Effects of Vehicle Impact Velocity, Vehicle Front-End Shapes on Pedestrian Injury Risk

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Effects of Vehicle Impact Velocity, Vehicle Front-EndShapes on Pedestrian Injury RiskYong Han a b , Jikuang Yang c d , Koji Mizuno b & Yasuhiro Matsui ea Department of Mechanical Engineering , Xiamen University of Technology , Xiamen , Chinab Department of Mechanical Science and Engineering , Nagoya University , Nagoya , Japanc State Key Lab of Advanced Design and Manufacturing for Vehicle Body , Hunan University ,Changsha , Chinad Department of Applied Mechanics , Chalmers University of Technology , Gothenburg ,Swedene National Traffic Safety and Environment Lab , Tokyo , JapanAccepted author version posted online: 13 Feb 2012.Published online: 30 Aug 2012.

To cite this article: Yong Han , Jikuang Yang , Koji Mizuno & Yasuhiro Matsui (2012) Effects of Vehicle Impact Velocity, VehicleFront-End Shapes on Pedestrian Injury Risk, Traffic Injury Prevention, 13:5, 507-518, DOI: 10.1080/15389588.2012.661111

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Traffic Injury Prevention, 13:507–518, 2012Copyright C©© 2012 Taylor & Francis Group, LLCISSN: 1538-9588 print / 1538-957X onlineDOI: 10.1080/15389588.2012.661111

Effects of Vehicle Impact Velocity, Vehicle Front-EndShapes on Pedestrian Injury Risk

YONG HAN,1,2 JIKUANG YANG,3,4 KOJI MIZUNO,2 and YASUHIRO MATSUI5

1Department of Mechanical Engineering, Xiamen University of Technology, Xiamen, China2Department of Mechanical Science and Engineering, Nagoya University, Nagoya, Japan3State Key Lab of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, China4Department of Applied Mechanics, Chalmers University of Technology, Gothenburg, Sweden5National Traffic Safety and Environment Lab, Tokyo, Japan

Objective: This study aimed at investigating the effects of vehicle impact velocity, vehicle front-end shape, and pedestriansize on injury risk to pedestrians in collisions with passenger vehicles with various frontal shapes.

Method: A series of parametric studies was carried out using 2 total human model for safety (THUMS) pedestrian models(177 and 165 cm) and 4 vehicle finite element (FE) models with different front-end shapes (medium-size sedan, minicar,one-box vehicle, and sport utility vehicle [SUV]). The effects of the impact velocity on pedestrian injury risk were analyzedat velocities of 20, 30, 40, and 50 km/h. The dynamic response of the pedestrian was investigated, and the injury risk tothe head, chest, pelvis, and lower extremities was compared in terms of the injury parameters head injury criteria (HIC),chest deflection, and von Mises stress distribution of the rib cage, pelvis force, and bending moment diagram of the lowerextremities.

Result: Vehicle impact velocity has the most significant influence on injury severity for adult pedestrians. All injuryparameters can be reduced in severity by decreasing vehicle impact velocities. The head and lower extremities are at greaterrisk of injury in medium-size sedan and SUV collisions. The chest injury risk was particularly high in one-box vehicleimpacts. The fracture risk of the pelvis was also high in one-box vehicle and SUV collisions. In minicar collisions, the injuryrisk was the smallest if the head did not make contact with the A-pillar.

Conclusion: The vehicle impact velocity and vehicle front-end shape are 2 dominant factors that influence the pedestriankinematics and injury severity. A significant reduction of all injuries can be achieved for all vehicle types when the vehicleimpact velocity is less than 30 km/h. Vehicle designs consisting of a short front-end and a wide windshield area can protectpedestrians from fatalities. The results also could be valuable in the design of a pedestrian-friendly vehicle front-end shape.

[Supplementary materials are available for this article. Go to the publisher’s online edition of Traffic Injury Prevention forthe following free supplemental resource: Head impact conditions and injury parameters in four-type vehicle collisions andvalidation result of the finite element model of one-box vehicle and minicar.]

Keywords Pedestrian injury risk; Vehicle front-end shape; Impact velocity; Pedestrian size; Finite element analysis

INTRODUCTION

A pedestrian is an extremely vulnerable road user who is ata high injury risk in road traffic collisions with motor vehicles.In high-income countries, the number of pedestrian fatalitiesaccounts for a small percentage of total traffic fatalities. Thepercentages of fatalities are as follows: 12 percent in the UnitedStates, 13.2 percent in Australia, 15 percent in Western Europe,and 35.2 percent in Japan but with a relatively small fatalities

Received 20 September 2011; accepted 23 January 2012.Address correspondence to Yong Han, Department of Mechanical En-

gineering, Xiamen University of Technology, 600 Ligong Rd., Houxi,Jimei District, Xiamen 361024, China. E-mail: [email protected];[email protected]

number of 1714 (Australian Transport Safety Bureau [ATSB]2009; European Commission [EC] 2009; National HighwayTraffic Safety Administration [NHTSA] 2009; National PoliceAgency [NPA] 2011). However, in low-income countries, thenumber of pedestrian fatalities accounts for a large percentageof total traffic fatalities In China, an average of 25,000 pedes-trian died in road traffic accidents each year from 2000 to 2007(Traffic Administration Bureau of China [TABC], 2000–2007),accounting for approximately 25 percent of all traffic fatalitieseach year. An analysis of the global accident data in Japan (NPA2011), the main body regions injured were the head and lowerextremities (16.1 and 34.9%, respectively); head injuries werethe dominant cause of fatal injuries (54.2%) followed by chestinjuries (16.2%).

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In recent years, many studies have focused on the dy-namic behavior of child and adult pedestrians in vehicle col-lisions (Crandall et al. 2006; Han et al. 2011; Kerrigan et al.2005, 2009; Liu and Wagner 2002; Liu and Yang 2003;Mizuno 2005; Untaroiu et al. 2008). However, the impact dy-namics and injury risk have not been extensively and com-prehensively studied for various types of vehicle collisions.K. Mizuno and Kajzer (2000) performed a preliminary studyof head injuries for different vehicle front-end shapes using ac-cident analysis and computer simulation using a simple multi-body pedestrian model and buck model of the vehicle. Kerriganet al. (2009) compared the head impact dynamics of the Polar IIpedestrian dummy and postmortem human surrogates (PMHSs)tests in small sedan and large sport utility vehicle (SUV) im-pacts. However, some other important injury regions such asthe chest and pelvis have not been comprehensively evaluated.Moreover, various vehicles with different front-end shapes anddifferent pedestrian sizes have not been extensively studied withregard to injury mechanisms and injury risk.

Pedestrian accident injury risk has been widely investigatedsince the 1960s, when it was found that almost 95 percent ofall pedestrian accidents occurred at an impact velocity less than50 km/h and that half occurred at an impact velocity lower than30 km/h (Ashton 1980; Ashton et al. 1977; Yaksich 1964). Ac-cident data from different countries and different periods wereanalyzed in order to study the severity of injuries and risk offatal injuries in pedestrian impacts (Anderson et al. 1995, 1997;Ashton 1980; Ashton et al. 1977; Cuerden et al. 2007; Han-nawald and Kauer 2004; Kong and Yang 2010; Oh et al. 2008;Rosen and Sander 2009; Walz et al. 1983; Yaksich 1964). Theresults from these studies indicated that vehicle impact velocityhas a significant effect on the injury severity among pedestri-ans. These results, obtained by analyzing accident data beforethe year 2000, indicated that the fatality risk to pedestrians washigh and ranged from 45 to 85 percent and from 5 to 22 percentat impact velocities of 50 and 30 km/h, respectively (Andersonet al. 1997; Ashton 1977; Ashton 1980). However, from theanalysis of accident data after the year 2000, the fatality riskto pedestrians decreased and ranged from 8 to 34 percent at animpact velocity of 50 km/h and from 2 to 7 percent at an impactvelocity of 30 km/h (Cuerden et al. 2007; Kong and Yang 2010;Oh et al. 2008; Rosen and Sander 2009). Though improved ve-hicle design could decrease the pedestrian fatalities and injuryrisk to pedestrians, a more accurate and detailed analysis of theeffect of vehicle front-end shapes and vehicle impact velocityon pedestrian fatality and injury risk should be undertaken.

The purpose of this study was to investigate the influenceof impact velocity and vehicle front-end shape on the dynamickinematic response and injury risk to pedestrians in vehicle colli-sions. Four types of vehicles (medium-size sedan, minicar, one-box vehicle, and SUV) and 2 adult pedestrian models (177 and165 cm, hereafter referred to as AM50th and AM165) were em-ployed within different impact velocity simulations. The overallkinematics and head impact conditions were analyzed for im-pacts at different velocities, as well as the injury parameters ofthe head, chest, pelvis, and lower extremities.

Figure 1 THUMS pedestrian models.

MATERIALS AND METHODS

The effect of impact velocity was examined by changing thevelocity from 20 to 50 km/h in increments of 10 km/h. Two finiteelement (FE) pedestrian models and 4 FE models for vehicleswith different front-end shapes were used. The pedestrian headimpact conditions were compared in 4 types of vehicle colli-sions. The injury parameters were compared in terms of headinjury criteria (HIC), rib deflection, von Mises stress distribu-tion of the rib cage, pelvic force, and maximum bending momentdiagram (MBMD).

Pedestrian FE ModelsThe total human model for safety (THUMS) adult male50th percentile pedestrian FE model (version 1.4) was used inthis study. The THUMS is a detailed model that has been vali-dated in terms of component response and whole-body kinemat-ics (Kawahara et al. 2007; Maeno and Hasegawa 2001; Yasukiand Yamamae 2010). To examine the effect of pedestrian size,the AM50th pedestrian model was scaled to a shorter one witha height of 165 cm and weight of 60 kg (AM165; see Figure 1).

Vehicle FE ModelsFour different vehicle front-end shapes (medium-size sedan,minicar, one-box vehicle, and SUV) were selected to represent4 vehicle types that are involved frequently in real-world acci-dents. The NCAC Honda Accord (model year 2000) and ToyotaRav4 (model year 1997) models were selected as the medium-size sedan and SUV, respectively. Both models were developedoriginally for frontal crashworthiness studies, and it was neces-sary for these models to be modified for use in the simulationof pedestrian collisions. The frontal structures forward of theA-pillar were extracted from the original models as shown inFigures 2a and 2d. The elements of the bumper, hood panel,cowl, and windshield were remeshed with a higher resolution.The mass and inertia of the parts removed from the frontal

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EFFECTS OF VEHICLE IMPACT ON PEDESTRIAN INJURY 509

Figure 2 Vehicle FE models.

structure were lumped into the extracted structure to make themodel weights the same as the original ones. The number ofelements for the medium-size sedan and SUV totaled 83,024and 190,455, respectively.

The minicar (Suzuki Alto, model year 2006) and one-boxvehicle (Honda Acty, model year 2006) FE models were de-veloped (Figures 2b and 2c). The geometrical shape of eachpart of the front structures was measured using a real car, andthen the meshes for both vehicles were generated and assembledinto the whole FE model. The material properties of the partsin both models were defined based on the tension test resultof the specimens of the vehicle components. The number ofelements for each car was 176,238 and 74,451. The validity ofboth models was evaluated by comparing the headform acceler-ations from headform impact tests (Han et al. 2011), as well asinjury parameters (acceleration, knee bending angle, and kneeshear displacement) measurements from legform impact tests.The detailed validation results are shown in Figures B1–B3 inAppendix B.

Setup of the SimulationsFor the analysis, 32 simulations were carried out for the 4 typesof vehicle–pedestrian collisions with impact velocities of 20, 30,40, and 50 km/h. Initially, the pedestrian model was positionedin the front of the vehicle at the vehicle centerline, and it wasset to be in a walking posture facing laterally with the left legforward (without a walking speed), and the arms were positionedforward. The contact friction coefficient between the humanbody and the vehicle was 0.65 and it was 0.7 between the shoesand ground.

RESULTS

Pedestrian KinematicsFigure 3 shows the kinematics of the AM165 pedestrian modelsfor head contact, in which the pedestrian was struck at an impactvelocity of 40 km/h. In the collision with the medium-size sedan,the initial contact occurred between the bumper and the lowerleg, the hood edge collided with the pelvis, and then the upperbody rotated along the contour of the front of the vehicle. Thenthe chest was struck at the top of the hood, and the head wasaccelerated in both forward and vertical directions and collidedwith the cowl at 109 ms (see Figure 3a). In the collision withthe minicar, the pedestrian kinematics were similar to that incollision with the medium-size sedan, but the upper body rotatedaround a relatively high center because the hood leading edgewas relatively high. The chest made contact with the rear part ofthe hood, and then the head impacted the windshield at 95 msdue to a relatively short front-end profile. In the collision withthe one-box vehicle, the lower extremity and pelvis were struckby the bumper and front panel at almost the same time. Thechest was directly impacted by the windshield frame, and thewindshield frame prevented rotation of the upper body. Thenthe head impacted the windshield at 47 ms. In the collision withthe SUV, the kinematic behavior of the pedestrian was similar tothat in the minicar collision; however, the pedestrian was rotatedlaterally and the back was struck by the top of the hood. Thehead impacted the top of the hood at 97 ms.

Head Dynamic ResponseFigure 4 shows the time history of the head resultant velocitiesfor the AM165 and AM50th pedestrians in collisions with the4 vehicle models at impact speeds of 20, 30, 40, and 50 km/h.

Figure 3 Head dynamic kinematics in collisions with 4 vehicle types.

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Figure 4 Head impact velocity in AM165 and AM50th pedestrian collisions (color figure available online).

In all simulations, though the head contact time was differentfor the various vehicle shapes, the high impact velocity ledto a short time of head contact, and the head contact time oftall pedestrians was delayed for all vehicle models. The headimpact velocities were higher than the vehicles’ initial impactvelocities for both pedestrian sizes in collisions with themedium-size sedan. However, for the other 3 vehicles, the headimpact velocities were lower than the vehicles’ initial impact ve-

locities. The shapes of the curves for head impact velocity–timehistories were similar in the minicar and the SUV collisionsbecause the heights of the hood leading edges were comparablebetween the 2 vehicles. In the one-box vehicle impact, the headimpact velocities decreased after collision due to the smalleramount of rotation of the pedestrian’s upper body.

The head impact velocities are shown in Figure 5. Thehead impact velocity had a linear correlation with the vehicle

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Figure 6 HIC of pedestrian in collisions with 4 vehicle types (color figure available online).

impact velocity, irrespective of the vehicle front-end shape. Al-though both vehicle velocity and vehicle shape affected thehead impact velocity, the vehicle impact velocity had a morepredominant influence on head impact velocity than vehiclefront-end shape.

Head and Injury RiskThe HIC was developed based on the Wayne State ToleranceCurve (WSTC) and is widely used as one of the most impor-tant injury parameters to predict the risk of injury to the head(Versace 1971). The HIC injury threshold is 1000, which pre-dicts 15.6 percent of AIS4+ (severe) head injury risk (Mertzet al. 1997). Figure 6 shows the HIC for the 2 pedestrian sizesin collisions with the 4 vehicle types. In the collisions involvingthe medium-size sedan and SUV, the pedestrian’s head madecontact with the rear of the top of the hood. With increasingvehicle collision velocity, the head contact location moved fur-ther rearward, causing the head to make contact with the topof the stiff cowl. The HIC increased significantly with vehicleimpact velocity for both the AM165 and AM50th pedestrians.The HIC was close to or exceeded 1000 when the impact veloc-

ity was 40 km/h. In the minicar and one-box vehicle collisions,the head made contact with the windshield, and the HIC did notincrease as much with vehicle impact velocity. In these colli-sions, the HIC was much less than 1000, even at an impactvelocity of 50 km/h. The simulation results show that in all 4types of vehicle collisions, the HIC was significantly reducedwhen the vehicle impact velocity was 30 km/h or less.

To compare the influence of vehicle shape and velocity onhead injury risk, the severe injury risk (AIS4+) was calculatedbased on the HIC for AM165 and AM50th pedestrians in colli-sions with the medium-size sedan, minicar, one-box vehicle, andSUV at various impact speeds of 20, 30, 40, to 50 km/h (NHTSA1995). According to the AbbreviatedInjuryScale (Associationfor the Advancement of Automotive Medicine [AAAM] 2005),AIS4 injuries include vault fractures, exposed or loss of braintissue, and small epidural or subdural hematomas. The resultsof the comparison is shown in Figure 7. The probability of hav-ing an AIS4+ head injury risk increased for impacts with themedium-size sedan and SUV when the impact velocity changedfrom 40 to 50 km/h and from 30 to 40 km/h for both theAM165 and AM50th pedestrians, respectively. In particular, for

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Figure 7 Risk of AIS4+ head injury in collisions with 4 vehicle types (color figure available online).

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Figure 8 Rib cage deflections for 2 pedestrian sizes in collisions with 4 vehicle types (color figure available online).

the medium-size sedan and SUV impacts, the head impact loca-tion moved from the top of the hood to the rear part/cowl area,where the structural stiffness was substantially higher. However,even for the medium-size sedan and SUV, a decrease in severehead injury risk to less thn 10 percent could be achieved if thevehicle impact velocity was limited to 30 km/h or less.

Rib Cage Injury Parameters and Injury RiskThe rib deflection of the eighth rib was selected to represent therib cage deformation and its injury risk in collision with differentvehicle front-end shapes. The injury assessment reference value(IARV) of the rib deflection is one of the significant injury pa-

rameters for evaluating occupant chest injury risk in side impactprotection. The IARV of the AM50th pedestrian is 42 mm andwas calculated to be 39.6 mm when scaled to the AM165 pedes-trian. Figure 8 illustrates the rib deflections of the eighth rib forthe 2 pedestrian sizes and 4 vehicle collisions. It can be seenthat the rib deflection increased with impact velocity except inthe minicar-to-AM50th pedestrian collision. However, the ribdeflections were too small to predict the rib fracture risk inimpacts with the medium-size sedan, minicar, and SUV. More-over, the von Mises stress distribution of the rib cage showed asmall injury risk for these vehicle types (see Figures 9a–9c). Be-cause the chest was impacted on the top of the hood (where the

Figure 9 Rib cage skeletal deformation and von Mises stress distribution in vehicle-to-pedestrian (165 cm) collisions at 40 km/h impact velocity (color figureavailable online).

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Figure 10 Comparison of chest injuries in vehicle-to-pedestrian collisions (color figure available online).

stiffness was evenly distributed overall), the top of the hood andthe hood panel deformed uniformly and absorbed the impact en-ergy efficiently during chest-to-hood contacts. In the impact withthe one-box vehicle, the rib deflection was the largest amongthe 4 vehicle types. In addition, the rib deflection was largerfor the AM165 pedestrian than the AM50th pedestrian. More-over, the von Mises stress distribution of the rib cage indicatedthat the stress on the lower rib cage was higher than 150 MPa atan impact velocity of 40 km/h (Figure 9c). In the collision withthe one-box vehicle, the upper chest of the small size pedestrianwas struck by the windshield and the lower chest was struckby the windshield frame, which has a relatively higher stiffness.As a result, a shear loading was applied to the chest, and localdeformation of the lower chest occurred. In contrast, the wholechest of the AM50th pedestrian was struck by the windshield,and the resulting rib cage deformation was small.

To investigate the influence of impact velocity on the risk ofrib cage fracture, the probability of an AIS4+ chest injury wascalculated by transforming the deflection of the eighth rib ofAM165 and AM50th pedestrian to injury potential according tothe logistic regression curve developed by Pintar et al. (1997).The results showed that the AIS4+ injury risk was much lowerfor the pedestrian impacted by vehicles other than the minicar,even at an impact velocity of 50 km/h (see Figure 10). TheAIS4+ injury risk to the AM165 pedestrian was high and in-creased with vehicle impact velocity in the impact with the one-box vehicle. Though the impact velocity was 30 km/h, the chestinjury risk was still high at 30 percent. However, for the AM50th,the chest injury risk was also small, because the whole thoraximpacted the windshield. These results show agreement withprevious accident analysis studies (Longhitano et al. 2005; K.Mizuno 2000).

Injury Risk to Head and ChestThe probability of injury to the head and chest was summedup to investigate the factors that affect the fatal injury risk bycomparing the fatal injury risk results from accident analysis(Anderson et al. 1997; Kong and Yang 2010; Rosen and Sander

2009). Anderson et al.’s curve was developed from analysisof vehicles that were produced before the year 1991; Kongand Yang’s curve was developed from analyzing the in-depthaccident data in Changsha city of China between 2003 and2009; an Rosen and Sander’s curve was developed by analyzingthe German In-Depth Accident Study (GIDAS) database after1999. The combined injury probabilities were calculated usingthe following formula (NHTSA 2008):

Psum = Phead + Pchest − Phead × Pchest (1)

where the P is the injury probability. The results for the AM165and AM50th pedestrian collisions are illustrated in Figures 11aand 11b, respectively. In impacts with the medium-size sedan,the severe injury risk for the AM165 pedestrian was small andmatched Kong and Yang’s curve. However, the injury risk in-creased at an impact velocity of 40 km/h in the AM50th pedes-trian collision and matched Anderson et al.’s curve. This out-come resulted because the AM165 head was struck by the topof the hood, whereas the AM50th head was struck by the cowlarea (where the stiffness was higher than the hood panel). In theSUV impact, the injury risk for both pedestrian sizes was highwhen the impact velocity was 40 km/h or greater. The resultsillustrate that a longer vehicle front-end length may cause anincrease in pedestrian fatalities. In the minicar, the fatal injuryrisk was low if the pedestrian’s head struck the center of thewindshield but not by the A-pillar. For the one-box vehicle, thefatal injury risk mainly depended on the chest injury risk forthe AM165 pedestrian. For the AM50th pedestrian, the fatal in-jury risk was small, because the head and chest were impactedby the windshield. The recent designs of a short front-end makeit more likely that the pedestrian’s head will contact the center ofthe windshield and therefore fatal injuries will not occur. How-ever, the main cause of severe head injuries was the windshield(Otte 2010; Xu et al. 2009), as well as the A-pillar and other stiffstructures. K. Mizuno and Kajzer (2000) found that AIS3+ headinjuries were frequently observed in one-box vehicles (minivan),

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Figure 11 Comparison of the sum of injury probabilities of head and chest (AIS4+) among the simulations and pedestrian fatal injury risk from accident analysis(color figure available online).

because the head struck the boundary of the windshield or theA-pillar. Further, Fredriksson et al. (2010) analyzed pedestrianaccidents using GIDAS and found that AIS3+ head injurieswere often caused by the boundary of the windshield.

Pelvis Force and Injury RiskEastridge and Burgess (1997) reported that pelvic fracturesare caused by lateral compression in vehicle-to-pedestrian ac-cidents. Yang (2005) indicated that the pelvis can be in-jured by lateral impact with a stiff hood leading edge, andthe compressive force to the pelvis is the dominant injurymechanism for a pelvic fracture. Bouquet and Ramet (1994)determined a tolerance level of the pelvis to be an 8 kN im-pact force. Otherwise, if larger than 8 kN, 50 percent of theAIS2+ pelvis injuries could occur (Bouquet et al. 1998). Inthe current study, the impact force of 8 kN was selected as areference value to predict pelvic fracture risk. The maximumpelvic forces of the simulations are presented in Figure 12.The maximum loadings applied to the pelvis increased withimpact velocity but were still smaller than the injury thresh-old in the collision with the medium-size sedan and minicar. Inthe one-box vehicle collision, the pelvis force was more than8 kN even at an impact velocity of 30 km/h, particularly for theAM50th pedestrian because the pelvis was struck with the highstiffness front-end structure. In the SUV collision, the pelvicfracture risk was also high because the pelvis made contact withthe stiff leading edge of the hood.

Lower Extremity Injury RiskIn this study, MBMDs were used to evaluate the injury risk of thethigh, knee, and tibia during lateral impact. Kerrigan et al. (2003)and Ivarsson et al. (2004) concluded that the threshold for 50percent injury risk for the AM50th thigh and knee joint was 447and 134 Nm (SD = 7), respectively. An injury threshold for thetibia of 340 Nm has been proposed in the discussion of the globaltechnical regulation of pedestrian protection (United NationsEconomic and Social Council 2007). Tibia and fibula fracturesare AIS2 injuries and femur fractures are AIS3 injuries. TheMBMD of the AM165 was selected to predict the injury risk.The bending moment increased with impact velocity, which isshown in Figure 13. In the collision with the medium-size sedan,the bumper height was similar to that of the knee joint height;the bending effect occurred at the knee joint, and therefore thebending moment of the knee joint and tibia were high, even at animpact velocity of 30 km/h. In the minicar impacts, the bendingmoment of the lower extremity was small even at an impactvelocity of 50 km/h, as result of the flat and smooth shape ofthe front bumper. In the collision with the one-box vehicle, thebending moments of the thigh and lower leg were small andthe long-bone fracture risk was estimated to be small. However,the bending moment of the knee joint was large enough tocause knee ligament failure. In the SUV collisions, the bendingmoment around the knee joint height was large in the rangeof 400 to 600 mm ground clearance. The bending moment ofthe femur was high in the SUV compared to the other vehicletypes.

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Figure 14 illustrates the injury risk of the AM165 pedes-trian tibia in collisions with the 4 types of vehicles, in whichthe injury risk curve used for the analysis was proposed bythe Japan Automobile Manufacturers Association (JAMA) inthe Global Road Safety Partnership (GRSP) meeting (JAMA2008, 2009). The injury risk of the tibia was high in the colli-sion with the medium-size sedan: 98 percent at a vehicle impactvelocity of 50 km/h, 62 percent at 40 km/h, and 56 percent at30 km/h. In the SUV impact, the injury risk of the tibia washigh enough to cause damage, and it could be inferred that the

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femur injury risk also would be high. For the collisions withthe one-box vehicle and minicar, the tibia injury risk was low atan impact velocity of 40 km/h or less. According to the results,the shape of the front end of the minicar can reduce the tibiafracture risk.

DISCUSSION

In vehicle-to-pedestrian collisions, the impact velocities ofthe various human body regions (e.g., head and chest) dependon the vehicle’s shape. The injury risk depends on the vehicleimpact velocity, impact location, and stiffness of the local struc-tures. The effects of vehicle impact velocity on pedestrian injuryrisk as a function of vehicle shape should be evaluated together.The simulation results presented in Figure 4 show that the headimpact conditions were different according to the vehicle shape.The head impact velocity in the collision with the medium-sizesedan was comparable to the initial impact velocity due to rota-tion of the upper body. In the collision with the one-box vehicle,the head impact velocity was lower than the initial vehicle ve-locity, which is similar to the findings of K. Mizuno and Kajzer(2000). Similar head impact velocities were observed in theminicar and SUV collisions because the AM50th pedestriansrotated around the hood leading edges in both vehicle collisionswith a similar rotation center height. Moreover, the head injuryrisk varied due to the different vehicle shapes and hood lengths.A long hood length and high hood leading edge resulted in a

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higher head injury risk in head-to-hood impacts in pedestriancollisions with the medium-size sedan and SUV. However, ashort hood length resulted in a low head injury risk in head-to-windshield impacts in pedestrian collisions with the minicar. Inaddition to the head impact conditions, the time to head impactcontact decreased with increased vehicle impact velocity from20 to 50 km/h. This result is beneficial for identifying the hoodpop-up properly in the field of active countermeasures with apedestrian detection sensor and an activating protective deviceto provide pedestrian protection (Fredriksson et al. 2010; Rosenet al. 2010).

Head injury risk has been analyzed in this study by consid-ering the HIC, which was developed according to the WSTC(using head linear acceleration). However, some fatal brain in-juries such as diffuse axonal injury (DAI) have been proved to becaused by head rotation acceleration resulting from the head’srotational movement and that concussion is caused by the com-bined effect of linear and rotational acceleration (Yang 2005).This is one limitation of this study that should be investigatedby using more advanced pedestrian FE models in the future.

According to the FE simulations, chest injuries not only de-pend on the striking vehicle’s impact velocity but the shape ofthe vehicle front end is an important factor in the injury outcome.The chest injury risk was small in impacts with the medium-size sedan, minicar, and SUV for the AM165 pedestrian model,but high injury risk occurred in collisions with the one-box ve-hicle. In addition, considering the rib cage deflection and vonMises stress distribution as shown in Figures 8 and 9, the ribcage deflections and rib fracture risk were lower in impacts withmedium-size sedan and minicar relative to impacts with theSUV. These findings are consistent with the results that the V∗Cvalues of the rib were smaller in the passenger car impacts thanthose observed in SUV impacts as reported by Crandall et al.(2006). However, the number of rib fractures in Crandall et al.’sstudy was greater than the potential rib fractures predicted in theFE simulations of the bonnet type car collisions, possibly be-cause the PMHS subjects were older subjects and the rib cageswere easily fractured in Crandall et al.’s study, even though themeasured V∗C was much lower than 1.0.

The pelvic impact force was extremely high in impacts withthe one-box vehicle and pelvic fracture risk was very high. Theforce levels were similar at impact velocities of 30, 40, and50 km/h for both pedestrian sizes. It is difficult to identify theinfluence of the impact velocity on the force level, possiblybecause the structural stiffness is a dominant factor in pelvicinjuries for this type of vehicle. The pelvic force increased withimpact velocity in the SUV impacts. The hood leading edgestiffness and impact velocity worked together to determine thepelvic injury risk.

The bending moment diagram of the lower extremity showsthat the impact velocity and bumper shape are the 2 most im-portant factors to determine the injury risk due to impact withthe bumper. A proper shape design could help to mitigate therisk of injury to the lower extremities. Further study should con-sider a “flat” and smooth bumper design and modification of the

properties of the structure for improved protection of the lowerextremities.

According to these results, an effective approach to pre-venting pedestrian fatalities and injuries would be decreasingthe vehicle impact velocity. The simulation results showed thatthe probability of fatal injury was 0 and the probability of se-vere injury would be less than 10 percent if the vehicle im-pact velocities were decreased to 30 km/h or less. Another ap-proach to improve pedestrian protection would be modifying thedesign of the front end. It is likely that a short front-end designwith a relatively high hood leading edge and large windshieldarea (like that of the minicar) could result in the head impactingcloser to the center area of the windshield, which would reducefatal head injuries in pedestrians.

In this study, the effect of muscle tone (active force) on injuryrisk was not investigated because detailed muscle models withmuscle tone were not represented. Because the present study wascarried out at impact speed of 20 to 40 km/h, it was assumedthat there may have been a small effect of active muscle forceon the pedestrian injury risk at high impact speeds. However, ifthe vehicle impact speed is low, for instance at 20 km/h or less,the effect of muscle active force may have a certain influenceon the pedestrian injury risk. This could be considered in a futurestudy.

Moreover, the pedestrian size selected in this study showsdifferent pedestrian responses and chest injuries in collisionswith the one-box vehicle; however, the small difference of thepedestrians’ size shows the similar pedestrian response and in-jury risk in impacts with the medium-size sedan, minicar, andSUV. The pedestrian size influenced the kinematics and injuryrisk for some vehicle types, and a wider range of pedestriansizes should be considered in future studies, including adult andchild pedestrians.

CONCLUSION

In this study, a series of FE simulations was conducted toanalyze the influence of the shape of front end of the strikingvehicle, vehicle impact velocity, and pedestrian size on the riskof injury to the head, chest, pelvis, and lower extremities. Theconclusions can be summarized as follows:

1. Vehicle impact velocity has the most significant influenceon injury severity in the adult pedestrian. All injury param-eters can be reduced if the vehicle impact velocity is below30 km/h. Reducing impact velocity was confirmed as a effec-tive approach to mitigate the severity of pedestrian injuriesin impacts with various type of vehicles. The simulation re-sults show that the probability of a fatal head injury is low atvelocities of 30 km/h or less.

2. Vehicle front-end shape and pedestrian size are importantfactors that influence the injury risk each pedestrian bodyregion. Head and lower extremities were at higher injuryrisk than other body regions in impacts with the medium-size sedan in this study. The pelvis had a high injury risk in

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impacts with the one-box vehicle and SUV. Chest injury riskwas high in collisions with the one-box vehicle and this resultwas in agreement with analyses of real pedestrian accidents.In addition, the shape of the front end correlates to rib cagedeflection in collisions with the one-box vehicle.

3. The FE simulation results showed that the shape of the frontend of the minicar is effective for pedestrian injury mitiga-tion. A front-end shape like that of the minicar can reducethe injury parameters of each adult pedestrian body region ifthe head does not make contact with the A-pillar.

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Documents

Effects of Vehicle Impact Velocity, Vehicle Front-End Shapes on Pedestrian Injury Risk