RESEARCH Passive Safety

20 ATZ worldwide 1/2003 Volume 105

By Werner Koch and

Mark Howard

Ganzheitlicher Ansatz zur

Verbesserung der Fußgängersicherheit

bei Fußgänger-Pkw-Unfällen

You will find the figures mentioned in this article in the German issue of ATZ 1/2003 beginning on page 58.

ComprehensiveApproach to Increased Pedestrian Safetyin Pedestrian-to-Car Accidents

At the Ford Forschungszentrum Aachen a finite element pedes-trian humanoid model for use in pedestrian accident simulationswas constructed that is capable of being used as vehicle engi-neering development tool. To further improve the understand-ing of the kinematics of pedestrian accidents and to optimisethe computer simulation program it is necessary to collect a setof highly detailed real world data. At present that data is eitherunavailable, or not sufficiently accurate for this purpose. Tomeet these targets an interdisciplinary study has been estab-lished. In parallel a demonstrator vehicle has been build toshow future technologies in pedestrian safety.

21ATZ worldwide 1/2003 Volume 105

1 Introduction

The statistics show that in Europe approx.41,000 people die every year as a result ofinjuries sustained in road traffic accidents(source: IRTAD 2000). Of these some 6,000are pedestrians (approx. 16 % of all road fa-talities). The total number of fatally injuredpedestrians per year has dropped since theend of the 70s, Figure 1.

Pedestrians are one of the major roaduser groups and it is therefore useful toconsider the proportion of pedestrian fatal-ities compared to other road user groups, asshown in Figure 2. Pedestrians haveformed the second largest group of road fa-talities for many years in Germany and inEurope (and still do in the UK) though pow-ered two/wheelers are now about thesame. In Japan the situation is even worse,where pedestrians form the largest groupof road fatalities.

Data from the German In-Depth Acci-dent Study (GIDAS) shows that in 77 % ofthe cases the collision partner is a passen-ger car. The majority of pedestrians killed(68 %), died in built-up areas. If non-fatalinjuries are included, this figure rises to 94% for urban regions. The fatality/non-fatal-ity ratio is however less favorable in ruraldistricts due to the generally higher speedsinvolved in collisions. The over-sixties andunder-tens were the groups most affected.The former group accounted for 48 % of allurban fatalities whilst 25 % of all serious ur-ban injuries involved people belonging tothe latter group.

Head, upper and lower extremities arethe most frequent injured body regions inpassenger car-to-pedestrian accidents.However, 62 % of pedestrian fatalities werecaused by head injuries. Brain contusionand scull fractures in general are mainlycaused by pedestrian head contact to wind-screen/A-pillar and hood. Non-live threat-ening injuries, e.g. soft tissue abrasion, armand leg fractures, as well as concussionswere generated by impact to vehicle frontend/bumper and road surface.

It is the responsibility of society and notone stakeholder (e.g. vehicle manufactur-ers) to implement pedestrian protection so-lutions. Each stakeholder has a limited areain which they have a primary influenceand can contribute to solutions (e.g. gov-ernments can influence public transport in-frastructures). This paper concentrates onthe research efforts of one stakeholder tounderstand the real nature of pedestrianaccidents by using a combination of realworld accident data and finite elementmodelling to create and analyse pedestrianaccident scenarios. The purpose of such anapproach is to achieve an improved under-

standing of real world pedestrian accidents.Other types of approach have been previ-ously described and published within aconceptual framework [8, 9, 10].

Having tried other research approaches,in 1996, the Ford Research Lab Aachen (FFA)offered a future vision for pedestrian safe-ty. This vision was to be able to create realworld pedestrian accident simulation mod-els in which an engineer would be able tosimulate any type of pedestrian accident bysimply entering pedestrian and vehiclecharacteristics. To achieve this vision, tworequirements were identified: more accu-rate vehicle models and a family ofscaleable pedestrian humanoids thatshould incorporate significant injury mech-anisms. This resulted in the creation of thepedestrian protection research programshown in Figure 3.

2 Finite Element HumanoidModels

The formulation of the humanoid modelthat was originally based on dummydatasets is gradually being upgraded tohave a flexible and fully deformable limbformulation. The construction is based on afinite element representation that can beemployed with vehicle finite element mod-els to allow greater interaction between thetwo models than can be achieved by a rigidelement formulation with contact stiffness.

The aim in the humanoid modelling wasto achieve a human like representationwith biofidelic properties, yet avoiding adetailed finite element construction. De-tailed models would require considerablecomputer processing time and prevent useof the models for the rapid assessment ofpedestrian protection technologies. Themodelling approach was to identify the pri-mary injury locations, which would needgreater attention as they represented themost frequent and more severe injurymechanisms [2, 13, 14].

The humanoid mathematical modelswere constructed to examine overall kine-matics and assess the likely occurrence ofinjury mechanisms (e.g. bone fracture, liga-ment rupture and soft tissue injuries). Forthese purposes and the detail model con-straints, the bones of the leg, for example,where modelled as a series of beams to rep-resent the femur and tibia – the patella andfibula were omitted for lateral impacts asrecommended by Bermond et al. [3]. Thebeams were formulated to give elastic de-formation up to the point where bone frac-ture would occur. Solid elements were at-tached to the beams at coincident nodes, al-lowing loads generated in the solids due tocompression to pass directly into the

beams. At the knee joint three rotationaland three translational springs wereutilised to represent the full ranges of mo-tion present in a human knee. Similarly, inthe neck model, a formulation with sevenrigid cervical vertebrae and a six degree offreedom joint between each was imple-mented in LS-DYNA3D, based on the Globalmodel developed by de Jager [12].

The development of a 50th percentile fi-nite element humanoid model is only part ofthe whole story for pedestrians. A process ofgenerating Humanoid models has been de-veloped. To aid further development of thefinite element humanoids, all generationstarts from a 'reference model' that is adjust-ed for size and shape based on the regressionequations in the GEBOD [4] program. Thegeneration process develops the mass andmoments of inertia for the models and pro-duces a keyword file that is immediately us-able with the LS-DYNA3D code. Techniquesfor scaling the joint and material propertiesare being developed in parallel with the geo-metric data. The scaling of these parametersis particularly important where they relatethe children and the elderly.

Consequently, humanoid models can begenerated that represent children from 3years to 15 years of age and adults – male orfemale – from 5th to 95th percentile. Pedes-trian size is very important for the overallkinematics and has therefore a strong in-fluence on the simulation findings. Up-dates of the GEBOD program anthropomet-rics database, for example with the comple-tion of the SAE's Caesar Project, will keepthe generated humanoids current with thesizes and shapes of the current population.The range of the models generated is illus-trated in Figure 4.

Simulations and parametric studies inprevious published work [6, 10] have indi-cated that two parameters, vehicle speedand bumper (car front) height, were thecause of the greatest variation in simula-tion results and predicted injury mecha-nisms. Overall body kinematics were great-ly influenced by the standing position(stance) of the humanoid. With the legs to-gether and the humanoid sideways to thecar, the humanoid generally fell sidewayswith little rotation about his vertical axis.However, with a fore and aft separation ofthe legs the humanoid rotated about hisvertical axis to finish face down or face upon the car. This might result in more severehead and neck injuries by removing theshoulder contact.

3 Humanoid Validation

To validate the finite element model of thehumanoid it was necessary to compare its

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22 ATZ worldwide 1/2003 Volume 105

response with data from impacts withpedestrians. Real world comparisons wouldbe ideal but the variability and element ofdoubt over some key information involvedin using real world data dictated against thismethod. Instead data from staged pedestri-an impacts using cadavers as pedestriansubstitutes were used by a Japanese re-search group and the particular data select-ed was reported by Ishikawa et al [11] in 1993and has subsequently been used by other re-searchers [1]. For the validation of the hu-manoid its responses were compared to theresults from two of the tests. These testswere chosen since they covered a wide sepa-ration in impact speeds and vehicle geome-try. Full details of the vehicle shapes and thecontact stiffness of the vehicle componentsused in the tests are given by Ishikawa [11].Figure 5 shows the test set-ups.

Validation of the humanoid model wasconducted on two levels; body segment tra-jectories and measured parameters (headresultant velocity). Body segment trajecto-ries were obtained from target markers po-sitioned on the head, pelvis, knee and footduring the cadaver tests. The trajectoriesand head velocities from the humanoidmodel were compared against the mea-sured trajectories and head velocities. Thecomparisons are shown in Figure 6 for boththe test configurations.

In the test T9 configuration the knee andfoot trajectories were particularly pleasingsince Ishikawa [11] and Yoshida et al [18]noted that their simulation models wereunable to predict leg trajectories at higherimpact severities and ascribed this to theinability of their models to predict dam-age/injuries.

The humanoid model has shown verygood validation in terms of trajectories forthe head, pelvis, knee and ankle against thetrajectories of the cadaver tests. The headvelocity has also shown very good valida-tion, with the humanoid model accuratelypredicting the time of impact between thehead and bonnet. The validation has beenconducted up to velocities of 39km/h,which makes the humanoid model suitablefor assessing many real world injury mech-anisms, unlike the proposed legislativesub-system impactors [5] that assess only afew. The humanoid model also has the ad-vantage of being able to assess the influ-ence of initial injury mechanisms on subse-quent injury mechanisms and trajectories.

4 The Vehicle Model

In parallel to the development of the hu-manoid models a detailed vehicle modeland pedestrian accident simulation modelswere developed.

The vehicle model shown in Figure 7consists of three distinct parts:1. There is a rear end structure that is mod-elled in a rigid material with a coarse mesh. 2. The main vehicle structure and some mi-nor vehicle components are modelled witha finer mesh and have standard materialproperties used in other types of model (e.g.frontal crash models). 3. The more important vehicle componentsfor pedestrian impact conditions are de-fined with material properties determinedby a systematic testing and modellingmethodology to characterise their behav-iour under pedestrian impact energy levels.

More details regarding the systematictesting and modelling procedure and thevehicle components validated have beenpreviously published [7, 17].

5 IMPAIR Study

Pedestrian-to-car accidents are being inves-tigated in a prospective interdisciplinarystudy in Berlin and Mecklenburg-Vorpom-mern. The objective of the study is to recon-struct pedestrian accidents for a better un-derstanding of the kinematics and to iden-tify causal correlations between injuriessustained by the pedestrian and the corre-sponding damages to parts of the vehicle.Furthermore improved biomechanical lim-it values of human tissues (e.g. skin, mus-cles, bones etc.) will be defined.

IMPAIR stands for "In-depth MedicalPedestrian Accident Investigation and Re-construction". Partners of the FFA in this re-search are DEKRA, Berlin Trauma Clinic(UKB), University of Greifswald and theFederal Highway Research Institute (BASt).The study will end after 36 months in July2004.

Besides the very high quality of the data,as a difference to previous investigations, acomplete set of data for every single caseshould be achieved. From what we know sofar neither tests with dummies nor withspecially prepared human bodies will re-flect the complexity of a pedestrian acci-dent realistically enough so that the datacould be used for computer simulation pro-grams.

A member of the DEKRA accident re-search unit will be alarmed by Berlin FireBrigade together with the ambulance and –using blue flashing lights and siren, Figure8 – reach the accident scene as quickly aspossible to gather all the technical informa-tion necessary. In parallel the emergencydoctor starts to document medical informa-tion on injuries. At the Berlin Trauma Clin-ic for 24 hours a day a medical doctor is oncall to continue data collection, includingbody measurements.

During interdisciplinary meetings thatare held on a regular basis each data setwill be discussed and the resulting overallkinematics are to be checked for plausibili-ty. Only cases with a complete reconstruc-tion will then be verified with computersimulation.

6 Demonstrator Vehicle

An outlook on possible changes in futuregenerations of vehicles has been demon-strated with a concept study based on threedifferent modules: a deployable hood sys-tem that operates mechanically in the caseof a pedestrian-car-collision, an optimisedfront end bumper system and an energyabsorbing headlamp. The Ford Research Labhas integrated the total concept into a stan-dard Ford Focus.

In the case of pedestrian contact to thefront of the vehicle the hood will be shiftedrearwards and lifted at the same time, asshown in Figure 9, in a fraction of a second.Thus, large part of the impact energy willbe absorbed. Furthermore the up shift ofthe hood provides additional space under-neath the bonnet between the pedestrianhead and components in the engine com-partment, reducing loads on head andshoulder area at the same time during theimpact. During a collision between two ve-hicles the system will not be activated andwill also be disabled with the ignitionswitched off.

The front-end bumper is manufacturedwith special foams, Figure 10, that is de-formable and able to absorb energy alreadyat very low energy levels. A stiff crush kerbunderneath the bumper represents a secondcontact point for the pedestrian lower leg toinsure that most of the loads applied are be-ing transferred into the leg below the knee.This will reduce the injury risk for the legs ingeneral and for the knee in particular.

The headlamp housings are made of adeformable material, Figure 11, to providean energy-absorbing device during an im-pact with the pedestrian upper leg or hip.The direct load on these body parts will bereduced. Sever injuries from a broken head-lamp glass will be largely avoided.

The biggest advantage of this intelligentsolution is that it doesn't need any sensortechnology to release or activate the sys-tem and that e.g. the deployable hood afteran activation might be pushed back manu-ally into its original position without caus-ing expensive repairs.

References

[1] Akiyama, A.; Yoshida, S.; Matsuhasi, T.; Ran-garajan, N.; Shams, T.; Ishikawa, H.; Konosu,A.: Development of Simulation Model and

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23ATZ worldwide 1/2003 Volume 105

Pedestrian Dummy, Advance in Safety Tech-nology, SP-1433. SAE paper 1999-01-0082,Detroit, Michigan, 1999

[2] Ashton, S. J.: Cause, Nature and Severity ofthe Injuries Sustained by Pedestrians Struckby the Fronts of Cars or Light Goods Vehicles.University of Birmingham, UK, PhD thesis,1978

[3] Bermond, F.; Ramet, M.; Bouquet, R.; Cesari,D.: A Finite Element Model of The PedestrianKnee-Joint in Lateral Impact. 1993 Internation-al IRCOBI Conference on The Biomechanicsof Impacts, September 8-10th, 1993, Eind-hoven, pp 117-129, 1993

[4] Cheng, H.; Obergefell, L.; Rizer, A.: Generatorof Body (GEBOD) Manual. AL/CF-TR-1991-0051, Air Force Material Command, Wright-Patterson Air Force Base, Ohio, 1991

[5] EEVC/CEVE: Improved test methods to evalu-ate pedestrian protection afforded by passen-ger cars. Working Group 17 Report, Brussels,1998

[6] Hardy, R.N.; Watson, J.W.; Howard, M.S.:Developments in the Simulation of RealWorld Car to Pedestrian Accidents using aPedestrian Humanoid Finite Element Model.In: International Journal of Crashworthiness(2000), Woodhead Publishing Ltd, Vol 5 No 1

[7] Howard, M. S.; Thomas, A. V.; Philipps, M.;Friesen, F.: A Systematic Modelling and Test-ing Methodology to Characterise VehicleComponents under Pedestrian Impact Condi-tions. Report für das Ministerium für Wis-senschaft und Forschung des Landes Nor-drhein-Westfalen, Februar 2000

[8] Howard, M. S.; Thomas, A.: An IntegratedApproach to Real World Pedestrian SafetyResearch. Aachener Kolloquium Fahrzeug undMotorentechnik, Aachen, October 1999

[9] Howard, M.S.; Thomas, A.: Pedestrian SafetyResearch – Past, Present, Future. EuromotorTagung Crashworthiness – Crash Tests – Sim-ulations, Institut für Kraftfahrwesen Aachen,Aachen, Dezember 1998

[10] Howard, M.S.; Watson, J.W.; Hardy, R.N.:The Simulation of Real World Car to Pedestri-an Accidents using a Pedestrian HumanoidFinite Element Model. In: International Journalof Crashworthiness (1998), Woodhead Pub-lishing Ltd, Vol 3 No 4

[11] Ishikawa, H.; Kajzer, J.; Schroeder, G.: Com-puter Simulation of Impact Response of theHuman Body in Car-Pedestrian Accidents.37th Stapp Car Crash Conference, San Anto-nio, Texas, 1993

[12] De Jager, M.K.J.: Mathematical Head-NeckModels for Acceleration Impacts. Universityof Eindhoven, PhD thesis, 1996

[13] Langwieder, K.; Danner, M.; Wachter, W.;Hummel, T.: Patterns of Multi-Traumatisationin Pedestrian Accidents in Relation to InjuryCombinations and Car Shape. Proceedings of8th International Technical Conference onExperimental Safety Vehicle, Wolfsburg, 1980

[14] MacLaughlin T.F.: NHTSA's Advanced Pedes-trian Protection Program, 11th InternationalConference on the Enhanced Safety of Vehi-cles, 1987, SAE Paper 876100

[15] Organisation for Economic Co-Operation andDevelopment (OECD): International Road Traf-fic and Accident Database (IRTAD), 2000

[16] Organisation for Economic Co-Operation andDevelopment (OECD): International Road Traf-fic and Accident Database (IRTAD), 1998

[17] Philipps M.; Howard M.: FE-Simulation vonFahrzeug-Fußgänger-Kollisionen.In: ATZ 101(1999), Nr. 7/8, S. 506-509

[18] Yoshida, S.; Matsuhashi, T.; Matsuoka, Y.:Simulation of Car-Pedestrian Accident forEvaluate Car Structure. Proceedings of 16th

International Technical Conference on theEnhanced Safety of Vehicles, Windsor, Cana-da, 1998

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