Compatibility problems in frontal, side, single car collisions and car-to-pedestrian accidents in Japan

Accident Analysis and Prevention 31 (1999) 381–391

Compatibility problems in frontal, side, single car collisions andcar-to-pedestrian accidents in Japan

Koji Mizuno a,b,*, Janusz Kajzer a

a Nagoya Uni6ersity, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japanb Traffic Safety and Nuisance Research Institute, Mitaka, 6-38-1 Shinkawa, Tokyo 181-0004, Japan

Received 15 December 1997; received in revised form 9 June 1998; accepted 2 December 1998

Abstract

Compatibility problems in car-to-car frontal, side, single car and car-to-pedestrian collisions in Japan are discussed using trafficaccident data. The number of serious and fatal injuries is investigated for the subject car and other cars, which are categorizedby their class and mass. The aggressivity of the cars is calculated by the number of fatalities, fatality rates and by the number ofcar registrations. The results show that in car-to-car frontal collisions, cars with a mass of 1150 kg are the most compatible amongthe current car population. In both car-to-car frontal and side collisions, the sports utility vehicle and mini car are found to bethe most incompatible car types with high and low aggressivity, respectively. On the other hand, the accident data show that thewagon and midsize sedan are the most compatible car types. The compatibility of fixed objects in the road environment with carsand cars with pedestrians is also discussed. In a single car collision with a fixed object, the guardrail is the most compatible objectand can reduce the fatality rate on prefecture roads by about 60%. The front geometry of the car has large effect on compatibilitywith a pedestrian. © 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Safety; Accident; Injury; Compatibility; Aggressivity

1. Introduction

In vehicle-to-vehicle collisions, the protection of alloccupants in the subject and other vehicle should beconsidered. This compatibility problem was first dis-cussed in connection with the Experimental Safety Ve-hicle (ESV) in 1970 and has not been solved yet.EC/EEVC has a leading responsibility for vehicle com-patibility, which is one of the International HarmonizedResearch Activities (IHRA) of ESV (NHTSA, 1996). Inthe United States, the National Highway Traffic SafetyAdministration (NHTSA) has started a research pro-gram on this subject (Hollowell and Gabler, 1996).However, in Japan, there seems to have been littleresearch on vehicle compatibility in this decade. Onereason is that it has been difficult to obtain accidentdata in Japan. Thus, the Institute for Traffic AccidentResearch and Data Analysis (ITARDA) which offerssome accident data was established in 1992.

Compatibility means that passenger vehicles of dis-parate size provide an equal level of occupant protec-tion in car-to-car collisions (NHTSA, 1996). The fielddata shows there are many vehicles which are incom-patible with other vehicles. The incompatibility is in-duced by the difference in the mass, stiffness andgeometry of both vehicles (Buzeman, 1997). Mass in-compatibility causes high acceleration of the lighter carin collision. Stiffness incompatibility allows large defor-mation for the less stiff vehicle, which results in the riskof compartment intrusion. Override of the car in colli-sion occurs by the incompatibility of the geometry ofboth vehicles. An incompatible vehicle induces highrisks for the occupants in the other vehicle, which canbe defined as aggressivity.

In a single vehicle collision, the stiffness and geome-try of the fixed objects in the road environment withwhich vehicles collide affect the fatality rate of theoccupants. Thus, this is considered as compatibility offixed objects with vehicle. There is also compatibility ofvehicle with pedestrian due to vehicle mass, stiffnessand geometry. The behavior of the pedestrian and the

* Corresponding author. Tel.: +81-422-413215; fax: +81-422-768603.

E-mail address: [email protected] (K. Mizuno)

0001-4575/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 0 1 -4575 (98 )00076 -1

K. Mizuno, J. Kajzer / Accident Analysis and Pre6ention 31 (1999) 381–391382

severity of injury can be affected by vehicle geometryand stiffness. The compatibility between vehicles, offixed objects with vehicle, and of vehicle with pedestri-ans has to be considered in order to decrease the totalnumber of fatalities in road accidents.

The purpose of this study is to identify the compati-bility problem of vehicle-to-vehicle, single vehicle andvehicle-to-pedestrian collisions in Japan based on acci-dent data using vehicle masses and classes. The compat-ibility problem should be examined for each countrybecause the traffic environment differs in each countryin terms of vehicle size, population, velocity, traveldistance and type of fixed objects in the roadenvironment.

2. Methodology

The distributions of fatalities are examined for alltypes of accidents by fatalities related to the subjectvehicle. The fatalities can be distinguished as internaland external in relation to the vehicle (Appel, 1996).The fatalities of the subject vehicle in vehicle-to-vehiclecollisions and in single vehicle accidents are classified asinternal fatalities, while those of other vehicles, motor-cyclists, cyclists and pedestrians are classified as exter-nal fatalities. The distribution of fatalities in all types ofaccidents is estimated by the number of internal andexternal fatalities per million registrations of the subjectvehicle.

The compatibility of vehicles in vehicle-to-vehiclefrontal collision is classified as the mass, stiffness andgeometry compatibility. In the present study, the masscompatibility is examined using the equation based ondelta-V. The geometry compatibility is discussed usingin-depth analysis of the accident. In real accidents, themass, stiffness and geometry incompatibility is com-bined. The combined effect of the mass, stiffness andgeometry are examined when the compatibility is ana-lyzed according to car class because each vehicle classhas a different distribution of these parameters.

The goal of vehicle compatibility in vehicle-to-vehiclefrontal collisions is to minimize the number of fatalitieswhile maintaining the injury rate of occupants in eachvehicle at the same level. Thus, in the current study toestimate compatibility in a vehicle-to-vehicle frontalcollision, a method is employed to determine the totalnumber of fatalities in both vehicles per accident whencomparing the ratio of the fatalities occurring in eachvehicle.

In the current study, the following methods are usedto estimate the aggressivity in vehicle-to-vehicle frontalcollisions:1. (Number of fatalities in other vehicles)/(number of

fatalities in subject vehicles),2. Percentage of fatalities in other vehicles,

3. Number of fatalities in other vehicles per millionsubject vehicle registrations.

Methods 1 and 3 were suggested by Hollowell andGabler (1996). Using method 1, the aggressivity of avehicle without the influence of human factors can bedescribed. If the crash velocity of the subject vehicle ishigh, the risk of injury to the occupants in othervehicles as well as in the subject vehicle is high. Thus,the influence of crash velocity on the aggressivity esti-mated by method 1 will be small. On the other hand,the aggressivity of the vehicle including the influence ofcrash velocity is estimated when the injury rate of thedriver in the other vehicles is used in method 2. If thecrash velocity of the subject vehicle is high, the aggres-sivity obtained by method 2 will be higher because thenumber of fatalities in the other vehicles will increase.The aggressivity estimated by method 3 includes theinfluence of travel distances, vehicle velocities and acci-dent rates, reflecting how they are used (Table 1).

The method of examining aggressivity depends onthe problem being investigated. For example, vehiclemanufacturers can use method 1 to estimate aggressiv-ity of vehicles because this method is related to thevehicle itself. Method 2, which includes the velocityeffect, is usable in studies dealing with road user behav-iors. Method 3 expresses the aggressivity of each regis-tered vehicle, so it can be used when insuranceproblems are investigated.

In side collisions, the vehicles are classified as strikingand struck. The aggressivity of striking vehicle in sidecollision is estimated by the fatalities caused in thestruck vehicles. The aggressivity can be defined for aside collision by changing ‘subject’ to ‘struck’ and‘other’ to ‘striking’ in methods 2 and 3 as follows:

2% Percentage of fatalities in struck vehicles,3% Number of fatalities in struck vehicles per million

striking vehicle registrations.

Table 1Effect of crash velocity and accident rate on aggressivity using eachmethod

K. Mizuno, J. Kajzer / Accident Analysis and Pre6ention 31 (1999) 381–391 383

Table 2Car classes (ITARDA, 1996a)

3. Car registration

The number of cars accounts for about half the totalvehicle registrations. Fig. 1 shows the distribution ofregistered cars according to their classes in 1992 and1995. The number of cars increased by 1.4 times from27 772 to 39 657 thousand. Moreover, the distributionof the car classes changed. The proportion of wagons,one-boxes and SUVs registered increased. On the otherhand, the proportion of sedans decreased, especiallysedan B from 19.5 to 15.8%. As the variety of cars isincreasing, it is thought that the proportion of thenumber of collisions between various classes of cars isincreasing. Thus, the compatibility for various types ofcars is becoming a topic of more important consider-ation now.

4. Distribution of fatalities

The distribution of fatalities was calculated fromaccident data in Japan. This distribution is examined bythe number of fatalities internal and external to thesubject car in various types of accidents. Fig. 2 showsthe number of fatalities in relation to the subject car permillion registrations.

Sports and specialty, SUV, one-box and sedan Ccause more external-type fatalities than any other vehi-cle type. SUV and sports and specialty, in particular,cause the most fatalities in the other car in car-to-carcollisions. Cyclists sustain more fatalities when struckby sports and specialty and one-box, while more pedes-trians die from accidents involving sports and specialtyand SUV.

From the analysis of distribution of fatalities, it isfound that the total number of fatalities with the minicar is lowest, so this car type could be considered as themost compatible vehicle. But this conclusion cannot bedrawn because mini cars are used for short-distancetravel at a relatively low velocity (ITARDA, 1996a),and also because the frequency of driver internal fatali-

If the aggressivity by methods 2% and 3% is high, thestriking vehicle is aggressive in side collisions.

In single vehicle collisions, the fatality rate varies bythe fixed objects in the road environment which includelight poles, road signs, guardrails, houses, bridge struc-tures or other immovable objects. This can be consid-ered as the compatibility of the fixed object with thevehicle. In the current study, the ratio of the number offatalities to that of all drivers involved in single vehiclecollisions is used as the fatality rate. However, in singlevehicle collisions, some drivers do not report the acci-dent to the police if they are not injured. Therefore, itis thought that there are a number of non-injureddrivers who are not included in the data on singlevehicle collisions. The number of non-injured drivers insingle vehicle collisions can be uncertain, so the methodusing the fatality rate is validated by comparing theresults of this method with those of calculation usingthe ratio of the number of fatalities to that of minorinjuries.

The compatibility of vehicles with pedestrians wasanalyzed using accident data from collisions wherepedestrians were hit by the front of the vehicle. Theprobability of fatal injury and the distribution of thebody regions injured for pedestrian were examined byvehicle mass and two contrary vehicle front shapes suchas the bonnet-type car and one-box vehicle.

Law enforcement accident data for the four yearsfrom 1992 to 1995 were used. The analysis is conductedonly for cars with a model year of 1988 or later. In thecurrent research, cars are categorized in eight classes—mini, sedan A, sedan B, sedan C, sports and specialty,wagon, one-box and sports utility vehicle (SUV). Carexamples with their classes are shown in Table 2. Onlyinjuries to drivers are examined to simplify the analysis. Fig. 1. Distribution of car registrations in Japan.


Fig. 2. Distributions of internal and external fatalities of the subjectcar for different accident types.

D61=m2

m1+m2

6c (2)

where D61 is the delta-V of car 1, 6c is the closing speed,and m1 and m2 is the mass of car 1 and 2, respectively.Substituting Eq. (2) for Eq. (1), the injury rate of driver1, R1, is given by

R1=) m2

m1+m2

6ca

)k=a

� m2

m1+m2

�k

(3)

where a= �6c/a �k.Before discussing Eq. (3), the relationship between

the coefficient a, closing speed and vehicle mass must beexamined. Fig. 3 shows the average velocity recognizedto be dangerous versus vehicle mass in car-to-car fron-tal and single-car collisions. The velocity recognized tobe dangerous is one of the items included in the acci-dent data, which is defined as the velocity at themoment the driver perceives the danger of accident. Itindicates the velocity before the driver brakes or steersto avoid the accident, and is compiled mainly fromdrivers’ testimony. For car-to-car frontal collisions, thevelocity recognized to be dangerous does not changesignificantly with vehicle mass (32.5–35.2 km/h) asshown in Fig. 3. On the other hand, in single-carcollisions, this velocity increases from 48.4 to 68.6 km/has the vehicle mass increases. The velocity recognized tobe dangerous is likely to be closely related to crashvelocity. Thus, the coefficient a is thought to be inde-pendent of vehicle mass in car-to car frontal collisionsin Eq. (3).

When Eq. (3) is applied to a real accident, theprobability of serious and fatal injury to the driver ofcar 1 can be calculated as shown in Fig. 4. The parame-ters k and a are calculated for seat belt wearing andinjury severity as shown in Table 3. Based on thismethod (Mizuno et al., 1997), the parameter k is ob-tained as 2.64 for the belted drivers sustaining fatal andserious injury. This value is almost the same as the 2.62shown by Evans (1994). However, he calculated the

ties in car-to-car collision is high. It is also necessary, inthe analysis of the compatibility, to exclude the influ-ence of factors which are not related to the car itself,such as driver behavior, car velocity and accident rate.

The number of fatalities with the sports and specialtycar in single car accidents is especially large, whichindicates that the crash velocity and the accident rateare higher than for any other car classes. As a result,the number of fatalities related to sports and specialtyis high for all types of accidents.

5. Compatibility of car in car-to-car frontal collision

5.1. Mass compatibility

Car mass is one of the most significant factors affect-ing driver injury in car-to-car collisions. It is wellknown that the fatality rate of the driver decreases withincreasing car mass. Evans and Frick (1993) found thatthe ratio of the injury rate in a lighter car to that in aheavier car may be expressed by the power ratio of thecar mass of the heavier car to that of the lighter car. Inthe current study, the individual injury rate is expressedby the average car mass ratio.

According to Joksh (1993) the injury rate R can beexpressed by delta-V (D6) as:

R= �D6/a �k (1)

where aand k are parameters obtained by curve fitting.For many head-on collisions, delta-V is approximatedfor a central collision. Assuming that the restitutioncoefficient is zero, the delta-V can be expressed usingthe average mass ratio as:

Fig. 3. Average velocity recognized to be dangerous by the driver andvehicle mass.


Fig. 4. Average mass ratio and probability of injury to driver in car1 (belted and unbelted driver). Fig. 5. Car mass and the driver fatality of subject and other car in

car-to-car frontal collision.

injury ratio of belted car drivers in the heavier car tothose in the lighter car, and considered all directions ofimpact.

The percentage of driver injuries in the subject carplus that in the other car corresponds to the driverfatalities per accident. From Eq. (3), we obtain

R1+R2=m1

k+m2k

(m1+m2)ka. (4)

R1+R2 has a minimum of 21−ka(k\1) when m1=m2. Thus, cars with equal masses are most compatiblein a collision since the injuries per accident are minimaland the injury rate is equal for both cars.

The percentage of driver fatalities in the subject andother car is shown in Fig. 5. As the mass of the subjectcar increases, the fatality rate of the driver in thesubject car decreases; on the other hand, that of thedriver in the other car increases. The sum of the per-centage of driver fatalities in the subject and the othercar indicates the number of driver fatalities per accidentwhere the subject cars are involved. When the car massis 1150 kg, the number of fatalities per accident takesthe minimum value while the fatality rate of the subjectcar and that of the other car are almost the same. Thus,the car with a mass of 1150 kg is considered mostcompatible in the current car population in Japan. Thecompatible car mass of 1150 kg is almost the same asthe average mass of cars in Japan, which is 1131 kg(Mizuno et al., 1997). This is because there is a highpossibility that the subject car with mass close to the

average will crash into the other car with a small massdifference from the subject car.

When the mass of the subject car is in the range of750 kgBmB1350 kg, the number of fatalities peraccident is small. However, when the subject car mass isless than 750 kg or greater than 1350 kg, the number ofdriver fatalities per accident increases. Thus, in order todecrease the total number of fatalities, it is necessary todesign lighter car while allowing for the safety of thedrivers in the subject cars, and to design heavier carwhile allowing for the safety of the drivers in the othercars.

5.2. Geometry compatibility

There are also stiffness and geometry differencesbetween cars. The geometry compatibility is discussedthrough in-depth analysis of accident. Fig. 6 shows thegeometry incompatibility between SUV and sedan A.The bumper and frame height of SUV is higher thanthat of sedan A. This incompatibility of frame heightcauses differences in the deformation mode betweencars.

Figs. 7 and 8 show crushed cars in a head-on colli-sion. Car A (SUV, 1820 kg) went beyond the centerlineof the road and collided with car B (Sedan A, 1160 kg)with 30% overlap. Car A overrode car B due to thedifference in the front frame height. The maximumcrush depth of car A was 43 cm and the passengercompartment remained intact. On the other hand, the

Table 3Parameter k and a

akSeat beltInjury severity

2.64 13.9BeltedFatal and serious2.49Unbelted 33.4

BeltedFatal, serious and minor 1.08 107.8123.50.955Unbelted

Fig. 6. Difference in height of front structure between two cars.


Fig. 7. Car A.

The fatality rate in mini cars is high when they collidewith a one-box or SUV. The total percentage of seriousinjury to the drivers in the one-box is high (3.69%)compared to that of fatal injury (0.18%). For example,the total percentage of fatal injury to the drivers insedan C (0.18%) is similar to that in a one-box. How-ever, the total percentage of serious injury (2.41%) islower than for one-box. This is related to the fact thatthe percentage of driver’s serious injury to his/her legsis higher for one-box due to large intrusion comparedto other car classes (ITARDA, 1996b). It is noted thatthere are no fatalities of the drivers in a SUV incar-to-car frontal collisions, though the percentage ofdriver fatalities is high when the other car is a SUV.

The number of driver fatalities in the subject andother car per thousand accidents is shown in Fig. 9. ForSUV and one-box, the total number of driver fatalitiesis large and the proportion of the fatalities in other carsis high, so SUV and one-box can be considered asincompatible cars. On the other hand, for mini cars, thenumber of fatalities in the subject car is the largest inall car classes. Therefore, mini cars cannot be said to becompatible in car-to-car frontal collisions. The wagonand sedan B are compatible cars in car-to-car frontalcollision because the proportion of the number of fatal-ities in the subject to that in other cars is almost thesame, and the total number of fatalities in the subjectand other cars per accident is small. However, thenumber of incompatible car types such as the SUV andone-box is increasing (Fig. 1), while that of the compat-ible sedan B car type is decreasing.

The aggressivity estimated by methods 1, 2 and 3 isshown in Figs. 10–12, respectively. In method 1, carscan be defined as aggressive when the aggressivity valueis greater than one, because the number of fatalities inthe other car is larger than in the subject car. Thereforebased on Fig. 10, the SUV, one-box, sedan C andsports and speciality can be described as aggressive.According to the analysis of Fig. 10, the aggressivityranking of the car itself is shown as:

miniBsedan ABsedan BBwagon

Bsports and specialtyBsedan CBone-box

BSUV.

The aggressivity of a car according to its class has asimilar tendency in the results using methods 1, 2 and 3except for the sports and specialty. The aggressivity ofthe sports and specialty is large when estimated bymethod 3 using car registrations, although it is not solarge when estimated by method 1 using the ratio ofdriver fatality for each car. This can be explained by thefact that the accident rate, crash velocity and traveldistance of sports and specialty are so high that thenumber of fatalities per car registrations becomeslarger. It is possible to consider that the sports and

front frame of car B could not absorb the crush energyas designed for a crash against a rigid barrier. The hooddeformed in the upward direction. The maximum crushdepth of car B was 60 cm, and the compartmentdeformed so that the bottom of the A-pillar movedbackward 10 cm and the intrusion of the dashboard onthe driver’s side was 10 cm.

Both drivers in the two cars failed to wear a seat belt.The driver in car B suffered a brain contusion (AIS 5)by contact with the windscreen, fractures of seven ribs(AIS 3) by contact with the steering wheel, and alsofractures of tibia (AIS 2) due to intrusion of the wholefront panel. On the other hand, the driver in car Asuffered a wrist fracture from the steering wheel (AIS2). There are reports of many other cases in whichthose incompatibilities of geometry cause very seriousdamage.

5.3. Compatibility analyzed by car class

Car classes have different mass, stiffness and geome-try distributions. The effects of mass, stiffness andgeometry are combined when the compatibility is ana-lyzed by car classes.

Table 4 shows the percentage of serious and fatalinjuries to the drivers in cars according to their class.

Fig. 8. Car B.


Fig. 9. Car compatibility.

Fig. 11. Car aggressivity calculated by method 2.

Fig. 12. Car aggressivity calculated by method 3.Fig. 10. Car aggressivity calculated by method 1.

Table 4Driver fatality (%) in car-to-car frontal collisions (1992–1995)a

Other

Subject Wagon One-box SUV TotalMini Sedan A Sedan B Sedan C Sports andspecialty

1.26 0.45Mini 1.310.220.03 0.640.720.390.26(7.84)(2.62) (8.50) (5.33)(4.59) (5.47) (6.87) (5.83) (6.01)

0.58 0.27Sedan A 0.04 0.20 0.20 0.38 0.36 0.34 0.66(5.14) (3.36)(4.39) (5.09)(3.38)(3.91)(1.42) (2.98) (3.34)

0.500.05 1.23 0.210.04 0.21 0.35 0.29 0.07Sedan B(3.12) (4.28) (4.39) (2.70)(0.87) (2.36) (2.81) (3.02) (3.58)0.06 0.30 0.68 0.180.04Sedan C 0.04 0.09 0.26 0.48

(4.90) (2.41)(2.61) (3.08)(3.10)(2.83)(2.35)(1.92)(1.05)0.13 0.39 0.32 0.23Sports and specialty 0.050.13 0.26 0.40 0.30

(6.55) (2.67)(0.72) (3.22)(2.05) (3.83)(3.51)(3.16)(2.68)0.000.11 0.38 0.170.04 0.07 0.22 0.38 0.84Wagon

(3.07) (3.70) (3.80) (2.23)(0.87) (1.84) (2.30) (2.38) (2.81)0.25 0.29 0.80 0.180.09One-box 0.17 0.00 0.17 0.29

(8.51) (3.69)(3.21) (6.03)(3.61)(4.80)(4.47)(2.27)(1.03)0.00 0.00 0.00 0.000.00 0.00SUV 0.000.00 0.00

(0.93) (1.51)(1.74) (2.93)(1.73)(0.47) (1.27) (2.66)(1.28)0.520.05 0.73 0.240.12 0.19 0.36 0.39 0.21Total

(3.46) (4.54) (5.46) (3.12)(2.63) (3.18) (3.66)(1.29) (3.84)

a Figures in parentheses refer to serious injury.


Table 5Driver fatality (%) in the struck car in side collisions (1992–1995)a

Striking

Sedan B Sedan C Sports and TotalWagon One-boxStruck Mini SUVSedan Aspecialty

0.24 0.53 0.48 0.87 1.09 0.78Mini 0.06 0.340.11(7.17) (6.72) (4.17)(5.46)(5.49)(3.14) (5.04)(3.62)(2.63)

0.55 0.350.48 0.09 0.71 0.70Sedan A 0.10 0.25 0.19(2.85) (3.55) (4.47) (3.72) (4.36) (6.07)(1.78) (2.44) (3.14)

0.780.480.170.29 0.250.410.13 0.25Sedan B 0.00(3.44) (2.73) (3.27) (4.10) (4.11) (2.88)(1.69) (2.69) (2.60)

0.36 0.62 0.23 0.43 0.321.34Sedan C 0.00 0.15 0.29(2.30) (2.42) (3.49) (1.95) (3.73) (5.21)(1.09) (2.03) (2.42)

0.610.650.940.45 2.420.210.39 0.50Sports and specialty 0.21(3.17) (2.81) (2.61) (4.28) (5.19) (2.65)(1.07) (2.16) (2.59)

0.34 0.00 0.26 0.00 0.00 0.73Wagon 0.00 0.00 0.11(8.76)(3.24)(0.70)(3.34) (2.18)(1.80)(1.45) (2.35)(1.30)

0.26 0.33 0.43 0.00 0.35 0.61One-box 0.00 0.00 0.20(2.50) (3.43) (2.56) (1.77) (2.10)(3.05)(1.07) (1.77) (1.67)

0.24 0.32 0.00 0.00 0.62 0.00SUV 0.00 0.00 0.15(0.00) (1.23 (0.00) (1.85))(2.77)(1.52) (1.91) (2.24)(2.36)

0.18 0.24 0.43 0.51 0.27 0.60 1.01Total 0.320.06(2.90)(3.23) (3.77) (3.09) (4.24) (5.39)(1.65) (2.36) (2.66)

a Figures in parentheses refer to serious injury.

specialty itself is not aggressive but when it is driven, ithas high aggressivity due to human factors.

Hollowell and Gabler (1996) shows that the aggres-sivity by method 3 in the US is 24 for sub-compact, 38for compact, 39 mid-size, 42 for large, 46 for minivansand 72 for SUV, which are greater than the valuesshown in Fig. 12. These differences cannot be explainedby assumptions in this study where only driver fatalitiesare considered, whereas in the US study all occupantfatalities in car-to-car collisions were included. This isrelated to the fact that in the US the cars travel withhigher velocity and over longer distances than in Japan.For example, the average travel distance of a car peryear is 17 862 km in US, against 10 130 km in Japan(1994) (IRF, 1995).

6. Compatibility of car in side collision

The probability of fatal and serious injuries to thedriver in the struck car of a side collision was examinedaccording to the striking and struck car class. Table 5shows the percentage of driver fatalities in a struck carin side collisions. The overall average fatal injury rate is0.32%, which is higher than in a car-to-car frontalcollision (0.24%). However, the total driver fatal injuryrate when a mini car is struck is 0.34% in side collision,which is lower than in a car-to-car frontal collision(0.45%). The driver fatality rate in a mini car sidecollision is lower than in a car-to-car frontal collision.

Figs. 13 and 14 show the aggressivity of the strikingcar on the fatal injury rate of the driver in the struck

Fig. 13. Car aggressivity in side collision by method 2%.

car using methods 2% and 3%. The order of aggressivityof each class has almost the same tendency as for acar-to-car frontal collision. The aggressivity of SUV isthe largest when estimated by methods 2% and 3%. It isthought that this is due to the incompatibility of theSUV owing to its large mass and stiffness in conjunc-tion with geometry. The front side members of the SUVare higher than the side sill of a mid-sized car (Shearlawand Thomas, 1996). The sports and specialty also has alarge aggressivity for drivers in the struck car, althoughthis is done due to its high velocity and accident rate.

As shown in Table 5, when the struck car is a minicar, the driver fatality rate in the struck car reaches alarge value of 0.34%, although the aggressivity of themini car is small (Figs. 13 and 14). On the contrary, theaggressivity of the SUV is the largest among all cartypes. Accordingly, in side collisions as in car-to-car


Fig. 14. Car aggressivity in side collision by method 3%.

Fig. 15. Relationship between fatality rate and car mass classified bydanger recognition velocity in single car collision.

Fig. 16. Fatality rate of the driver in single car collision with fixedobject classified by location of impact (excluding expressway).

frontal collisions, the mini car and the SUV are alsoconsidered incompatible car types.

When sedan B is struck from the side, the driverfatality rate is 0.25%, which is less than the overallaverage fatal injury rate of 0.32%. When the strikingcar is sedan B, the fatality rate in the struck car is0.24%, which is also less than the overall average fatalinjury rate of 0.32%. Thus, the authors consider thatsedan B is a compatible car type in a side collisionbecause when sedan B strikes or is struck, the driverfatality rate in the struck car is less than the overallaverage fatality rate. For the same reason, a wagon isalso regarded as a compatible car type in side collisions.

7. Single car collision

The compatibility of the fixed object with the car ina single car collision was examined by car mass, classand the fixed object. Fig. 15 shows the relationshipbetween the fatality rate and the mass of a bonnet-typecar by the velocity recognized at danger by drivers.When analyzing the total fatality rate involved in singlecar collisions without considering car velocity, the fatal-ity rate increases as the car mass increases. However, ifthe fatality rate is calculated under the condition thatthe collision velocity is less than 50 km/h, the fatalityrate decreases with car mass. The accident conditionsare different between light and heavy cars—the heavycar is more likely to crash at a high velocity. Thus,when normalizing by crash velocity, the fatality ratedecreases with car mass. The same phenomenon wasalso demonstrated by Evans (1984) using the ratio ofdriver fatalities to pedestrian fatalities.

In a single car collision, the fixed object with whichthe car collides has a large effect on the fatality rate ofthe driver. Fig. 16 shows the fatality rate of the driverin single car collisions, classified by the fixed object and

the location of the impact. Accidents on expresswaysare excluded because the crash velocity and the fixedobject are far different from that of general roads.

In a single car collision with a fixed object, thefatality rate is high when either side of the vehiclecollides with the fixed object. This means that the sideof a car has little compatibility with fixed objects.Especially, when the car crash is on the driver side intorather thin objects, such as a light pole, road sign, orcentral reserve/median strip, the fatality rate is high, atmore than 25%. For these thin objects, the fatality ratein a driver-side impact is about twice that in a passen-ger-side impact. When the impact location is on thedriver side of the car, it is considered that these objectscause a direct intrusion into a small area of the door,leading to the high fatality rate for the driver. Thefatality rate in a collision with a guardrail is 19.0% onthe driver side, 14.0% on the passenger side, and 3.8%in front, which is low for all impacted locations of thecar examined. The guardrail shows the highest compat-ibility of the fixed objects. The compatibility of thebridge, light pole, road sign and the central strip islower. By putting up guardrails along roads, the driverfatality rate can be reduced by about 60% and the roadenvironment will be more compatible for cars.


Fig. 17. Fatality rate of the driver in single car collision with fixedobject classified by road type (excluding expressway).

Fig. 18. Pedestrian fatality rate and car mass classified by the velocityrecognized to be dangerous for bonnet type car.

are too high compared to those of pedestrian. Thus, inthis range of car mass, the fatality rate of the pedestrianis not affected by car mass. However, when the car massis greater than 1400 kg, the fatality rate increases. Thisis because the SUV accounts for a great portion of heavycars. It is assumed that not the mass but the geometryincompatibility of SUV causes the high fatality rateamong pedestrians.

Geometry incompatibility of cars has large effects onpedestrian injury. In pedestrian-car accidents, the aver-age velocity recognized to be dangerous for the one-box(23.7 km/h) is lower than that for the bonnet-type car(26.2 km/h). In spite of the low crash velocity of theone-box, the probability of fatal injury of a pedestrianwhen struck by a one-box (4.76%) is higher than with abonnet-type car (3.91%). Fig. 19 illustrates the distribu-tion of pedestrian injuries per thousand accidents bybody region, injury severity and car shapes. To excludehigh velocity collisions especially for bonnet-type carswith pedestrian, the danger recognition velocity waslimited to 40 km/h or less. The head is a dominant bodyregion in fatalities for both the bonnet-type car andone-box. The number of fatalities caused by head, chestand abdomen injuries is about three times larger forone-box than for bonnet-type car. In a serious injury, thenumber of head and chest injuries, which can be causeof death, is larger for the one-box than for a bonnet-typecar. Therefore, it is considered that the front shape of theone-box is more aggressive for a pedestrian than that ofa bonnet-type car. However, the shape of bonnet-type caris aggressive to the pedestrian leg because the number ofserious leg injuries is large in crashes with this type ofcar. The distribution of minor injuries by body regionshows a tendency similar to serious injuries. The numberof minor injuries to the head with a bonnet-type car islarge as that with a one-box.

As the front shape of the vehicle body affects thedistribution of pedestrian injuries, modification of thegeometry of the car’s front shape can be effective inincreasing the compatibility between car and pedestrian.

For each type of road, the velocity distribution of thecars and probability of crash with certain fixed objectsis different. Fig. 17 indicates that the fatality rate dependson road types. The fatality rate decreases as the roadwidth is smaller, no matter what kind of road; national,prefecture or municipal. These results are related to thevelocity distribution of the cars. For all types of roads,a guardrail is effective in reducing the fatality rate of thedriver. The fatality rate due to the guardrail is low:national highway (5.36%), municipal roads (4.26%). Thisdemonstrates that the guardrail has a high compatibilityeven at different velocities. Therefore, construction of aguardrail, especially on roads where cars travel fast (i.e.,national highways and prefecture roads) can preventcollisions with other fixed objects and decrease the driverfatality rate.

To investigate the influence of unreported single acci-dents on the calculation of the fatality rate, the resultsfrom Figs. 15–17 were compared with the results calcu-lated using the ratio of the number of fatalities to thatof minor injuries. It was found that the method used inthe current study gave similar results as when calculatedby the fatality ratio. This means that to investigatecompatibility in single car collisions with fixed objects inJapan, the fatality rate, including the total number ofdrivers involved, can be used.

8. Pedestrian-car accidents

The compatibility of cars with pedestrian involves themass, stiffness and geometry of the car. The relationshipbetween car mass and pedestrian fatality rate was exam-ined for a bonnet-type car. Fig. 18 shows the pedestrianfatality rate and car mass by the velocity recognized tobe dangerous by drivers. As car velocity has large effecton pedestrian injury, the pedestrian fatality rate increaseswith car velocity. Whereas, at the same range of carvelocity, the fatality rate is almost constant when the carmass is less than 1400 kg. The mass and stiffness of car


Fig. 19. Distribution of pedestrian injuries per thousand accidents bybody region, car shape and injury severity for the velocity recognizedto be dangerous 540 km/h.

2. In car-to-car frontal and side collisions, SUV andmini cars are the least compatible car types withhigh and low aggressivity to other cars, respectively.

3. Sedan B and the wagon are considered compatiblecars in car-to-car frontal and side collisions. Incar-to-car frontal collisions, the proportion of thenumber of fatalities in the subject cars to that inother cars is almost the same and the total fatalitiesin the subject and other cars are few. In a sidecollision, the driver fatality rate in the struck car isless than the overall average in all cases where sedanB or wagon is the striking or struck car.

4. The guardrail is the most compatible fixed objectstruck by a single car. It can reduce the fatality rateon the prefecture roads by about 60%. If the otherfixed objects in the road environment are equippedwith the guardrail, many drivers’ lives could besaved.

5. Geometry incompatibility of the car with a pedes-trian has a large effect on the distribution of pedes-trian injuries.

References

Appel, H., 1996. Compatibility guideline for passive safety, for activesafety and even for the total transportation system. FISITA.

Buzeman D.G. (1997) Car-to-car and single car crash compatibility:individual effects of mass, structure, stiffness and geometry, Thesisfor the Degree of Licentiate of Engineering, ISBN 91-7197-536-5,Chalmers University of Technology.

Evans, L., 1984. Driver fatalities versus car mass using a new exposureapproach. Accident Analysis and Prevention 16, 19–36.

Evans, L., 1994. Driver injury and fatality risk in two-car crashes versusmass ratio inferred using Newtonian mechanics. Accident Analysisand Prevention 26 (5), 609–616.

Evans, L., Frick, M.C., 1993. Mass ratio and relative driver fatality riskin two-vehicle crashes. Accident Analysis and Prevention 25,213–224.

Hollowell, W.T., Gabler, H.C. 1996. NTHSA’s vehicle aggressivity andcompatibility program. Proceedings of 15th International TechnicalConference on Enhanced Safety Vehicles, DOT HS 808 465, pp.576–592.

IRF, 1995. World Road Statistics, Geneva, International RoadFederation.

ITARDA, 1996a. The relationship between traffic accidents, driver andvehicle, Research Report (in Japanese).

ITARDA, 1996b. Report of accident investigation and analysis in 1996(in Japanese).

Joksh, H.C., 1993. Velocity change and fatality risk in a crash? A ruleof thumb. Accident Analysis and Prevention 25 (1), 103–104.

Mizuno, K., Umeda, T., Yonezawa, H., 1997. The relationship betweencar size and occupant injury in traffic accidents in Japan. SAE Paper970123.

NHTSA, 1996. Report on international harmonized research agenda.Proceedings of 15th International Technical Conference on En-hanced Safety Vehicles, DOT HS 808 465, pp. 2082–2124.

Shearlaw, A., Thomas, P., 1996. Vehicle to vehicle compatibility in realworld accidents. Proceedings of 15th International Technical Con-ference on Enhanced Safety Vehicles, DOT HS 808 465, pp.607–616.

9. Conclusions

The compatibility issue is discussed for car-to-carfrontal, side collisions and for a single car striking afixed object or pedestrian using traffic accident data inJapan. The results are as follows.1. A car with a mass of 1150 kg is considered the most

compatible among the current car population inJapan in terms of car-to-car frontal collisions. Thiscompatible car mass coincides with the averagemass of cars. .

Documents

Compatibility problems in frontal, side, single car collisions and car-to-pedestrian accidents in Japan