

.014676

N.

N.

N.

N.

N.

h.

N.

M.

N.

N.

12:00

12:00

12:00

12:00

12:00

12100

1:00

1:00

18 Off

1:00

1:00

llOO

1:00

1:00

1:00

1100

1:00

1:00

12:00

12:00

12100

12100

12:00

12J00

12:00

12i00

TLI

1

5

A.

A.

5.3

5.9

2.3

6.1

5.2

6.1

0

5.3

A.

A.

1.3

4.4

A.

A.

4.8

5.7

A.

A.

0

5.9

A.

A.

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

OBLIQUE

OBLIQUE

OBLIQUE

OBLIQUE

OBLIQUE

OBLIQUE

OBLIQUE

OBLIQUE

OBLIQUE

OBLIQUE

OBLIQUE

OBLIQUE

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

CHEST

53.4

27.8

46.5

20.6

46.5

29.5

29

40.2

39

36.6

36.7

46.4

17.7

16.6

59.7

39

33.1

27.8

52.6

72.8

UNK.

32.8

45.7

49.1

35.9

21.6

FRONT,SOMEONE BEHIND

FRONT,SOMEONE BEHIND

BACK SEAT PASSENGER

BACK SEAT PASSENGER

FRONT, NO6OOY BEHINO

FRONT, N0800T BEHIND

FRONT, NOBOOT BEHINO

FRONT, NOBODY BEHINO

FRONT, NOBODY BEHINO

FRONT, NOBODY 8EHIN0

FBONT,SOMEONE BEHINO

FRONT.SCHEONE BEHINO

BACK SEAT PASSENGER

BACK SEAT PASSENGER

FRONT,SOMEONE BEHINO

FRONT,SOMEONE BEHIND

BACK SEAT PASSENGER

BACK SEAT PASSENGER

FR3NT,S0ME0NE BEHINO

FRONT,SCMEONE BEHINO

BACK SEAT PASSENGER

SACK SEAT PASSENGER

FRONT,SOMEONE BEHINO

FRONT,SOMEONE BEHIND

BACK SEAT PASSENGER

BACK SEAT PASSENGER

CHESTE CHESTL

30 53.4

27.8 19

N. A. N. A.

ftm A*r N . A .

N . A. N. A.

N. A. N. A.

N . A. N. A.

N. A. N. A.

N. A. N. A.

N. A. K. A.

36.7 28

46.4 10

N. A. N. A.

N. A. N. A.

35 59.7

39 7

N. A. N. A.

N. A. N. A.

38 52.6

45 72.8

CSX

338

144

159

59

191

140

130

195

189

176

202

275

44

62

1012

231

67

93

329

320

N. A. N. A. UNK.

N. A. N. A.

40 45.7

49.1 19

N. A. N. A.

N. A. N. A.

79

300

257

136

62

LFEM

1276

1922

398

268

2925

1355

3445

3136

3841

1738

2108

3524

812

375

4403

4173

508

468

1344

2692

589

290

2191

3814

339

438

RFEM

712

1364

616

463

780

2436

515

1798

323

2148

509

2356

503

429

567

3378

516

476

440

2156

619

602

6 24

3195

613

422
SLEO TEST RESULTS 10158 THURSDAY, JULY 10, 1986

OBS TESTNO LOCKED TARGET SPEEQ • V SLEOG POOF SEATPOS

CO

P

79

10

81

82

83

84

08$

T9

80

81

82

83

84

3018

3018

3018

3018

3021

3021

LR

LEFT

RIGHT

LEFT

RIGHT

LEFT

RIGHT

NO S6L

NO SSL

NO SBL

NC SSL.

WITH SSL

WITH S8L

HIC

167

3068

113

413

696

1629

HEAO

33.0

67.0

41.6

72.5

61.8

9S.2

22.5

2 2. S

22.5

22.5

30.0

30.0

HEAOE

N.

M.

U.

u.

33

67

A.

A.

A.

A.

22. 5

22.5

22.5

22.5

29.1

29.1

HEADL

N.

N.

N.

N.

IS

0

A.

A.

A.

A.

24.4 11.0

24.4 11.0

24.4 11.0

24.4 11.0

30.1 14.5

30.1 14.5

NSC

0.002383

0.020120

0.002053

0.004326

0.003621

0.006699

12100

12200

12

12

12

12

TLI

6

5.8

N. A.

N. A.

5.1

5.2

:oo

:oo

:oo

:oo

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

FRONTAL

CHEST

37.1

48.9

29.3

20.7

68.8

84.6

FRONT,SOMEONE BEHIND

FRONT,SOMEONE BEHIND

BACK

BACK

FRONT

FRONT

CKESTE

37.1

48.9

N. A.

N. A.

N. A.

N. A.

SEAT PASSENGER

SEAT PASSENGER

, NOBODY BEHIND

, NOBOOY BEHIND

CHESTL CSI

24 16T

20 294

N. A. 114

N. A. 71

N. A. 587

N. A. 1310

LFEK

1280

2849

457

404

1841

38 59

RFEN

359

2120

658

417

738

5129
APPENDIX B

DETAILED STATE DATA TABULATIONS

185
TABLE B-l

WASHINGTON STATE 1973-77: K + A INJURY RATES,

ALL OCCUPANTS

2 DOOR VS. 4 DOOR SEDANS AND HARDTOPS OF THE SAME MAKES AND MODELS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Excludes station wagons, convertibles, and any make/models

that were produced only with 2 doors or only with 4 doors)

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Injuries

359

291

505

423

740

617

999

993

954

723

1256

1320

1165

832

1516

1526

N of

Persons

10,667

11,379

13,505

15,644

21,547

23,935

26,385

34,165

26,752

28,548

32,318

44,543

31,419

33,483

37,730

51,859

Injuries per

100 Persons

3.37

2.56

3.74

2.70

3.43

2.58

3.79

2.91

3.57

2.53

3.89

2.96

3.71

2.48

4.02

2.94

Gross

Red. (X)

24

28

25

23

29

24

33

27

NET RED.

FOR SBL*

Z Test

+5%

Z=0.49

-2%

Z=0.32

-77.

Z-1.14

-9°/.

Z=1.55

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha « .05 - i.e., Z greater than

1.96).

186
TABLE B-2

WASHINGTON STATE 1973-77: K + A + B INJURY RATES,

ALL OCCUPANTS

2 DOOR VS. 4 DOOR SEDANS AND HARDTOPS OF;THE SAME MAKES AND MODELS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Excludes station wagons, convertible!!, and any make/models

that were produced only with 2 doors or only with 4 doors)

MY Before/

After SBL NET RED.

N of Install, in n of N of Injuries per Gross FOR SBL*

Doors 2 Door Cars Injuries Persons 100 Persons Red. (%)

Z Test

4 last 1 before 1350 10,667 12.66

first 1 after 1205 11,379 10.59 16

2 last 1 before 1995 13,505 14.77

first 1 after 1979 15,644 12.65 14 -11

Z=0.48

4 last 2 before 2830 21,547 13.13

first 2 after 2485 23,935 10.38 21

2 last 2 before 4045 26,385 15.33

first 2 after 4279 34,165 ,12.52 18 -3%

Z-0.99

4 last 3 before 3592 26,752 13.43

first 3 after 2949 28,548 10.33 23

2 last 3 before 5042 32,318 15.60

first 3 after 5534 44,543 12.42 20 -47.

Z-1.15

4 last 4 before 4351 31,419 13.85

first 4 after 3445 33,483 10.29 26

2 last 4 before 6022 37,730 15.96

first 4 after 6393 51,859 12.33 23 -4 %

Z=1.41

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha - .05 - i.e., Z greater than

1.96).

187
TABLE B-3

WASHINGTON STATE 1973-77: OVERALL INJURY RATES,

ALL OCCUPANTS

2 DOOR VS. 4 DOOR SEDANS AND HARDTOPS OF THE SAME MAKES AND MODELS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Excludes station wagons, convertibles, and any make/models

that were produced only with 2 doors or only with 4 doors)

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Injuries

2,697

2,645

3,694

3,934

5,569

5,453

7,290

8,421

6,944

6,472

8,953

10,917

8,294

7,551

10,533

12,665

N of

Persons

10,667

11,379

13,505

15,644

21,547

23,935

26,385

34,165

26,752

28,548

32,318

44,543

31,419

33,483

37,730

51,859

Injuries per

100 Persons

25.28

23.24

27.35

25.15

25.84

22.78

27.63

24.65

25.96

22.67

27.70

24.51

26.40

22.55

27.92

24.42

Gross

Red. (%)

8

8

12

. 11

13

12

15

13

NET RED.

FOR SBL*

Z Test

none

Z=0.00

- 1%

Z-0.55

- 1%

Z=0.65

-27.

Z-1.31

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha • .05 - i.e., Z greater than

1.96).

188
TABLE B-4

WASHINGTON STATE 1973-77: EJECTEES PER 1OOO OCCUPANTS,

ALL SEATING POSITIONS

2 DOOR VS. 4 DOOR SEDANS AND HARDTOPS OF THE SAME MAKES AND MODELS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Excludes station wagons, convertibles, and any make/models

that were produced only with 2 doors or only with 4 doors)

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Ejectees

71

50

130

111

146

89

295

246

195

112

366

299

240

126

434

354

N of

Occ.

10,667

11,379

13,505

15,644

21,547

23,935

26,385

34,165

26,752

28,548

32,318

44,543

31,419

33,483

37,730

51,859

Ejectees per

1000 Occ.

6.66

4.39

9.63

7.10

6.78

3.72

11.18

7.20

7.29

3.92

11.33

6.71

7.64

, 3.76

11.50

6.83

Gross

Red. (%)

34

26

45

36

46

41

51

41

NET RED.

FOR SBL*

Z Test

-12%

Z=0.49

-17%

Z-1.00

-10%

Z=0.68

-20%

Z-1.42

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha « .05 - i.e., Z greater than

1.96).

189
TABLE B-5

NASHINGTON STATE 1973-77: K + A INJURY RATES,

FRONT SEAT OCCUPANTS WITH NOBODY SITTING BEHIND THEM

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Injuries

506

456

700

712

1106

943

1436

1554

1681

1314

2067

2414

2200

1642

2515

3058

N of

Persons

20,853

21,053

26,067

29,691

42,522

43,336

50,204

61,500

62,140

61,866

70,729

94,632

77,403

78,682

82,637

120,943

Injuries per

100 Persons

2.43

2.17

2.69

2.40

2.60

2.18

2.86

2.52

2.71

2.12

2.92

2.55

2.84

2.09

3.04

2.53

Gross

Red. (!)

11

11

16

12

21

13

27

17

NET RED.

FOR SBL**

Z Test

none

Z=0.00

- 6%

Z=0.96

-11%

Z=2.26*

-13%

Z=2.95*

*Statistically significant effect (two-sided alpha - .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

190
TABLE B-6

WASHINGTON STATE 1973-77: K + A + B INJURY RATES,

FRONT SEAT OCCUPANTS WITH NOBODY SITTING BEHIND THEM

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 afterlast 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Injuries

2,166

1,935

3,121

3,311

4,713

3,936

6,329

6,898

7,060

5,563

9,104

10,542

9,079

7,003

10,855

13,424

N of

Persons

20,853

21,053

26,067

29,691

42,522

43,336

50,204

61,500

62,140

61,866

70,729

94,632

77,403

78,682

82,637

120,943

Injuries per

100 Persons

10.39

9.19

11.97

11.15

11.08

9.08

12.61

11.22

11.36

8.99

12.87

11.14

11.73

8.90

13.14

11.10

Gross

Red. (%)

12

7

18

11

21

13

24

16

NET RED.

FOR SBL**

Z Test

- 5%

Z-1.34

- 9%

Z-3.11*

-9%

Z=4.09*

-11%

Z=5.50*

*Statistically significant effect (two-sided alpha - .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

191
TABLE B-7

WASHINGTON STATE 1973-77: OVERALL INJURY RATES,

FRONT SEAT OCCUPANTS WITH NOBODY SITTING BEHIND THEM

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Injuries

4,493

4,342

6,178

6,678

9,501

8,809

12,142

13,828

13,990

12,480

17,226

21,235

17,763

15,746

20,275

27,219

N of

Persons

20,853

21,053

26,067

29,691

42,522

43,336

50,204

61,500

62,140

61,866

70,729

94,632

77,403

78,682

82,637

120,943

Injuries per

100 Persons

21.55

20.62

23.70

22.49

22.34

20.33

24.19

22.48

22.51

20.17

24.35

22.44

22.95

20.01

24.54

22.51

Gross

Red. (%)

4

5

9

7

10

8

13

8

NET RED.

FOR SBL**

Z Test

+ 1%

Z=0.35

- 2 %

Z-1.26

-3°/.

Z-1.95

- 5%

Z=3.94

*Statistically significant effect (two-sided alpha - .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

192
TABLE B-8

WASHINGTON STATE 1973-77: K + A INJURY RATES,

FRONT SEAT OCCUPANTS WITH SOMEBODY SITTING BEHIND THEM

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/

After SBL NET RED.

N of Install, in n of N of Injuries per Gross FOR SBL*

Doors 2 Door Cars Injuries Persons 100 Persons Red. (%)

Z Test

4 last 1 before 54 990 5.45

first 1 after 34 1053 3.23 41

2 last 1 before 81 1162 6.97

first 1 after 69 1354 5.10 27 -23%

Z=0.79

4 last 2 before 112 2018 5.55

first 2 after 78 2116 3.68 34

2 last 2 before 163 2264 7.20

first 2 after 149 2820 5.28 27 -10%

Z=0.55

4 last 3 before 163 2971 ' 5.49

first 3 after 112 3076 3.64 34

2 last 3 before 222 3268 6.79

first 3 after 22! 4249 5.20 23 -15%

Z=0.94

4 last 4 before 197 3623 5.44

first 4 after 139 3975 3.50 36

2 last 4 before 261 3812 6.85

first 4 after 266 5351 4.97 27 -13%

Z=0.88

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha * .05 - i.e., Z greater than

1.96).

193
TABLE B-9

WASHINGTON STATE 1973-77: K + A + B INJURY RATES,

FRONT SEAT OCCUPANTS WITH SOMEBODY SITTING BEHIND THEM

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/

After SBL NET RED.

N of Install, in n of N of Injuries per Gross FOR SBL*

Doors 2 Door Cars Injuries Persons 100 Persons Red. (%)

Z Test

4 last 1 before 163 990 16.46

first 1 after 134 1053 12.73 23

2 last 1 before 262 1162 22.55

first 1 after 252 1354 18.61 17 -7%

Z=0.48

4 last 2 before 337 2018 16.70

first 2 after 297 2116 14.04 16

2 last 2 before 532 2264 23.50

first 2 after 525 2820 18.62 21 +6%

Z=0.63

4 last 3 before 510 2971 17.17

first 3 after 430 3076 13.98 19

2 last 3 before 743 3268 22.74

first 3 after 769 4249 18.10 20 +2%

Z=0.29

4 last 4 before 621 3623 17.14

first 4 after 555 3975 13.96 19

2 last 4 before 855 3812 22.43

first 4 after 954 5351 17.83 21 +2%

Z=0.35

'Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha - .05 - i.e., Z greater than

1.96).

194
TABLE B-10

WASHINGTON STATE 1973-77: OVERALL INJURY RATES,

FRONT SEAT OCCUPANTS WITH SOMEBODY SITTING BEHIND THEM

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/

After SBL NET RED.

N of Install, in n of N of Injuries per Gross FOR SBL*

Doors 2 Door Cars Injuries Persons 100 Persons Red. (%)

Z Test

4 last 1 before 300 990 30.30

first 1 after 277 1053 26.31 13

2 last 1 before 398 1162 34.25

first 1 after 443 1354 32.72 4 -10%

Z=l.03

4 last 2 before 615 2018 30.48

first 2 after 571 2116 26.98 11

2 last 2 before 810 2264 35.78

first 2 after 930 2820 32.98 8 -4 %

Z=0.62

4 last 3 before 906 2971 30.49

first 3 after 825 3076 26.82 12

2 last 3 before 1155 3268 35.34

first 3 after 1344 4249 31.63 11 -27.

Z=0.33

4 last 4 before 1111 3623 30.67

first 4 after 1071 3975 26.94 12

2 last 4 before 1343 3812 35.23

first 4 after 1697 5351 31.71 10 -27.

Z=0.50

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha • .05 - i.e., Z greater than

1.96).

195
TABLE B-ll

HASHINGTON STATE 1973-77: K + A INJURY RATES,

BACK SEAT OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/

After SBL NET RED.

N of Install, in n of N of Injuries per Gross FOR SBL*

Doors 2 Door Cars Injuries Persons 100 Persons Red. (%)

Z Test

4 last 1 before 45 1100 4.09

first 1 after 32 1195 2.68 35

2 last 1 before 78 1312 5.95

first 1 after 62 1488 4.17 30 -7%

Z=0.24

4 last 2 before 87 2247 3.87

first 2 after 78 2403 3.25 16

2 last 2 before 153 2563 5.97

first 2 after 139 3115 4.46 25 +11%

Z«0.60

4 last 3 before 136 3296 4.13

first 3 after 98 3469 2.83 32

2 last 3 before 207 3663 5.65

first 3 after 210 4664 4.50 20 -16%

Z=0.93

4 last 4 before 171 4029 4.24

first 4 after 128 4473 2.86 33

2 last 4 before 234 4275 5.47

first 4 after 248 5881 4.22 23 -14%

Z-0.91

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha « .05 - i.e., Z greater than

1.96).

196
TABLE B-12

WASHINGTON STATE 1973-77: K + A + B INJURY RATES,

BACK SEAT OCCUPANTS

2 DOOR VS. 4.DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Injuries

183

137

249

241

369

307

518

505

554

423

732

736

693

569

854

907

N of

Persons

1100

1195

1312

1488

2247

2403

2563

3115

3296

3469

3663

4664

4029

4473

4275

5881

Injuries per

100 Persons

16.64

11.46

18.98

16.20

16.42

12.78

20.21

16.21

16.81

12.19

19.98

15.78

17.20

12.72

19.98

15.42

Gross

Red. (°/.)

31

15

22

20

27

21

26

23

NET RED.

FOR SBL*

Z Test

-24%

Z=1.57

- 3%

Z-0.33

-9°/.

Z=1.09

-4%

Z-0.62

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha = .05 - i.e., Z greater than

1.96).

197
TABLE B-13

WASHINGTON STATE 1973-77: OVERALL INJURY RATES,

BACK SEAT OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Injuries

329

291

414

430

658

609

845

881

983

883

1194

1302

1234

1147

1404

1648

N of

Persons

1100

1195

1312

1488

2247

2403

2563

3115

3296

3469

3663

4664

4029

4473

4275

5881

Injuries per

100 Persons

29.91

24.35

31.55

28.90

29.28

25.34

32.97

28.28

29.82

25.45

32.60

27.92

30.63

25.64

32.84

28.02

Gross

Red. (7.)

19

8

13

14

15

14

16

15

NET RED.

FOR SBL*

Z Test

-12%

Z-1.28

+1%

Z-0.14

none

Z=0.07

-27,

Z=0.40

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha - .05 - i.e., Z greater than

1.96).

198
TABLE B-14

WASHINGTON STATE 1973-77: K + A INJURY RATES,

BELTED OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/ '

After SBL NET RED.

N of Install, in n of N of Injuries per Gross FOR SBL*

Doors 2 Door Cars Injuries Persons 100 Persons Red. (%)

Z Test

4 last 1 before 54 3,688 1.46

first 1 after 49 4,056 1.21 17

2 last 1 before 72 4,310 1.67

first 1 after 85 5,298 1.60 4 -161

Z=0.60

4 last 2 before 113 7,256 1.56

first 2 after 112 8,504 1.32 15

2 last 2 before 150 8,233 1.82

first 2 after 180 11,073 1.63

4 last 3 before 165 10,076 1.64

first 3 after 170 12,628 1.35

2 last 3 before 188 11,094 1.69

first 3 after 306 17,516 1.75

4 last 4 before 193 11,777 1.64

first 4 after 225 16,633 1.35 17

2 last 4 before 209 12,146 1.72

first 4 after 388 23,084 1.68 2 -21%

Z=l.30

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha - .05 - i.e., Z greater than

1.96).

11

18

-3

-6%

2-0.31

-25%

2-1.59

199
TABLE B-15

WASHINGTON STATE 1973-77: K + A + B INJURY RATES,

BELTED OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION Of SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Injuries

322

321

420

555

685

674

884

1103

985

1050

1205

1799

1155

1366

1365

2351

N of

Persons

3,688

4,056

4,310

5,298

7,256

8,504

8,233

11,073

10,076

12,628

11,094

17,516

11,777

16,633

12,146

23,084

Injuries per

100 Persons

8.73

7.91

9.74

10.48

9.44

7.92

10.74

9.96

9.78

8.31

10.86

10.27

9.81

8.21

11.24

10.18

Gross

Red. (%)

9

-8

16

7

15

5

16

9

NET RED.

FOR SBL*

Z Test

-19%.

Z-1.74-

-11%

Z-1.47

-11%

Z-1.91-

-8%

Z-1.57

''Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha « .05 - i.e., Z greater than

1.96).

200
TABLE B-16

WASHINGTON STATE 1973-77: OVERALL INJURY RATES.

BELTED OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Injuries

784

881

1015

1254

1625

1821

1991

2544

2292

2727

2707

4113

2683

3570

3035

5464

N of

Persons

3,688

4,056

4,310

5,298

7,256

8.504

8,233

11,073

10,076

12,628

11,094

17,516

11,777

16,633

12,146

23,084

Injuries per

100 Persons

21.26

21.72

23.54

23.66

22.40

21.41

24.18

22.97

22.75

21.59

24.40

23.48

22.78

21.46

24.99

23.67

Gross

Red. (%)

-2

-1

4

5

5

4

6

5

NET RED.

FOR SBL*

Z Test

+2X

Z=0.29

none

Z=0.16

-1%

Z-0.41

Z-0.18

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha - .05 - i.e., Z greater than

1.96).

201
TABLE B-17

TEXAS 1972-74: K + A INJURY RATES,

ALL DRIVERS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Injuries

814

771

1060

1038

1824

1543

2115

2290

2798

2199

3027

3602

3686

2871

3702

4873

N of

Persons

48,362

48,704

50,374

59,932

96,243

104,156

96,173

132,753

139,221

156,389

131,212

214,049

177,570

207,986

153,825

291,275

Injuries per

100 Persons

1.68

1.58

2.10

1.73

1.89

1.48

2.20

1.73

2.01

1.41

2.31

1.68

2.08

1.38

2.41

1.67

Gross

Red. (%)

6

18

22

22

30

27

34

30

NET RED.

FOR SBL**

Z Test

+12%

Z=2.02*

none

Z=0.08

- 4%

Z-l.ll

- 5%

Z=l.35

"Statistically significant effect (two-sided alpha - .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

202
TABLE B-18

TEXAS 1972-74: K + A + B INJURY RATES,

ALL DRIVERS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/

After SBL

N of Install, in n of N of

Doors 2 Door Cars Injuries Persons

last 1 before 2,873 48,362

first 1 after 2,654 48,704

last 1 before 3,597 50,374

first 1 after 3,684 59,932

last 2 before 6,091 96,243

first 2 after 5,416 104,156

last 2 before 7,068 96,173

first 2 after 8,100 132,753

last 3 before 9,123 139,221

first 3 after 7,753 156,389

last 3 before 9,878 131,212

first 3 after 12,964 214,049

last 4 before 12,076 177,570

first 4 after 10,101 207,986

last 4 before 11,917 153,825

first 4 after 17,331 291,275

Injuries per

100 Persons

5.94

5.45

7.14

6.15

6.33

5.20

7.35

6.10

6.55

4.96

7.53

6.06

6.80

4.86

7.75

5.95

NET RED.

Gross FOR SBL**

Red. (%)

Z Test

14

18

17

24

+67.

Z-1.83

Z-0.43

20

29

23

-6%

Z=3.08*

-1%

Z=4.15°*

*Statistically significant effect (two-sided alpha - .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

203
TABLE B-19

TEXAS 1972-74: OVERALL INJURY RATES,

ALL DRIVERS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Injuries

4,854

4,629

5,609

6,108

9,947

9,562

10,931

13,404

14,681

13,818

15,274

21,491

19,190

18,074

18,216

28,822

N of

Persons

48,362

48,704

50,374

59,932

96,243

104,156

96,173

132,753

139,221

156,389

131,212

214,049

177,570

207,986

153,825

291,275

Injuries per

100 Persons

10.04

9.50

11.13

10.19

10.34

9.18

11.37

10.10

10.55

8.84

11.64

10.04

10.81

8.69

11.84

9.90

Gross

Red. (%)

5

8

11

11

16

14

20

16

NET RED.

FOR SBL**

Z Test

+3*

Z=1.29

none

Z=0.01

- 3%

Z-1.91

- 4%

Z=2.86*

*Statistically significant effect (two-sided alpha

than 1.96)

.05 - i.e., Z greater

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

204
TABLE B-20

TEXAS 1972-74: K + A INJURY RATES,

DRIVERS IN FRONTAL CRASHES

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/

After SBL NET RED.

N of Install, in n of N of Injuries per Gross FOR SBL**

Doors 2 Door Cars Injuries Persons 100 Persons Red. (%)

Z Test

4 last 1 before 518 19,837 2.61

first 1 after 494 19,650 2.51 4

2 last 1 before 623 21,830 2.85

first 1 after 590 25,470 2.32 19 +16%

Z=2.03*

4 last 2 before 1167 40,324 2.89

first 2 after 970 41,292 2.35 19

2 last 2 before 1275 42,321 3.01

first 2 after 1291 55,933 2.31

4 last 3 before 1801 58,819 3.06

first 3 after 1361 61,150 2.23

2 last 3 before 1837 58,179 3.16

first 3 after 2006 89,442 2.24

4 last 4 before 2384 75,946 3.14

first 4 after 1760 80,771 2.18

2 last 4 before 2278 68,639 3.32

first 4 after 2733 121,184 2.26

^Statistically significant effect (two-sided alpha » .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

23

27

29

31

32

+6%

Z-1.00

+2%

Z=0.48

+n

Z-0.51

205
TABLE B-21

TEXAS 1972-74: K + A + B INJURY RATES,

• DRIVERS IN FRONTAL CRASHES

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Injuries

1742

1603

2139

2138

3731

3255

4281

4678

5633

4641

6017

7393

7507

5961

7322

9888

N of

Persons

19,837

19,650

21,830

25,470

40,324

41,292

42,321

55,933

58,819

61,150

58,179

89,442

75,946

80,771

68,639

121,184

Injuries per

100 Persons

8.78

8.16

9.80

8.39

9.25

7.88

10.12

8.36

9.58

7.59

10.34

8.27

9.88

7.38

10.67

8.16

Gross

Red. (X)

7

19

15

17

21

20

25

24

NET RED.

FOR SBL**

Z Test

+8%

Z=l.83

+3%

Z=0.98

- 2%

Z-0.33

-It

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha - .05 - i.e., Z greater than

1.96).

206
TABLE B-22

TEXAS 1972-74: OVERALL INJURY RATES,

• • DRIVERS IN FRONTAL CRASHES

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Injuries

2,472

2,356

2,943

3,055

5,186

4,783

5,860

6,747

7,739

6,908

8,247

10,752

10,245

8,927

9,949

14,440

N of

Persons

19,837

19,650

21,830

25,470

40,324

41,292

42,321

55,933

58,819

61,150

58,179

89,442

75,946

80,771

68,639

121,184

Injuries per

100 Persons

12.46

11.99

13.48

11.99

12.86

11.58

13.85

12.06

13.16

11.30

14.18

12.02

13.49

11.05

14.49

11.92

Gross

Red. (%)

4

11

10

13

14

15

18

18

NET RED.

FOR SBL**

Z Test

+8%

Z=2.16*

+3X

Z«=l .32

+U

Z=0.59

none

Z-0.18

*Statistically significant effect (two-sided alpha •..05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

207
TABLE B-23

NEW YORK STATE 1974: INOURY RATES BY INJURY TYPE, ALL OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

LAST YEAR BEFORE VS. FIRST YEAR AFTER INSTALLATION OF SBL IN 2 DOOR CARS

4 Doo r Car s 2 Doo r Car s

o

CO

Body

Region

Head

Neck/back

Head

Leg

Head

Arm

Torso

HEAD

Leg

Arm

Head

HEAD

Leg

All over

Arm

Leg

LEG

ARM

TORSO

Injury

Type

minor bleed.

pain

pain

pain

contusion

pain

pain

SEVERE BLEED.

contusion

minor bleed.

abrasion

CONCUSSION

minor bleed.

pain

contusion

abrasion

FRACTURE

FRACTURE

INTERNAL

Overall

n of

Occurrences

2780

2029

1539

942

787

683

613

545

327

288

266

238

189

191

145

136

237

104

98

I

I

I

I

I

I

I

PRE

(9682

Occ.)

Inj.

Rate*

75.50

53.19

39.25

26.93

20.66

19.11

15.29

14.46

8.78

7.64

6.09

5.78

5.06

4.85

4.44

4.13

2.89

1.96

2.79

POST

(10991

Occ.)

Inj.

Rate*

59.87

50.59

39.76

22.84

18.38

18.65

18.01

11.55

7.28

5.73

7.19

4.91

3.91

5.82

.3.37

3.18

3.64

2.09

1.91

Gross

Red.

(%)

21

5

-1

12

11

2

-18

20

17

25

-18

15

23

-20

24

23

-26

-7

31

I

I

I

I

I

I

I

PRE

(7721

Occ.)

Inj.

Rate*

85.35

50.90

40.15

26.94

24.61

17.74

13.86

16.97

9.58

8.94

6.48

6.22

6.35

5.96

4.02

3.63

3.76

4.02

3.11

POST

.(9881

"Occ.)

Inj

Rate*

74.08

57.18

41.70

23.38

19.73

15.79

16.19

14.88

8.91

8.30

7.89

8.10

4.86

3.44

3.44

3.34

4.05

3.14

2.63

Gross

Red.

(%)

13

-12

-4

13

20

11

-17

12

7

7

-22

-30

23

42

14

8

-8

22

15

I

I

I

I

I

I

I

NET RED.

FOR SBL

(%)**

_g

-18

-3

+1

+10

+9

+ 1

-10

-12

-24

-3

-53

+ 1

+52

-13

-19

+14

+27

-24

Z

Test

1.23

1.90

.25

.18

.74

.60

.05

.60

.52

.90

.12

1.63

.03

2.49#

.36

.52

.45

.78

.52

*Injury rate per 1000 occupants

**Note: positive numbers indicate that the SBL group (2 door cars) had greater improvements than the

control group (4 door cars); negative numbers indicate that SBL did worse than the control group.

Statistically significant effect (two-sided alpha = .05 - i.e., Z greater than 1.96)
TABLE B-24

NEW YORK STATE 1974: INOURY RATES BY IN0URY TYPE, ALL OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

LAST 2 YEARS BEFORE VS. FIRST 2 YEARS AFTER INSTALLATION OF SBL IN 2 DOOR CARS

Body

Region

Head

Neck/back

Head

Leg

Head

Arm

Torso

HEAD

Leg

Arm

Head

HEAD

Leg

All over

Arm

Leg

LEG

ARM

TORSO

Injury

Type

minor bleed.

pain

pain

pain

contusion

pain

pain

SEVERE BLEED.

contusion

minor bleed.

abrasion

CONCUSSION

minor bleed.

pain

contusion

abrasion

FRACTURE

FRACTURE

INTERNAL

Overall

n of

Occurrences

5594

4128

3094

1871

1621

1356

1204

1169

652

583

563

486

403

390

300

282

269

205

199

I

I

I

I

I

I

I

I

4 D

PRE

(18182

Occ.)

Inj.

Rate*

75.18

52.47

39.21

25.30

22.27

17.71

15.56

17.60

9.07

7.15

7.20

5.83

5.61

5.39

4.62

4.29

3.63

2.47

2.36

o o r C

POST

(23170

Occ.)

Inj.

Rate*

57.96

52.91

39.71

22.53

18.00

18.17

16.31

10.01

7.21

5.65

6.56

5.05

4.19

5.44

3.54

3.11

3.19

2.33

2.37

a r s

Gross

Red.

(X)

23

-1

-1

11

19

-3

-5

43

21

21

9

13

25

_1

23

28

12

6

none

I

T

i

I

I

I

I

I

I

2 D o

PRE

(14511

Occ.)

Inj.

Rate*

89.45

49.20

40.18

26.46

23.22

17.71

14.61

21.71

9.23

9.03

7.79

7.03

6.62

4.89

3.79

4.00

3.65

3.38

2.83

or C a

POST

(21952

Occ.)

Inj

Rate*

72.25

56.21

40.00

23.00

21.05

16.22

15.08

13.76

8.47

8.70

7.61

7.33

4.92

4.33

3.60

3.37

3.46

2.60

2.73

r s

Gross

Red.

(X)

19

-14

none

13

9

8

-3

37

8

4

2

-4

26

12

5

16

5

23

3

I

T

1

I

I

I

I

I

I

NET RED.

FOR SBL

(%)**

-5

-13

+2

+2

-12

+11

+2

-11

-16

-22

_7

-20

none

+12

-24

-16

-8

+18

+4

Z

Test

.89

2.01#

.24

.26

1.16

1.04

.13

.92

.91

1.18

.41

1.01

.02

.63

.92

.63

.31

.73

.14-

*Injury rate per 1000 occupants

**Note: positive numbers indicate that the SBL group (2 door cars) had greater improvements than the

control group (4 door cars); negative numbers indicate that SBL did worse than the control group.

^Statistically significant effect (two-sided alpha = .05 - i.e., Z greater than 1.96)
TABLE B-25

NEW YORK STATE 1974: INJURY RATES BY INJURY TYPE, ALL OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

LAST 3 YEARS BEFORE VS. FIRST 3 YEARS AFTER INSTALLATION OF SBL IN 2 DOOR CARS

Body

Region

Head

Neck/back.

Head

Leg

Head

Arm

Torso

HEAD

Leg

Arm

Head

HEAD

Leg

All over

Arm

Leg

LEG

ARM

TORSO

Injury

Type

minor bleed.

pain

pain

pain

contusion

pain

pain

SEVERE BLEED.

contusion

minor bleed.

abrasion

CONCUSSION

minor bleed.

pain

contusion

abrasion

FRACTURE

FRACTURE

INTERNAL

Overall

n of

Occurrences

7905

5957

4509

2662

2383

1987

1774

1698

974

832

820

693

578

552

432

392

367

290

286

I

T

1

I

I

I

I

I

I

4 D

PRE

(22900

Occ.)

Inj.

Rate*

76.55

51.97

39.48

25.46

21.66

18.73

16.33

18.69

8.86

7.34

7.38

5.72

5.68

4.93

4.67

4.24

3.58

2.23

2.36

o o r C

POST

(34673

Occ.)

Inj.

Rate*

55.63

53.27

39.74

22.27

18.26

17.68

16.67

9.58

7.70

5.02

6.32

4.96

4.33

5.19

3.37

2.88

3.26

2.28

2.36

a r s

Gross

Red.

(X)

27

-3

-1

13

16

6

-2

49

13

32

14

13

24

-5

28

32

9

-2

none

I

T

i

I

I

I

I

I

I

2 D

PRE

(18424

Occ.)

Inj.

Rate*

88.80

50.21

39.62

26.22

24.48

18.07

15.03

23.77

9.50

9.88

7.71

7.11

6.35

4.45

3.91

4.01

3.58

3.31

2.82

o o r C a

POST

(36712

Occ.)

Inj

Rate*

70.47

54.34

40.78

22.44

21.87

16.67

14.85

13.62

8.96

8.39

7.90

7.05

4.93

4.82

3.70

3.30

2.89

2.70

2.67

r s

Gross

Red.

(X)

21

-8

-3

14

11

8

1

43

6

15

-2

1

22

-8

5

18

19

19

5

I

T

1

I

I

I

I

I

I

NET RED.

FOR SBL

(X)**

-9

-6

-2

+2

-6

+2

+3

-12

-9

-24

-20

-14

-2

-3

-31

-21

+ 11

+20

+6

Z

Test

1.98#

1.03

.37

.28

.70

.25

.33

1.14

.62

1.52

1.24

.85

.11

.16

1.37

.94

.56

.93

.22

*Injury rate per 1000 occupants

**Note: positive numbers indicate that the SBL group (2 door cars) had greater improvements than the

control group (4 door cars); negative numbers indicate that SBL did worse than the control group.

^Statistically significant effect (two-sided alpha = .05 - i.e., Z greater than 1.96)
ro

TABLE B-26

NEW YORK STATE 1974: INJURY RATES BY INJURY TYPE, ALL OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

LAST 4 YEARS BEFORE VS. FIRST 4 YEARS AFTER INSTALLATION OF SBL IN 2 DOOR CARS

Body

Region

Head

Neck/back

Head

Leg

Head

Arm

Torso

HEAD

Leg

Arm

Head

HEAD

Leg

All over

Arm

Leg

LEG

ARM

TORSO

Injury

Type

minor bleed.

pain

pain

pain

contusion

pain

pain

SEVERE BLEED.

contusion

minor bleed.

abrasion

CONCUSSION

minor bleed.

pain

contusion

abrasion

FRACTURE

FRACTURE

INTERNAL

Overal1

n of

Occurrences

9709

7572

5717

3318

3023

2447

2233

2104

1244

1041

1017

863

729

694

559

495

458

357

336

I

T

1

I

I

I

I

I

I

4 D

PRE

(25387

Occ.)

Inj.

Rate*

77.01

51.84

39.27

24.78

22.33

18.40

16.39

19.46

8.86

7.56

7.52

5.83

5.63

4.73

4.65

4.21

3.51

2.21

2.32

o o r C

POST

(46177

Occ.)

Inj.

Rate*

54.05

52.84

38.72

22.15

17.78

17.04

16.33

9.29

7.77

5.28

6.13

4.74

4.22

4.92

3.44

2.97

3.01

2.14

2.10

a r s

Gross

Red.

(%)

30

-2

1

11

20

7

none

52

12

30

19

19

25

-4

26

30

14

3

10

I

T

1

I

I

I

I

I

I

2 D o

PRE

(20269

Occ.)

Inj.

Rate*

88.41

50.18

39.57

26.10

25.21

18.21

15.44

24.17

9.72

9.77

8.09

7.25

6.27

4.74

4.19

4.19

3.60

3.11

3.01

or C a

POST

(51029

Occ.)

Inj

Rate*

67.92

54.85

41.74

22.28

22.03

16.15

14.70

13.54

9.07

7.98

7.43

6.84

5.17

4.92

3.86

3.25

3.08

2.72

2.33

r s

Gross

Red.

(%)

23

-9

-5

15

13

11

5

44

7

18

8

6

17

-4

8

22

15

12

23

I

T

i

I

I

I

I

I

I

NET RED.

FOR SBL

(%)**

-9

-7

-7

+5

-10

+4

+4

-17

-6

-17

-13

-16

-10

none

-24

-10

+ 1

+10

+14

Z

Test

2.21#

1.43

1.20

.64

1.23

.51

.50

1.82

.52

1.21

.90

1.03

.62

.01

1.23

.51

.01

.45

.69

*Injury rate per 1000 occupants

**Note: positive numbers indicate that the SBL group (2 door cars) had greater improvements than the

control group (4 door cars); negative numbers indicate that SBL did worse than the control group.

^Statistically significant effect (two-sided alpha = .05 - i.e., Z greater than 1.96)

APPENDIX C

DETAILED FARS DATA TABULATIONS

213
TABLE C-l

FARS 1975-85: FATALITY RATES PER MILLION "EXPOSURE"* YEARS,

FRONT SEAT OCCUPANTS WITH NOBODY SITTING BEHIND THEM

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN-2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Deaths

2113

2147

2413

3195

3640

4783

4177

7248

4192

6217

4892

10516

4555

7980

5219

13388

Millions

of

Exposure

Years

11.328

13.026

9.690

12.527

22.180

29.007

18.032

28.138

27.466

35.913

22.547

37.676

32.110

43.930

25.079

45.254

Deaths per

Million Yrs.

188.0

164.8

249.0

255.0

164.1

164.9

231.6

257.6

152.6

173.1

217.0

279.1

141.9

181.7

208.1

295.8

Gross

Red. (X)

12

-2

hone

-11

-13

-29

-28

-42

NET RED.

FOR SBL**

Z Test

-17%

Z=3.80*

-117.

Z=3.46*

-13%

Z=4.76*

-11%

Z=4.23*

*Statistically significant effect (two-sided alpha • .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

^"Exposure years" equal the number of vehicle years (from Table 4-1) multiplied by the proportion of occupants who are sitting in the front seat with

nobody behind them (in Washington State - see Tables 3-1 and B-5).

214
TABLE C-2

FARS 1975-85: FATALITY RATES PER MILLION "EXPOSURE"** YEARS,

FRONT' SEAT OCCUPANTS WITH SOMEBODY SITTING BEHIND THEM

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/ Millions

After SBL of NET RED.

N of Install, in n of Exposure Deaths per Gross FOR SBL*

Doors 2 Door Cars Deaths Years Million Yrs. Red. (1)

• Z Test

4 last 1 before ' 216 0.659 327.8

first 1 after 238 0.777 306.1 7

2 last 1 before 274 0.550 498.4

first 1 after 377 0.676 557.6 -12 -20°/.

Z=l.47

4 last 2 before 355 1.307 271.7

first 2 after 513 1.663 308.4 -14

2 last 2 before 474 1.015 467.0

first 2 after 834 1.497 557.2 -19 -5%

Z=0.55

4 last 3 before 389 1.616 240.7

first 3 after 654 2.091 312.7 -30

2 last 3 before 559 1.298 430.8

first 3 after 1182 1.957 604.0 -40 -8°/.

Z=0.93

4 last 4 before 433 1.857 233.2

first 4 after 856 2.597 329.7 -41

2 last 4 before 592 1.443 410.2

first 4 after 1481 2.295 645.3 -57 -11%

Z=l.39

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha - .05 - i.e., Z greater than

1.96).

**"Exposure years" equal the number of vehicle years (from Table 4-1) multiplied by the proportion of occupants who are sitting in the front seat with

somebody behind them (in Washington State - see Tables 3-1 and B-8).

215
TABLE C-3

FARS 1975-85: FATALITY RATES PER MILLION "EXPOSURE"** YEARS,

BACK SEAT OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/ Millions

After SBL . of NET RED.

N of Install, in n of Exposure Deaths per Gross FOR SBL*

Doors 2 Door Cars Deaths Years Million Yrs. Red. (7.)

Z Test

4 last 1 before 134 0.734 182.5

first 1 after 201 0.885 227.0 -24

2 last 1 before 176 0.619 284.4

first 1 after 255 0.739 345.2 -21 +2%

Z-0.16

4 last 2 before 237 1.448 163.7

first 2 after 442 1.882 234.9 -43

2 last 2 before 312 1.152 270.9

first 2 after 597 1.645 362.9 -34 +7%

Z=0.64

4 last 3 before 264 1.784 147.9

first 3 after 565 2.351 240.3 -62

2 last 3 before 373 1.456 256.2

first 3 after 852 2.149 396.5 -55 +5%

Z=0.50

4 last 4 before 298 ' 2.046 145.6

first 4 after 712 2.903 245.3 -68

2 last 4 before 389 1.619 240.3

first 4 after 1090 2.519 432.7 -80 -It

Z=0.74

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate;

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha » ,05 - i.e., Z greater than

1.96).

**"Exposure years" equal the number of vehicle years (from Table 4-1) multiplied by the proportion of occupants who are sitting in the back seat (in

Washington State - see Tables 3-1 and B-ll).

216
TABLE C-4

FARS 1975-85: EJECTEES* PER MILLION "EXPOSURE"** YEARS,

FRONT SEAT OCCUPANTS WITH NOBODY SITTING BEHIND THEM

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Ejectees

456

410

784

1088

808

940

1345

2429

957

1265

1628

3514

1071

1622

1756

4393

Millions

of

Exposure

Years

11.328

13.026

9.690

12.527

22.180

29.007

18.032

28.138

27.466

35.913

22.547

37.676

32.110

43.930

25.079

45.254

Ejectees per

Million Yrs.

40.6

31.5

80.9

86.9

36.4

32.4

74.6

86.3

34.8

35.2

72.2

93.3

33.4

36.9

70.0

97.1

Gross

Red. (%)

22

-7

11

-16

-1

-29

-11

-39

NET RED.

FOR SBL**

Z Test

-38%

Z=3.94*

-30%

Z=4.48*

-28%

Z=4.69*

-25%

Z-4.65"

*Statistically significant effect (two-sided alpha - .05 - I.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

#Persons who were ejected in a crash that was fatal to somebody, but who,

themselves, were not necessarily killed.

^"Exposure vears" equal the number of vehicle years (from Table 4-1) multiplied by the proportion of occupants who are sitting in the front seat with

nobody behind them (in Washington State - see Tables 3-1 and B-5).

217
TABLE C-5

FARS 1975-85: EOECTEES# PER MILLION "EXPOSURE"## YEARS,

FRONT' SEAT OCCUPANTS WITH SOMEBODY SITTING BEHIND THEM

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

o

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Ejectees

73

54

120

164

111

121

207

359

124

166

248

504

140

217

261

619

Millions

of

Exposure

Years

0.659

0.777

0.550

0.676

1.307

1.663

1.015

1.497

1.616

2.091

1.298

1.957

1.857

2.597

1.443

2.295

Electees per

Million Yrs.

110.8

69.5

218.3

242.6

84.9

72.7

203.9

239.9

76.7

79.4

191.1

257.5

75.4

83.6

180.8

269.7

Gross

Red. (%)

37

-11

14

-17

-3

-35

-11

-49

NET RED.

FOR SBL**

Z Test

-77%

Z=2.66*

-37%

Z=2.01*

-30%

Z=1.87

-35%

Z=2.27*

*Statistically significant effect (two-sided alpha * .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

^Persons who were ejected in a crash that was fatal to somebody, but who,

themselves, were not necessarily killed.

^"Exposure years" equal the number of vehicle years (from Trible 4-1) multiplied by the proportion of occupants who are sitting in the front seat with

somebody behind them (in Washington State - see Tables 3-1 and B-8).

218
TABLE C-6

FARS 1975-85: EOECTEES# PER MILLION "EXPOSURE"^ YEARS,

BACK SEAT OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/ Millions

After SBL of NET RED.

N of Install, in n of Exposure Ejectees per Gross FOR SBL*

Doors 2 Door Cars Ejectees Years Million Yrs. Red. (%)

• . • . Z Test

4 last 1 before 68 0.734 92.6

first 1 after, 52 0.885 58.7 37

2 last 1 before 100 0.619 161.6

first 1 after 108 0.739 146.2 ' 10 -43%

Z-1.55

4 last 2 before 100 1.448 69.1

first 2 after 129 1.882 68.5 1

2 last 2 before 175 1.152 152.0

first 2 after 238 1.645 144.7 5 +47.

. Z=0.25

4 last 3 before 110 1.784 ,61.6

first 3 after 164 2.351 69.8 -13

2 last 3 before 208 1.456 142.9

first 3 after 343 2.149 159.6 -12 +1%

Z=0.08

4 last 4 before 130 2.046 63.5

first 4 after 209 2.903 72.0 -13

2 last 4 before 217 1.619 134.1

first 4 after 433 2.519 171.9 -28 -13%

Z«=0.89

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha » .05 - i.e., Z greater than

1.96).

^Persons who were ejected in a crash that was fatal to somebody, but who,

themselves, were not necessarily killed.

^"Exposure years" equal the number of vehicle years (from Table 4-1) multiplied by the proportion of occupants who are sitting in the back seat (in

Washington State - see Tables 3-1 and B-ll).

219
TABLE C-7

FARS 1975-85: FATALITY RATES IN CRASHES INVOLVING FIRES**,

• PER MILLION VEHICLE YEARS, ALL OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Fire

Crash

Deaths

71

91

84

144

111

201

149

319

126

274

171

452

137

369

180

591

Millions

of Car

Years

12.632

14.688

10.858

13.942

24.934

32.552

20.198

31.280

30.866

40.356

25.300

41.782

36.013

49.249

28.141

50.068

Deaths per

Million Yrs.

5.62

6.20

7.74

10.33

4.45

6.18

7.38

10.20

4.08

6.79

6.76

10.82

3.80

7.47

6.40

11.80

Gross

Red. (%)

-10

-34

-39

-38

-66

-60

-96

-85

NET RED.

FOR SBL*

Z Test

-21%

Z=0.92

none

Z-0.02

+4%

Z-0.27

+61

Z.0.47

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha • .05 - i.e., Z greater than

1.96).

**Persons who were killed in a vehichle that caught fire. FARS, however, does

not specify whether the person's fatal injury was due to the fir? or to other

causes.

220
TABLE C-8

FARS 1975-85: FATALITY RATES IN CRASHES INVOLVING FIRES**,

• PER MILLION "EXPOSURE"* YEARS,

FRONT SEAT OCCUPANTS WITH NOBODY SITTING BEHIND THEM

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/ n of

After SBL Fire Millions NET RED.

N of Install, in Crash of Car Deaths per Gross FOR SBL*

Doors 2 Door Cars Deaths Years Million Yrs. Red. (%)

., Z Test

4 last 1 before 64 11.328 5.70

first 1 after 69 13.026 5.30 7

2 last 1 before 73 9.690 7.53

first 1 after 112 12.527 8.94 -19 -280.

Z=1.06

4 last 2 before 101 22.180 4.55

first 2 after 168 29.007 5.79 -27

2 last 2 before 129 18.032 7.15

first 2 after 263 28.138 9.35 -31 -3'/.

Z=0.16

4 last 3 before 114 27.466 4.15

first 3 after 233 35.913 6.49 -56

2 last 3 before 147 22.547 6.52

first 3 after 374 37.676 9.93 -52 +2%

Z=0.18

4 last 4 before 125 32.110 3.89

first 4 after 307 43.930 6.99 -80

2 last 4 before 155 25.079 6.18

first 4 after 480 45.254 10.61 -72 +4%

Z«0.32

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha - .05).

**Persons who were killed in a vehichle that caught fire. FARS, however, does

not specify whether the person's fatal injury was due to the fire or to other

causes.

""'Exposure years" equal the number of vehicle years (from Table 4-1) multiplied by the proportion of occupants who are sitting in the front seat with

nobody behind them (in Washington State - see Tables 3-1 and B-5).

221
TABLE C-9

FARS 1975-85: FATALITY RATES IN CRASHES INVOLVING FIRES**,

PER MILLION "EXPOSURE"* YEARS,

FRONT SEAT OCCUPANTS WITH SOMEBODY SITTING BEHIND THEM

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install. 1n

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Fire

Crash

Deaths

2

12

4

15

3

22

8

22

4

25 •

9

33

4

38

10

46

Millions

of Car

Years

0.659

0.777

0.550

0.676

1.307

1.663

1.015

1.497

1.616

2.091

1.298

1.957

1.857

2.597

1.443

2.295

Deaths per

Million Yrs.

3.04

15.44

7.28

22.19

2.30

13.23

7.88

14.70

2.48

11.95

6.94

16.86

2.15

14.64

6.93

20.04

Gross

Red. (%)

-408

-205

-476

-87

-383

-143

-579

-189

NET RED.

FOR SBL*

Z Test

+40%

Z=0.54

+68%

Z*1.57

+50%

Z-1,06

+57%

Z-V.3B

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha - .05).

**Persons who were killed in a vehichle that caught fire. FARS, however, does

not specify whether the person's fatal Injury was due to the fire or to other

causes.

^"Exposure years" equal the number of vehicle years (from Table 4-1) multiplied by the proportion of occupants who are sitting in the front seat with

somebody behind them (in Washington State - see Tables 3-1 and B-8).

222
TABLE C-10

FARS 1975-85: FATALITY RATES IN CRASHES INVOLVING FIRES**,

• PER MILLION "EXPOSURE"* YEARS,

BACK SEAT OCCUPANTS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Fire

Crash

Deaths

5

10

7

17

7

11

12

34

8

16

15

45

8

24

15

65

Millions

of Car

Years

0.734

0.885

0.619

0.739

1.448

1.882

1.152

1.645

1.784

2.351

1.456

2.149

2.046

2.903

1.619

2.519

Deaths per

Million Yrs.

6.81

11.29

11.31

23.02

4.84

5.85

10.42

20.67

4.48

6.81

10.30

20.94

3.91

8.27

9.27

25.81

Gross

Red. (%)

-66

-103

-21

-98

-52

-103

-111

-178

NET RED.

FOR SBL*

Z Test

-23%

Z=0.29

-6,4%

Z»0.85

-34%

Z=0.56

-32%

Z-0.55

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

. that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha - .05).

**Persons who were killed in a vehichle that caught fire. FARS, however, does

not specify whether the person's fatal injury was due to the fire or to other

causes.

^"Exposure years" equal the number of vehicle years (from Table 4-1) multiplied by the proportion of occupants who are sitting in the back, seat (in

Washington State - see Tables 3-1 and B-ll).

223
TABLE C-ll

FARS 1975-85: FATALITY RATES PER MILLION VEHICLE YEARS,

OCCUPANTS OF COMPACT AND INTERMEDIATE SIZED CARS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Deaths

785

786

1913

2778

1483

1651

3345

6404

1690

2126

3888

9466

1872

2767

4067

12119

Millions

of Car

Years

3.411

3.818

5.978

8.236

7.297

7.976

10.814

18.511

8.859

9.601

13.483

25.059

10.425

11.714

14.431

29.960

Deaths per

Million Yrs.

230.1

205.9

320.0

337.3

203.2

207.0

309.3

346.0

190.8

221.4

288.4

377.7

179.5

236.2

281.8

404.5

Gross

Red. (%)

11

-2

-11

-16

-31

-32

-44

NET RED.

FOR SBL**

Z Test

-18%

Z-2.80*

-10%

Z=2.25*

-13%

Z=3.21*

-91

Z-2.49*

*Statistically significant effect (two-sided alpha - .05 - i.e., Z greater

than 1.96)

**Note: positive numbers Indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

224
TABLE C-12

FARS 1975-85: FATALITY RATES PER MILLION VEHICLE YEARS,

' OCCUPANTS OF FULL SIZED AND LUXURY CARS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/

After SBL Millions NET RED.

N of Install. 1n n of of Car Deaths per Gross FOR SBL*

Doors 2 Door Cars Deaths Years Million Yrs. Red. (%)

Z Test

4 last 1 before 1678 9,221 182.0

first 1 after 1800 10.870 165.6 9

2 last 1 before 950 4.880 194.7

first 1 after 1049 5.706 183.8 6 -4%

Z=0.73

4 last 2 before 2749 17.596 156.2

first 2 after 4087 24.633 165.9 -6

2 last 2 before 1617 9.500 170.2

first 2 after 2275 12.751 178.4 -5 +1%

Z-0.32

4 last 3 before 3154 21.921 143.9

first 3 after 5310 30.727 172.8 -20

2 last 3 before 1935 11.984 161.4

first 3 after 3084 16.520 186.7

4 last 4 before 3412 25.469 134.0

first 4 after 6781 37,565 180.5

2 last 4 before 2132 14.111 151.1

first 4 after 3840 19.901 193.0

*Note: positive numbers Indicate that the SBL group (2 door cars) had greater

Improvements than the control group (4 door cars); negative numbers Indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha - .05 - I.e., Z greater than

1.96).

-16

-35

-28

Z=

z-

+4%

0.94

+5%

1.54

225
TABLE C-13

FARS 1975-85: FATALITY RATES PER MILLION VEHICLE YEARS,

OCCUPANTS OF GENERAL MOTORS CARS

2 DOOR VS. 4 DOOR SEDANS AND HARDTOPS OF THE SAME MAKES AND MODELS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Excludes station wagons, convertibles, and any make/models

that were produced only with 2 doors or only with 4 doors)

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Deaths

1200

1135

1285

1464

1770

2555

1902

3342

2051

3275

2225

4662

2245

4131

2406

6087

Millions

of Car

Years

5.639

5.800

4.796

5.259

11.199

13.282

8.895

12.399

13.906

17.131

10.856

16.507'

16.445

20.679

12.298

20.204

Deaths per

Million Yrs.

212.8

195.7

267.9

278.4

158.0

192.4

213.8

269.6

147.5

191.2

204.9

282.4

136.5

199.8

195.6

301.3

Gross

Red. (%)

8

-4

-22

-26

-30

-38

-46

-54

NET RED.

FOR SBL**

Z Test

-137.

Z=2.15*

- 4 %

Z-0.82

-6%

Z-l.60

-5%

Z-T.43

*Statistically significant effect (two-sided alpha - .05 - i.e., Z greater

than 1.96) ' ;. L ' '• -•••.••-

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numb*,<rs indicate

that SBL did worse than the control group.

226
TABLE C-14

FARS 1975-85: FATALITY RATES PER MILLION VEHICLE YEARS,

OCCUPANTS OF FORD MOTOR CO. CARS

2 DOOR VS. 4 DOOR SEDANS AND HARDTOPS OF THE SAME MAKES AND MODELS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Excludes station wagons, convertibles, and any make/models

that were produced only with 2 doors or only with 4 doors)

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Deaths •t

464

531

432

632

931

1229

936

1395

1186

1903

1153

2950

1363

2822

1335

4046

Millions

of Car

Years

2.185

2.480

2.068

2.702

4.081

5.394

4.092

5.895

5.899

7.321

5.586

10.523

7.420

10.270

7.305

13.601

Deaths per

Million Yrs.

212.3

214.1

208.9

233.9

228.2

227.8

228.7

236.7

201.0

259.9

206.4

280.3

183.7

274.8

182.7

297.5

Gross

Red. (%)

-1

-12

none

-3

-29

-36

-50

-63

NET

FOR

Z

Z=

Z«=

Z=

ZRED.

SBL*

Test

-11%

1.17

-At

0.57

- 5%

0.97

-9%

1.85

*Note: positive numbers indicate that the- SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha - .05 - i.e., Z greater than

1.96).

227
TABLE C-15

FARS 1975-85: FATALITY RATES PER MILLION VEHICLE YEARS,

OCCUPANTS OF CHRYSLER CORP. CARS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

n of

Deaths

500

607

429

671

1037

1269

844

1541

Millions

of Car

Years

2.703

3.795

1.745

2.870

4.421

7.934

3.352

6.415

Deaths per

Million Yrs.

185.0

160.0

245.9

233.8

187.8

159.9

251.8

240.2

Gross

Red. (%)

14

5

15

5

NET RED.

FOR SBL*

Z Test

-10%

Z-1.10

-12%

Z-1.90

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha - .05 - i.e., Z greater than

1.96).

228
TABLE C-16

FARS 1975-85: EOECTEES# PER MILLION VEHICLE YEARS,

OCCUPANTS OF COMPACT AND INTERMEDIATE SIZED CARS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install. in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Ejectees

198

138

111

1063

358

316

1230

2394

419

418

1454

3510

484

539

1516

4368

Millions

of Car

Years

3.411

3.818

5.978

8.236

7.297

7.976

10.814

18.511

8.859

9.601

13.483

25.059

10.425

11.714

14.431

29.960

Ejectees per

Million Yrs.

58.0

36.1

121.6

129.1

49.1

39.6

113.7

129.3

47.3

43.5

107.8

140.1

46.4

46.0

105.1

145.8

Gross

Red. (%)

38

-6

19

-14

8

-30

1

-39

NET RED.

FOR SBL**

Z Test

-70%

Z*4.44*

-41%

Z-4.05*

-41%

Z=4.55*

-40%

Z=4.87*

*Statistically significant effect (two-sided alpha - .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

^Persons who were ejected in a crash that was fatal to somebody, but who,

themselves, were not necessarily killed.

229
TABLE C-17

FARS 1975-85: EOECTEES** PER MILLION VEHICLE YEARS,

' OCCUPANTS OF FULL SIZED AND LUXURY CARS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Ejectees

399

378

in

297

661

874

497

632

772

1177

630

851

857

1509

718

1077

Millions

of Car

Years

9.221

10.870

4.880

5.706

17.596

24.633

9.500

12.751

21.921

30.727

11.984

16.520

25.469

37.565

14.111

19.901

Ejectees per

Million Yrs.

43.3

34.8

56.8

52.1

37.6

35.5

52.3

49.6

35.2

38.3

52.6

51.5

33.6

40.2

50.9

54.1

Gross

Red. (%)

20

8

6

5

-9

2

-19

-6

NET RED.

FOR SBL*

Z Test

-14%

Z-1.20

none

Z-0.03

+10%

Z=l.46

+11%

Z»l.78

*Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group. None of the observed net effects

were statistically significant (two-sided alpha - .05 - i.e., Z greater than

1.96).

**Persons who were ejected in a crash that was fatal to somebody, but who,

themselves, were not necessarily killed.

230
TABLE C-18

FARS 1975-85: EOECTEES* PER MILLION VEHICLE YEARS,

OCCUPANTS OF GENERAL MOTORS CARS

2 DOOR VS. 4 DOOR SEDANS AND HARDTOPS OF THE SAME MAKES AND MODELS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Excludes station wagons, convertibles, and any make/models

that were produced only with 2 doors or only with 4 doors)

MY Before/

After SBL Millions NET RED.

N of Install. 1n n of of Car Ejectees per Gross FOR SBL**

Doors 2 Door Cars Ejectees Years Million Yrs. Red. (7.)

Z Test

4 last 1 before 275 5.639 48.8

first 1 after 212 5.800 36.6 25

2 last 1 before 418 4.796 87.2

first 1 after 447 5.259 85.0 2 -30%

Z-2.31*

4 last 2 before 398 11.199 35.5

first 2 after 480 13.282 36.1 -2

2 last 2 before 620 8.895 69.7

first 2 after 994 12.399 80.2 -15 -137.

Z-1.45

4 last 3 before 465 13.906 33.4

first 3 after 601 17.131 35.1 -5

2 last 3 before 755 10.856 69.5

first 3 after 1392 16.507 84.3 -21 -167.

Z-1.89

4 last 4 before 525 16.445 31.9

first 4 after 775 20.679 37.5 -17

2 last 4 before 827 12.298 67.2

first 4 after 1816 20.204 89.9 -34 -147.

Z=l.84

*Stat1st1cally significant effect (two-sided alpha « .05 - I.e., Z greater

than 1.96)

**Note: positive numbers Indicate that the SBL group (2 door cars) had greater

Improvements than the control group (4 door cars); negative numbers Indicate

that SBL did worse than the control group.

^Persons who were ejected In a crash that was fatal to somebody, but who,

themselves, were not necessarily killed.

231
TABLE C-19

FARS 1975-85: EJECTEES* PER MILLION VEHICLE YEARS,

OCCUPANTS OF FORD MOTOR CO. CARS

2 DOOR VS. 4 DOOR SEDANS AND HARDTOPS OF THE SAME MAKES AND MODELS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

(Excludes station wagons, convertibles, and any make/models

that were produced only with 2 doors or only with 4 doors)

N of

Doors

4

2

4

2

4

2

4

2

MY Before/

After SBL

Install, in

2 Door Cars

last 1 before

first 1 after

last 1 before

first 1 after

last 2 before

first 2 after

last 2 before

first 2 after

last 3 before

first 3 after

last 3 before

first 3 after

last 4 before

first 4 after

last 4 before

first 4 after

n of

Ejectees

106

99

147

248

221

275

335

516

293

477

419

1039

361

678

492

1384

Millions

of Car

Years

2.185

2.480

2.068

2.702

4.081

5.394

4.092

5.895

5.899

7.321

5.586

10.523

7.420

10.270

7.305

13.601

Ejectees per

Million Yrs.

48.5

39.9

71.1

91.8

54.2

51.0

81.9

87.5

49.7

65.2

75.0

98.7

48.6

66.0

67.3

101.8

Gross

Red. (%)

17

-29

6

-7

-31

-32

-36

-51

NET RED.

FOR SBL**

Z Test

z.

z.

Z-.

I*

-57%

-2.59*

-14%

-1.11

none

.0.04

-11%

=1.28

*Statistically significant effect (two-sided alpha - .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

^Persons who were ejected in a crash that was fatal to somebody, but who,

themselves, were not necessarily killed.

232
TABLE C-20

FARS 1975-85: EJECTEES# PER MILLION VEHICLE YEARS,

OCCUPANTS OF CHRYSLER CORP. CARS

2 DOOR VS. 4 DOOR CARS,

BEFORE VS. AFTER INSTALLATION OF SBL IN 2 DOOR CARS

MY Before/

After SBL Millions NET RED.

N of Install, in n of of Car Electees per Gross FOR SBL**

Doors 2 Door Cars Electees Years Million Yrs. Red. (%)

Z Test

4 last 1 before 150 2.703 55.5

first 1 after 140 3.795 36.9 34

2 last 1 before 151 1.745 86.5

first 1 after 233 2.870 81.2 6 -41%

Z-2.19*

4 last 2 before 274 4.421 49.6

first 2 after 274 7.934 34.5 30

2 last 2 before 297 3.352 88.6

first 2 after 561 6.415 87.4 1 -42%

Z=3.14*

*Statistically significant effect (two-sided alpha - .05 - i.e., Z greater

than 1.96)

**Note: positive numbers indicate that the SBL group (2 door cars) had greater

improvements than the control group (4 door cars); negative numbers indicate

that SBL did worse than the control group.

^Persons who were ejected in a crash that was fatal to somebody, but who,

themselves, were not necessarily killed.

233

REFERENCES

Cl] The Abbreviated Injury Scale (AIS). Morton Grove, 111.: American

Association for Automotive Medicine, 1976.

[2] Ball, John; Northrop, Gaylord; Knoop, Jim; and Sweeton, Edward.

Statistical Evaluation of the Effectiveness of Federal Motor

Vehicle Safety Standard 207: Seat Back Locks (Only). Technical

Report No. DOT HS 805 659. Washington: National Highway Traffic

Safety Administration, [1980].

C3] Carlson, Lloyd E., and Leonard, Paul. Cost Evaluation of Federal

Motor Vehicle Safety Standards 108 and 207. Technical Report No.

DOT HS 807 017. Washington: National Highway Traffic Safety

Administration, [1986].

[4] Code of Federal Regulations, Title 49. Washington: Government

Printing Office, 1981, Part 571.207.

[5] Code of Federal Regulations, Title 49. Washington: Government

Printing Office, 1981, Parts 572.1-572.11.

[6] "Collision Deformation Classification." SAE Recommended Practice

No. 0224 MAR80 in 1985 SAE Handbook, vol. 4. Warrendale, Pa.:

Society of Automotive Engineers, 1985.

[7] Costenoble, Kayla, and Northrop, Gaylord M. Review of Nine Federal

Motor Vehicle Safety Standards. Report No. 4238/4239-601.

Hartford: Center for the Environment and Man, [1978].

[8] CPIR Revision 3. HSRI Accident Data System Codebook No. 80-1. Ann

Arbor: Highway Safety Research Institute, [1980].

[9] Federal Register 46, 17 February 1981, 13193.

[10] Federal Register 50, 8 January 1985, 1036.

[11] Final Regulatory Impact Analysis. Average Fuel Economy Standard for

Model Years 1987-88. Passenger Cars. Washington: National Highway

Traffic Safety Administration, [1986].

[12] Final Regulatory Impact Analysis. MY 1987 Light Truck Fuel

Economy. Washington: National Highway Traffic Safety

Administration, [1985].

[13] First Annual National Vehicle Population Profile, as of July 1.

1975 - Domestic Cars - Area: U.S. Summary. Detroit: R. L. Polk,

1976.

235
[14] Garrett, John. W. Comparison of Door Opening Frequency In 1Q Q

Cars with Earlier Model U.S. Cars. Report No. DOT HS 800 231.

Washington: National Highway Traffic Safety Administration, [1969].

[153 . Remarks to C. J. Kahane (NHTSA), March 31, 1986.

[16] Gladstone, R.; Harvey, M. R.; arid Lesczhik, J. Weight and Consumer

Price Components of the 1980 General Motors Chevrolet Citation and

the 1981 Chrysler Plymouth Reliant, vol. 1. Report No. DOT HS 806

295. Washington: National Highway Traffic Safety Administration,

[1982].

[17] . Weight and Consumer Price Components of the 1980 General

Motors Chevrolet Citation and the 1981 Chrysler Plymouth Reliant,

vol. 2. Troy, M1ch.: Autosafety Engineering Corporation, [1982].

[18] Goryl, Michael E., and CynecM, Michael J. Restraint System Usage

1n the Traffic Population. 1984 Annual Report. Report No. DOT HS

806 714. Washington: National Highway Traffic Safety

Administration, [1985].

[19] Grush, Ernie S. (Ford). Remarks to C. J. Kahane (NHTSA), April 7,

1986.

[20] Harvey, M. R.; Lesczhik, J. A.; and McLean, R. F. Cost Evaluation

for Nine Federal Motor Vehicle Safety Standards. Volume IV. FMVSS

202 & 207. Report No. DOT HS 805 318. Washington: National

Highway Traffic Safety Administration, [1979].

[21] Humphrey, Richard F. (General Motors). Remarks to C. 0. Kahane

(NHTSA), March 24, 1986.

[22] Kahane, Charles 0. An Evaluation of Child Passenger Safety: The

Effectiveness and Benefits of Safety Seats. Technical Report No.

DOT HS 806 890. Washington: National Highway Traffic Safety

Administration, [1986].

[23] . An Evaluation of Federal Motor Vehicle Safety Standards

for Passenger Car Steering Assemblies: Standard 203 - Impact

Protection for the Driver; Standard 204 - Rearward Colgmn

Displacement. Technical Report No. DOT HS 805 705. Washington:

National Highway Traffic Safety Administration, [1981].

[24] . An Evaluation of Head Restraints - Federal Motor Vehicle

Safety Standard 202. Technical Report No. DOT HS 806 108.

Washington: National Highway Traffic Safety Administration, [1982].

[25] . An Evaluation of Side Marker Lamps for Cars. Trucks and

Buses. Technical Report No. DOT HS 806 430. Washington: National

Highway Traffic Safety Administration, [1983].

236
[26] . An Evaluation of Side Structure Improvements in Response

to Federal Motor Vehicle Safety Standard 214. Technical Report No.

DOT HS 806 314. Washington: National Highway Traffic Safety

Administration, [19823.

[27] . An Evaluation of Windshield Glazing and Installation

Methods for Passenger Cars. Technical Report No. DOT HS 806 693.

Washington: National Highway Traffic Safety Administration, [1985].

[28] . "The National Highway Traffic Safety Administration's

Evaluations of Federal Motor Vehicle Safety Standards." Paper No.

840902 in SAE Transactions. 1984. Warrendale, Pa.: Society of

Automotive Engineers, 1984.

[29] . A Preliminary Evaluation of Two Braking Improvements for

Passenger Cars - Dual Master Cylinders and Front Disc Brakes.

Technical Report No. DOT HS 806 359. Washington: National Highway

Traffic Safety Administration, [198;i].

[30] Kelleher, Barbara J., and Walsh, Michael J. "Sled Test Comparisons

of Child Restraint Performance." Paper No. 780903 in Twenty-Second

Stapp Car Crash Conference. Warrendale, Pa.: Society of Automotive

Engineers, 1978.

[31] Khadilkar, Anil V. (MCR Technology). Remarks to C. J. Kahane

(NHTSA), April 2 and April 4, 1986.

[32] Khadilkar, Anil V., and Forrest, Stephen M. Sled Tests for

Evaluating Child Safety Seat Effectiveness. Submission to NHTSA

Docket No. 85-16-N01-002. Goleta, Cal.: MCR Technology, [1984].

[33] McHenry, Raymond R. Extensions and Refinements of the CRASH

Computer Program. Part 2. Report No. DOT HS 801 838. Washington:

National Highway Traffic Safety Administration, [1976].

[34] Monthly Progress Report December 1984 - Sled Tests for Evaluating

Federal Motor Vehicle Safety Standard Number 207 - "Seating

Systems". Submission to NHTSA Docket No. 87-01-N01-002. Goleta,

Cal.: MCR Technology, [1985].

[35] Monthly Progress Report November 1984 - Sled Tests for Evaluating

Federal Motor Vehicle Safety Standard Number 207 - "Seating

Systems". Submission to NHTSA Docket No. 87-01-N01-002. Goleta,

Cal.: MCR Technology, [1984].

[36] Monthly Progress Report October 1984 - Sled Tests for Evaluating

Federal Motor Vehicle Safety Standard Number 207 - "Seating

Systems". Submission to NHTSA Docket No. 87-01-N01-002. Goleta,

Cal.: MCR Technology, [1984].

[37] MVMA Motor Vehicle Facts & Figures '82. Detroit: Motor Vehicle

Manufacturers Association, 1982.

237
[38] The National Traffic and Motor Vehicle Safety Act of 1966. Public

Law 89-563, as amended, 15 United States Code 1381-1431.

[393 1985 SAE Handbook, vol. 4. Warrendale, Pa.: Society of Automotive

Engineers, 1985.

[40] 1968 SAE Handbook. New York: Society of Automotive Engineers, 1968.

[41] Partyka, Susan C. NCSS - The Analyst's Companion. Report No. DOT

HS 805 871. Washington: National Highway Traffic Safety

Administration, [1981].

[42] Severy, Derwyn M.; Brink, Harrison M.; Baird, Jack D.; and

Blaisdell, David M. "Safer Seat Designs." Paper No. 690812 in

Thirteenth Stapp Car Crash Conference. New York: Society of

Automotive Engineers, 1969.

[43] Test Report for Tests Conducted 1n January-February 1985 - Sled

Tests for Evaluating Federal Motor Vehicle Safety Standard Number

207 - "Seating Systems". Submission to NHTSA Docket No.

87-01-NO!-002. Goleta, Cal.: MCR Technology, [1985].

[44] Vehicle Damage Scale for Traffic Accident Investigators. Chicago:

National Safety Council, 1971.

[45] Ward's 1985 Automotive Yearbook. Detroit: Ward's Communications,

1985.

[46] Ward's 1984 Automotive Yearbook. Detroit: Ward's Communications,

1984.

[47] Ward's 1983 Automotive Yearbook. Detroit: Ward's Communications,

1983.

[48] Ward's 1972 Automotive Yearbook. Detroit: Ward's Communications,

1972.

[49] Ward's 1968 Automotive Yearbook. Detroit: Ward's Communications.

1968.

[50] Ward's 1965 Automotive Yearbook. Detroit: Ward's Communications,

1965.

[51] Work Plan: Sled Tests for Evaluating Federal Motor Vehicle Safety

Standard No. 207 - "Seating Systems". Goleta, Cal.: MCR

Technology, [1984].

238