Perspective on Impact Biomechanics from Traffic Accident Analysis. M. Mackay. 1. Biomechanics and Its Impact on Human Life: From Gene Expression to Organ ...
J. Kajzer, E. Tanaka, H. Yamada (Eds.)
Human Biomechanics and Injury Prevention With 217 Figures, Including 3 in Color
Springer
Contents
Preface
;.
V
Contributors
XI
Perspective on Impact Biomechanics from Traffic Accident Analysis M. Mackay
1
Biomechanics and Its Impact on Human Life: From Gene Expression to Organ Physiology R.M.Nerem „
13
Recent Advances in the Biomechanics of the Head and Neck A.I. King, K.H. Yang
21
The Tibia Index: A Step in the Right Direction J.R. Crandall, J.R. Funk, R.W. Rudd, LJ. Tourret
„
29
The Biomechanics of Frontal and Lateral Collision D.Kallieris
41
Influence of Human Spinal Deformation on Minor Neck Injuries for Low Speed Rear Impacts K. Ono, S. Inami, K. Kaneoka, Y. Kisanuki
51
"Hybrid" Approach to Modelling of Biomechanical Systems C.Rzymkowski
59
Current Status of Finite Element Human Model Using PAM-CRASH Y. Matsuoka, K. Ando
65
Finite Element Model for Simulation of Muscle Effects on Kinematic Responses of Cervical Spine in Low-Speed Rear-End Impacts A. Wittek, K. Ono, J. Kajzer
71
A Biomechanical P.E.E.E.P. Show R.H. Eppinger
77
The Development of Chest Protection X. Trosseille, J.-Y. Foret-Bruno, E. Song, P. Baudrit
85
The Activities and Research Projects of the Ministry of Transport and Traffic Safety & Nuisance Research Institute Y.Nanto
93 VII
Perspective on Impact Biomechanics from Traffic Accident Analysis Murray Mackay Birmingham Accident Research Centre, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom. Summary. This paper begins with a brief review of the development of crash investigation techniques from the 1950s to the present day. Early work at Cornell University in the United States led to studies in Australia, Sweden and the United Kingdom and a general recognition of the fundamental importance of a detailed knowledge of the nature and circumstances of actual trauma to traffic accident victims. Today a number of "countries have on-going structured sample date collection programmes which provide an important means of assessing the effectiveness of biomechanical advances which are implemented through changing car design and regulations. Notably, the NASS/FARS programmes in the United States, by being freely available electronically, have become a valuable source to other countries. Such research illustrates an important yet still poorly understood aspect of impact biomechanics; that of variation within the population at risk. Examples are given of this issue in terms of head, neck, thoracic, leg and abdominal injury studies to show that a factor of three exists between the weakest and strongest elements of the normal population exposed to impact injury. Age and gender effects are discussed. The optimisation of protection recognising the two distributions of crash severity and impact tolerance variations leads to the conclusion that focusing on very severe collisions does not result in good overall protection. The links with experimental biomechanics are illustrated and the paper concludes with proposals for new research directions with increasingly detailed instrumentation of vehicles and more realistic modelling of the human frame. Keywords: Injuries, populations, optimisation, variability, biomechanics.
INTRODUCTION The purpose of this chapter is not to give specific recent research findings from current analyses of data on actual traffic collisions and consequent injuries; that aspect is covered by other contributors to this volume. Rather its aim is to review the evolution of the sector of traffic safety research as it impinges on our knowledge of the biomechanics of impact injuries. Historically the investigation of transport accidents has varied markedly by the different modes of travel, but in all modes the early focus was not to study causes of injury, but to examine the causes of the event. This applied to all the modes, air, road, rail and marine. Hence, many early studies give very cursory attention as to how injuries actually occurred, and this is still reflected in a number of the data systems used today. For example, it is still the standard practice in many hospital systems to describe the "mechanism of the injury" as "pedestrian hit by car" or "motorcyclist" or, somewhat more advanced, "car occupant in rear seat unrestrained". Clearly such descriptors provide almost no insight into the actual mechanisms of the specific injuries which have been sustained.
M. Mackay Indeed there is a useful parallel between such broadbrush descriptions of causes of injuries and similar sweeping descriptions of the "cause of the accident". A road accident investigation would cease with the conclusion that the cause of the accident was "inattention" or "driving too fast for the conditions". Such conclusions do no more than describe the obvious, without giving any real insights into why such behaviour occurred. This is still true of many police data bases around the world in the road accident area. In the early days of rail, air and marine investigations there was still that same sort of mind-set by the investigating authorities. Rail crash investigations for the last 150 years have often closed the file with the insightful conclusion that the driver failed to obey a red danger signal. Marine investigations have similarly concluded that the snip ran aground because of a navigational error. In the world of aircraft, Ken Mason, an eminent aviation pathologist, noted that "the concept of pilot error has put back crash investigation by a generation". With the focus of an investigation being to establish the cause of an accident and often to designate blame, the less obvious underlying factors of how an injury actually occurred and how it might have been prevented got scant attention. In the 1930s, in the aviation and rail sectors, the complexity of the analysis of the causes of an accident began to be recognised, with a greater appreciation of background factors and an increasing understanding of the frailties of human behaviour however well trained and supervised. In parallel with this more sophisticated approach to the investigation of causal factors, was an increasing insight into the specifics of how people in crashes actually got killed or injured. It was in that period in aviation that the biomechanics of impacts first began to be investigated objectively. It took another generation for such an approach to take root in the road crash arena.
EARLY KNOWLEDGE ON THE BIOMECHANICS OF IMPACTS An implicit recognition of impact biomechanics exists in all of us and is engendered from an early age. Soft, deformable structures are benign; sharp, rigid objects, like kitchen knives, deserve caution. From the earliest of times, the underlying principles of impact biomechanics. have been recognised: structures designed to maximise trauma are hard and concentrate loads, such as spears and clubs, while conversely, shields and armour absorb and distribute loads and protect vulnerable parts of the anatomy. Hippocrates, writing around 400 BC, noted that for head injuries, padding was valuable: "Of those who are wounded in the parts about the bone or in the bone itself, by a fall, he who falls from a very high place upon a very hard and blunt object is in most danger of sustaining a fracture and contusion of the bone, and of having it depressed from its natural position. Whereas, he that falls upon more level ground, and upon a softer object is likely to suffer less injury in the bone, or it may not be injured at all." *
Like most subjects, impact biomechanics have evolved from early observations of natural phenomena, through an experimental period to a theoretical framework that outlines general laws and precepts. Hugh de Haven is normally credited with the first insights into human tolerance of crash loads. During World War I, he was involved in a mid-air collision. While convalescing, he realised that his survival was due to the maintenance of the integrity of his cockpit that, together with a safely harness, protected him from the localised contacts and catastrophic injuries that killed the other pilot. He also observed that his own serious abdominal injuries related to the buckle of his harness, causing a severe internal haemorrhage with laceration of the liver. Crash-protective design, rather than capricious good fortune, had ensured his survival.
Impact Biomechanics from Traffic Accident
3
In 1942 De Haven analysed the circumstances of eight people who fell from considerable heights, seven of whom survived. Speeds at impact ranged from 37 to 59 mph and the objects struck consisted of fences, a wooden roof, soft ground, and in two cases the bonnets of cars. Decelerations at impact were estimated and the groundwork for whole body tolerance was laid. Subsequently Snyder et al (1977) developed this approach into a most useful methodology for obtaining such data. In 1941, Sir Hugh Cairns published a paper on fatalities occurring among Army dispatch riders. He showed that for those who wore helmets the head injuries were relatively mild. Later work by Cairns (1946) showed that following the compulsory wearing of helmets by Army motorcyclists there was a progressive fall in the death rate. He also noted that most blows on helmets were to the front and side rather than to the crown. The consequences of that observation have been reflected 30 years later in the evolution of the jet-style and full-faceiislmets of today and the demise of the cradle suspension inside the helmet. ( John Lane in Australia in 1942 noted that aircraft should be certified in two ways: they shouki be both airworthy and crashworthy, and so the term 'crashworthiness' was born, but its application to automobile design did not begin until some 20 years later. The experimental period of biomechanics got under way after World War n, with cadaveric studies by Gurdjian (1945) examining head injury in the main, and volunteer studies conducted by Stapp (1951). The great contribution by Stapp was to show that the primary forces acting in the majority of car collisions are entirely survivable if the packaging of the human frame is satisfactory. >He showed that accelerations of 30 G for up to 0.5 s were entirely tolerable with only reversible soft tissue injuries occurring. At 45 G, signs of concussion and retinal haemorrhage begin to show. These accelerations were measured on the seat of the dynamic sledge. The accelerations experienced by the head itself were much greater. In the 1950s, Severy and Mathewson (1954) were developieg the techniques of experimental crash testing with instrumented dummies and high-speed film analysis. By the mid-1960s, a body of knowledge had developed that gave insights into the general frequencies of traffic collisions and injuries, some understanding of the actual mechanisms that generate the injuries, and some means whereby the forces and accelerations applied to car occupants could be modified. What was largely missing was accurate information on the tolerance of the actual human frame to specific impact loadings, ideas of the likely benefits that could be obtained from practical changes in car design, and what the penalties in design terms would be. In parallel with the increasing amount of experimental work on both cadavers and animals, studies of real-world trauma continue. An elegant paper by Sheldon (1960), entitled 'On the natural history of falls in old age', showed how the routine observations of a practising clinician could lead to new insights into the aetiology and mechanisms of injury, particularly for long-bone fractures in the elderly. In experimental biomechanics Yamada (1963, 1970) presented an immense amount of data which described the properties of various human tissues, bone, skin, nerve fibres, cartilage and connective tissues in terms of their basic engineering parameters. He demonstrated the marked anisotropic characteristics of many tissues, particularly bones, the viscous natures and rate dependency of many tissues and variations in properties through the population according to gender, age and other factors. This is an area which still deserves greater recognition. Pioneering work on seat belts was conducted in Sweden where, by 1960, some 50 per cent of private cars had belts fitted. The appropriate elongation and geometrical characteristics of belt systems were evaluated experimentally by Aldman (1962), who demonstrated the importance of
4
M. Mackay
correct anatomical positioning and dynamic properties appropriate to the deformation and geometrical characteristics of specific car designs. The subject of biomechanics in relation to car-occupant crash protection grew rapidly in the 1950s and 1960s and became institutionalised with an extraordinarily important legislative act in the USA. As a result of government hearings that illustrated the great potential of crash protective design, in 1966 the National Highway Safety Bureau was created by act of Congress and it initiated a set of standards controlling the performance of cars in terms of their crashworthiness. The effects of those standards have reverberated through the automotive world ever since. They have been copied, modified, adopted by almost every country with a significant car population and they have changed car design from a free market, styling dominated activity to one in which certification, or passing the standards, with all the attendant engineering problems, is of prime importance in the priorities of car manufacturers. The scientific basis of these first crash performance standards was not well founded; many of the requirements were informed guesswork only. With the benefit of hindsight, quite extraordinarily few major mistakes were made, but what has also become clear is that the subject is a very complex one. The real world of collisions contains many surprises. Common sense has that most curious property of being more correct retrospectively than prospectively, and a major gap was left as the subject became more under the control of government and industry and away from the individual efforts of the early workers. That increasing problem was the absence of sufficiently detailed and representative real world crash data.
THE DEVELOPMENT OF TRAFFIC ACCIDENT DATABASES One of the pioneering achievements of Hugh de Haven in the 1950s was establishing the Automotive Crash Injury Research (ACIR) programme at Cornell University in the United States. Working with local and state police, specific sample studies were conducted in which the police agreed to collect more data on an accident than was the traditional standard practice. With this methodology greater knowledge was developed on the frequencies and consequences of various factors such as ejection, door opening rates, car size, seat belt effectiveness, windshield glass performance and other crash related factors. But the limitations of police and insurance company data bases as a source of information on the specifics of injury mechanisms became apparent. Following this, in the 1960s, individual workers started to investigate collisions themselves using detailed, multi-disciplinary at the scene and follow up procedures. Teams of engineers and doctors, ait Universities in Michigan, California, Adelaide and Birmingham began such studies. n
From these developments the usefulness of detailed analysis of occupant kinematics, their contacts with interior structutres and the performance of those structures in mitigating the severity of injuries was appreciated. That lead to a proliferation world-wide of in-depth crash investigation research and to the establishment of longitudinal studies in France, Germany, Sweden, Britain and the United States, which now provide an invaluable source of data covering over 20 years of crash and injury outcome details. In particular the United States programmes of PARS, NCSS, NASS and its successors CDS and GES, because the data are freely available under the Freedom of Information Act of the United States and because of the Internet, are now a valuable research resource used by
Impact Biomechanics from Traffic Accident
5
.
crash injury researchers all over the world. Such openness with the data paid for by taxpayers could well be emulated elsewhere. The current CIREN programme in the United States is a return to the detailed in-depth studies of the 1960s, where new insights into the biomechanics of injuries are obtained based on very detailed analysis of a few or even single collisions. Inevitably however, such small samples, biased towards casualties admitted to trauma centres, mean that the general applicability of the findings must be examined carefully. Following the development of these accident data bases specific parameters have evolved which are used to describe the two •fundamental aspects of a prash. These are the severity of the crash itself and the severity of the outcome in terms of injuries. In the early days crash severity was defined qualitatively based on photographs of crashed cars, for example in the TAD and ACER scales and Moreland's Damage index. Attempts were then made by Campbell and Patrick to relate a specific collision to its Equivalent Barrier Speed and by Mackay somewhat more widely to an equivalent test speed, (Mackay 1968). By the early 1970s the calculation of the change in velocity (delta V) at the centre of gravity of the vehicle in a givsn collision and relating it to the energy and momentum equations and stiffness characteristics from crash tests was established, particularly by Centre, and delta V is still the preferred parameter for gauging collision severity today (Centre, 1972). Alternatives however, have been proposed. For belt restrained occupants, particularly belt load and hence peak deceleration is a more appropriate parameter and mean deceleration is undoubtedly a better parameter than delta V for most vehicle to vehicle collisions. Today however, new technology is on offer, which can give a much more detailed description of a collision. On-board crash recorders are a realistic proposition for wide scale introduction. They have the ability to record objectively the shape of the time/deceleration history and many other events relating to inflatable restraints, vehicle attitudes with time, pre-impact braking, restraint use and occupant position. Such devices would enhance enormously the accuracy, objectiveness and detail of ongoing accident data bases and thus provide a major input into our knowledge of injury biomechanics in actual accidents. The second fundamental parameter relates to measuring the outcomes of a collision. In the early days injury scaling was almost totally subjective, with descriptors such as slight, incapacitated, bleeding, unconscious, serious, being used by both police and researchers. With the rise of in-depth crash investigation teams in the 1960s, almost every research group had its own injury severity scale. Agreement and uniformity of terminology arrived with the Abbreviated Injury Scale (AIS), which has been refined to the current dictionary (AIS 98) and is now accepted and used world-wide. That acceptance enormously enhanced the value of accident data bases by allowing comparisons and joint studies to be made. The AIS however is only one dimension of injury severity, that of threat to life, and clearly for the categories of AIS 1, 2 and for many AIS 3 injuries the threat to life is now a major consideration. Hence, many of those injuries are classified in an agreed category of severity, but without any obvious outcome comparison. With an increasing recognition of the importance of disabilities which can arise from injuries particularly in the AIS 1, 2 and 3 categories, there is a clear need for a disability scale, matching the definitions of injuries described in the AIS and thus applicable to accident data bases throughout the world Petrucelli, et al (1983).
M. Mackay
BIOMECHANICAL INSIGHTS FROM TRAFFIC ACCIDENT ANALYSIS Impact biomechancs draws on knowledge from several types of research. Laboratory studies used volunteers, physical dummies, computer modelling of lumped mass or FEM systems, cadavers and specific human tissue segments, animal studies and analysis of actual traffic crashes. This latter category has the big advantage of dealing with real people in actual accidents, but with the attendant disadvantages of all the uncertainties of the reconstruction process. The potential of crash recorder technology is enormous in this area as has been demonstrated by (Melvin, et al 1998) in the context of instrumenting crashes at Indianapolis. Data from that work challenges the injury assessment criterion for lateral loading of the thorax, but the nature of the population exposed, healthy young males, may well be a reasonable explanation for the differences in outcomes for such loadings compared to cadaver studies. It is one example of the importance of population variations, considered below. Traffic accident analysis, focused on crashes occurring under everyday circumstances has the unique advantage of providing the epidemiology of impact trauma. Such analyses provide the basic data on frequency and severity of actual injuries, and with careful reconstruction, the specific of how those injuries occur. Frequency issues. The widest use of traffic accident analyses is to describe the basic epidemiology of impact trauma, the patterns of injury to various road users, the frequencies of various events and the outcomes in terms of body regions injured, the severity of these injuries and their causes in terms of contacts made with structures of the vehicle. Such studies establish the priorities for further research, monitor changing injury patterns with time, and give warnings of new mechanisms of injury and crash characteristics. Hundred of such studies have been published over the last 30 years. Logically they should be the basis for establishing new vehicle safety regulations and monitoring existing ones, and providing insights for vehicle designers as to the effectiveness in the real world of their designs. This approach has only relatively recently been recognised in many parts of the world, and in Europe for example there is no comparable data base to CDS and PARS, Such studies as are conducted are small scale and not typical of E.U. collisions generally. Specific studies from BARC in the United Kingdom have described patterns of injuries over the years; the relative injury potential of tempered compared with laminated windscreens, still a live issue in Europe as a tempered windscreen is still allowed and is common in light trucks and in the after-market; the roll of intrusion and crash severity on lower limb injuries; the increased incidence of diffuse axonal injury in lateral collisions compared to frontal crashes; the limits of current seat belts and the injuries which arise in those limiting conditions; and studies on the effectiveness of many design changes such as improved door latches, side door beams, energy-absorbing steering columns, seat back strength and yield characteristics, and patterns and frequencies of injuries to other road users and the causes of those injuries, particularly for pedestrians and motorcyclists. These specific projects are listed to illustrate how in-depth studies of traffic crashes, if designed to be representative of the relevant parameters to be studied by using appropriate sampling techniques, can produce valid insights into the biomechanics of impact injuries. Incidence data. A neglected area of accident analysis is the use of simple observational studies of people in cars. Often however such studies can give useful insights into specific mechanisms of injury. For example Crandall et al (1996) took video observations of foot positions during driving and braking demonstrating, amongst other things, that heel separation from the floorpan increased with decreasing stature, with the result of increased foot and ankle injury risk.
Impact Biomechanics from Traffic Accident
7
Parkin et al (1993) developed a technique of recording the head position of drivers and passengers in relation to forward structures for populations in the United Kingdom and the USA, and Mackay et al (1997) quantified how small females, in particular, sit significantly closer to the steering wheel than is suggested by the standard fifth percentile female Hybrid m dummy position. Similar observational studies have recorded the incidence with which drivers under straight road conditions place a forearm across the cover of a hub-mounted airbag. Such studies can be helpful in setting boundary conditions for risk assessments and experimental investigations. Individual investigations. The clinical literature is full of papers describing individual, unusual cases of trauma, and its origins. Similarly, individual collisions when analysed in detail may give insights into unusual mechanisms of injury, and may give an early indication of an emerging more general issue. Some detailed analyses of injuries occurring when airbags deploy for example, can give insights into when during the crash an airbag actually inflates. Populations at risk. An under-researched area is the relationship of injury risks for the exposed population. This relates to both the characteristics of collisions and the variations of the biomechanical and anthropometric properties of the population. It is well known that collision severity in all configurations varies in a markedly skewed manner with many low speed events and very few at very high energy levels. Figures 1 and 2 illustrate the delta V distributions for frontal collisions for occupants injured firstly in the range of AIS 2-6, and secondly for fatalities only. Figure 3 illustrates a similar skewed distribution for rear end collisions (Parkin et al 1995) for all levels of injury severity. Secondly, an extensive literature exists concerning human response to impact forces, mostly conducted in an experimental context. A general conclusion from that body of knowledge is that for almost any parameter, there is a variation of at least a factor of 3 for the healthy population exposed to impact trauma in traffic collisions. That variation applies to variables which are relatively well researched such as the mechanical properties of bone strength, cartilage, ligamentous tissues and skin. It is likely to be even greater when applied to gross anatomical regions such as the thigh in compression, the thoracic cage, the neck or the brain. How such variability is demonstrated in populations of collisions is less well understood. Data from a ten year period of the European Co-operative Crash Injury Study (CCIS) for restrained front seat occupants are given in Figures 4 and 5. The methodology of that work has been described by Mackay, (1985). Figure 4 illustrates the effect of age on injury outcome in terms of the frequency of AIS 2 and greater injuries for three age groups1. Data are presented for frontal impacts involving a principal direction of force (PDF) of 11 to 1 o'clock, controlling for crash severity by equivalent test speed (ETS), generally close to the delta V of the collision. The 60+ age group especially shows greater vulnerability than the younger groups. As a broad generalisation one may conclude that for the same injury severity, the younger age groups must have a velocity change of some 10 km/hr more than the elderly. The effect is more marked if a more severe injury level is chosen. Figure 5 illustrates the cumulative frequencies for the three age groups for injuries of AIS 4 and greater. Figure 6 shows similar frequency curves for crash severity by sex of occupant. Thus at a velocity change of 48 km/hr (30 mph), some 2/3 of male and some 80% of female AIS 2+ injuries have occurred. As a starting point, therefore, as well as specific body weight and sitting position, a combination of age, sex and biomechanical variation could be developed as a predictor of the tolerance of a specific subset within the population range.
M. Mackay
^ 100000
5-
^ $5
n
| I 1• .
x ^
01-05 05-10 11-15 16-20 21-25 26-30 31-35 36-40 41-45 46-50 51-55 >=56 Totml D«IU V (mph)
Total Number of Fatal Vth\ -. M
N
Fig. I Data V for all injury passenger vehicles'" (frontal accidents) Notes: (I) injury vehicles include vehicles with occupant injuries of AIS severity 2 through 6. (2) Includes all sedan, coupe, hatchback, convertible, and station wagon passenger car with at least one injured occupant. Source: "National Accident Sampling System, 1988-93 Continous Sampling System, " U.S. Department of Transportation, National Highway Traffic Safety Administration. 3500
S 3000 Jt
S53 N^ ^
^
0
C3 0 0 =3
0
ui a
o ui ca o o o
.
en o
1
11-15
16-20
^ ^ ^$
sS$
^ ^§ ^
5s$ 5s§ s$s; w §S §$5 $$; $& ^ 21-25
25-30
31-35
35-40
Total Delta V(mph)
S 1S 1
41-45
45-50
51-55
>-56
Fig. 2 Delta V for all fatal passenger vehicles'" (frontal accidents) Notes: (1> includes all sedan, coupe, hatchback, convertible and station wagon passenger cars with at least one occupant fatality. Source: "National Accident Sampling System, 1988-93 Continous Sampling System, " U.S. Department of Transpiration, National Highway Traffic Safety Administration.
90+
m
Banded ETS (km/rir)
80-89 ra I
70-79
60-69 fjjjjjf"'""""
0Serious a Sligl It ijuied •Unlr
t
50-59 im™ mnp, 40-49
^ Jjfr^yj^
.
30-39 20-29
10-19 0-9
0
10
20
30
Number of front ocupants
Fig. 3 Police severity of front occupants in impacts by ETS (B'ham '93).
40
50
Impact Biomechanics from Traffic Accident
J:is
So tree
itud i
1! 83-- 932
,."•" .•
/
'/
/
£ so I
f
.
W
CM
CM
CM
to
ETS - Crash'3 - kmli
Fig. 4 Crash speed distributions for frontal impacts (PDF of 11 to 1 o'clock) to drivers (by age groups) who experienced injuries with a MAIS >= 2.
So jrce
JIS
y is 83--
-tud
992
I '"***"*
...-•"" ;
,••*"'
I
f
«
; /
Age-16-29(n-17) Age-30-59 (n-35)
40
/
; /
Age-60-*- (n-1 6)
/
,/ '/
X
S/
,„••
.-* x>-'"
v=-:"
/ O> •*
o
-
10
-
o
w
o
10
o
ft VO
O) U7
ID
ID
. ! ' . ' •
.•
O
r—
•
ETS-knVh
Fig. 5 Crash speed distributions for frontal impacts (PDF of 11 to 1 o'clock) to front seat occupants (by age groups) who experienced injuries with a MAIS >= 4. 100 Sot rc« CCIi Jtu< rl> S3-1!92
/
L/
;
75
/7
X
; 50
X
/ ,' rnlil
X
7 ^>lc(n=61)
•*> X
*/
I
Tanile (1=134)
ETS-km/h
Fig. 6 Crash speed distributions for frontal impacts to front seat passengers (by sex) who experienced injuries with a MAIS >= 2.
10
M. Mackay
Then there are anthropometric considerations. Current dummies and modelling cover the 5th percentile female to 95th percentile male range. Assuming for simplicity that males and females are exposed equally and that there are few males smaller than the 5th percentile female or females larger than the 95th percentile male, these conventional limits put 2.5% (1 in 40) of the small population and 2.5% of the larger population beyond those limits; 5% or 1 hi 20 overall. Table 1 gives the 1% and 99% ranges for height, sitting height and weight. These data show what would be required if the design parameters were extended to cover this wider range, so that only 1 in 50 of car occupants would be outside the design parameters (Soc. Actuaries, 1979). Table 1. Population ranges for height, sitting and weight
Adult
Height ins/cm
l%ile female 5%ile female 95%ile male 99%ile male
57/145 59/150 73/185 75/190
Sitting Height ins/cm 28/72 29/75 37/93 38/96
Weight Ibs/kg 82/37 90/41 225/102 236/107
More importantly, it is implicitly assumed in current vehicle designs that height (or sitting height) and hence sitting position is collinear with the weight of the occupant. In fact, there are data available to suggest that the relationship between height and weight are rather complex. For example, the body mass index (BMT) (i.e., the ratio of weight hi kilograms to height hi meters squared) varies to a greater degree hi women than in men, and particularly at the 75th percentile and above, women have higher BMIs than men. In addition, the prevalence of overweight increases with age, more with females than males (Williamson, 1993). Thus the factors of gender and age, used as a surrogate for biomechanical tolerance, height, weight and BMI, sitting position and posture can all be quantified in population terms for appropriate subsets of the occupant population exposed to injury risk in the crash populations illustrated with examples in Figures 1-3. The optimisation of protection recognising the twin aspects of human variability and crash severity exposure remains a major research challenge.
NEW RESEARCH DIRECTIONS From this brief and superficial review of how traffic accident analysis contributes to biomechanical knowledge, some directions for new research are proposed. In-depth, representative databases. With the exception of the United States, there is an absence of fundamental data about the details of crash frequencies, mechanisms of injury and occupant characteristics, in terms of comprehensive longitudinal studies. Relatively small scale research projects exist in Japan, the United Kingdom, Sweden, Germany and France, but there is little compatibility and integration between them. Given the magnitude of the traffic injury problems
Impact Biomechanics from Traffic Accident
n
world-wide the absence of detailed investigation and monitoring of crashes is striking. Compared to other travel modes the quality of the investigation of road traffic deaths and injuries is poor. Injury reference values. Conceptually these are still considered as single point, pass or fail criteria. For some parameters, for example the HIC and the TTI, there is some data that allows injury probabilities to be related to different values of these parameters, but there is little data which actually examines the consequences of a particular IRV on an actual population. For example, does an HIC of 1000 have the same injury risk for males as for females? Accident data suggests it does not. *
Optimisation of protection. Given the two populations of human variability and crash severity exposure, the minimisation of injuries as a result of the interaction of these two populations becomes a complex matrix. Optimising design for single point requirements in crashes which represent the upper extremes of the crash severity spectrum will not minimise injuries for everyone and will leave the vulnerable segments of the population exposed to conditions of less than optimal design for them. Human versus dummy response. It is well recognised that current dummies aim to replicate human response to crash loads at discreet crash severities, mainly at about 50 km/hr. For lower speeds the dummy response will not be like that of the human, as is illustrated by the work on rearend impacts. Developing transfer functions from dummy to human at a range of crash severities will be a useful area of biomechanical research; there is no reason to suggest that those functions will be linear. Road users other than vehicle occupants. On a global basis, car occupants are a small proportion of all road casualties. More research on the biomechanics of pedestrian and motorcyclist injuries must be a top priority.
REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
R. G. Snyder, D. R. Foust and B. M. Dowman. Studies of Impact Tolerance through FreeFall Investigation. Un. Mich. 1977. HSRI Report No.77. H. Cairns. Crash Helmets. Br. Med. J. 1946. No.4470. p322-328. E. S. Gurdjian. Experimental Studies on the Mechanism of Head Injury. Res. Bull. Ass. Nerv.Ment.Dis. 1945. Vol.24. p48. J. P. Stapp. Human Exposure to Linear Acceleration. Aero. Med. Lab. Air Force Report. 1951. Report 5912, 2. D. M. Severy and J. H. Mathewson. Automobile Barrier Impacts. 1954. Nat. Res. Council Pub. No.334. p39. J. H. Sheldon. On the Natural History of Falls in Old Age. 1960. Brit. Med. J. Vol.10. p!685. H. Yamada. Human Biomechanics. Kyoto Profectural University of Medicine, Kyoto, Japan. 1963. H. Yamada. Strength of Biological Materials. Ed. F.G. Evans. Williams and Wilkins Co. Baltimore. 1970. B. Aldman. Biodynamic Studies of Impact Protection. Acta. Physiol. Scand. 1962. Stockholm. No.56. p!92. H. de Haven. Accident Survival - Airplane and Passenger Automobile. Proc. Sym. Packaging the Passengers. 1952. Soc. Auto. Engrs. New York.
12
M. Mackay
[11] G. M. Mackay. Injury and Collision Severity. Proc. 12th Stapp Conf. 1968. Soc. Auto. Engrs. New York. Paper 680779. P207-219. [12] P. Ventre. Homogeneous Safety Amid Helerogeneous Car Population? Proc. 3rd E.S.V. Conf. 1972. N.H.T.S.A., Washington D.C. p2-39-57. [13] E. Petrucelli, J. D. States and L. M. Hames. The Abbreviated Injury Scale: Evolution, Usage and Future Adaptability. Ace. Anal. & Prev. 1981. No.13. p29-35. [14] J. W. Melvin, K. J. Baron, W. C. Little and T. W. Gideon. Biomechanical analysis of Indy car Race Crashes. Proc. Stapp Conf. S.A.E. 1998. p247-266. [15] J. R. Crandall, P. G. Martin, C. R. Bass, P. C. Dischinger and A. R. Burgess. Foot and Ankle Injury. Proc. 40th Conf. Am. Ass. Automotive Med. 1996. Pl-18. [16] S. Parkin, G. M. Mackay and A. Cooper. How Drivers Sit in Cars. Proc. Am. Ass. Automotive Med. 1993. P375-388. [17] G. M. Mackay, A. M. Hassan and J. R. Hill. Adaptive Restraints - their Characteristics and Benefits. Autotec. 1997. Published by Inst. Mech. Engrs. Paper C524/190. P37-52. [18] S. Parkin, G. M. Mackay, A. M. Hassan and R. Graham. Rear End Collisions and Seat Performance. Proc. Ace. Ad. Auto. Med. Conf. Chicago. 1995. P231-244. [19] G. M. Mackay, S. J. Ashton, M. Galer and P. Thomas. Methodology of In Depth Studies of Car Crashes in Britain. Proc. Conf. On Accident Investigation Methodologies. Soc. Auto Engrs. 1985. Paper 850556. [20] Society of Actuaries. Build and Blood Pressure Study. London. 1979. [21] D. F. Williamson. Descriptive Epidemiology of Body Weight and Weight Change in U.S. Adults. Ann. Intern. Med. 1993. Vol. 119. Pt2. p646-9.
