ITARDA 24th Work Shop Report2021
The 24th Traffic Accidents Investigation Analysis Research Presentation Meeting

Tendency and Features of Vehicle Rollover Accidents~ Analysis Based on Micro Data ~Hiroki Kobayashi
Investigator, Tokyo Traffic Accident Investigation Office1

ITARDA 24th Work Shop Report

1. Introduction
The research conducted by our center in 2003 shows a tendency that occupant's injuries become more serious invehicle rollover accidents. (ITARDA, 2006). However, due to the change of vehicle model constitution andenvironmental changes such as spread of safety equipment and advance of aging population, situations may havebeen changed from that time. Therefore, we analyzed rollover accidents based on the latest accident data andcompared the results with past research and studied changing points and their factors.
2. Analysis method
2.1 Use data
If the Road Traffic Accident Statistics

due to constraints of tabulation items.

Truck, etc.
SUV
Minivan (mV)Van
Ordinary sedan car (Car)Super tall wagon (SHk)

Popular name (example)
Land cruiser, CS-X, Juke
Alphard, Selena, Freed
High ace, Town ace
Camry, PRIUS, Fit
N-BOX, Tanto
N-WGN, Move, Dayz

Therefore, we performed analysis using

Tall (light wagon)

Alto, Mira

"micro data", which are data ofLight sedan car (k)Light van (kV)Light truck (kT)High Z, Every
Carry, Acty
Land cruiser, CS-X, Juke(macro data) of the National Police
Agency is used, it is difficult to perform
detailed analysis of rollover accidents

accidents investigated and tabulated byour center. The targets in the analysis

Remarks

Overall height
1,700 mm or more
Overall height
1,600 mm or more

Fig.1 Classification of vehicle models

are single vehicle accidents and vehicle-vehicle accidents of four-wheeled vehicles that occurred from 1993 to 2003 (old data) and from 2010 to 2020
(new data) with the vehicle models classified into ten kinds, mainly medium sized vehicles and small sizedvehicles (Fig.1).
2.2 Comparison between micro data and macro data
Whereas the macro data recorded all casualty accidents in Japan, micro data mainly collected the data of serious
accidents that occurred around Tsukuba City, Ibaraki Prefecture and may not represent accidents that occurred inJapan. Therefore, we made a comparison by focusing on "overturn accidents”, which are the only common
denominators of micro data and micro data in relation to rollover accidents. Overturn accidents refer to rollover
accidents without a collision that becomes a cause of accident. The overturn ratio of both data (Fig.2) show that,
although the overturn ratio of micro data is generally high, both data has the same tendency as to comparison
between vehicle models (medium sized and small sized) and time series (old and new data). Therefore, wedetermined that the micro data can be used for relative comparison between vehicle models and new and old data
and an analysis of occurrence factors.

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Overturn ratio (number of overturned units / number of units of single vehicle accidents)
Macro data

Old

Micro data

Old

New

Medium sized vehicles

NewOldSmall sized vehicles

New

Medium sized vehicles

Old

New

Small sized vehicles

Fig.2 Comparison of overturn ratio between macro data and micro data

3. Present state of rollover accidents
Fig.3 shows the variation with time of rollover ratios of small sized vehicles, medium sized vehicles, and all
vehicle models (small sized and medium sized) obtained from the micro data. The rollover ratio that added up
single vehicle accidents and vehicle-vehicle accidents of all vehicle models increases to double. By vehicle model,
the rollover ratio of small sized vehicles increases more than that of medium sized vehicles. In the case of small
sized vehicles, vehicle-vehicle accidents increased 2.5 times and single vehicle accidents increased 1.2 times fromold to new data, which shows that an increase of the rollover ratio of single vehicle accidents is lower than that ofvehicle-vehicle accidents.
Rollover ratio(number of rollover units / number of units involved in accidents)
Single vehicle
ccidents
Vehicle-vehicle
accidents
Total

Small (old)Small (new)Medium sized Medium sized All vehicle
vehicle (old) vehicle (new) models (old)All vehicle
models (new)Fig.3 Variation with time of the rollover ratio by vehicle model

4. Study
We focus on the tendencies found in Chapter 3 - "the tendency that the rollover ratio of all vehicle models
increased from old data to new data" and "the tendency that an increase in rollover ratio of single vehicle
accidents is lower than that of vehicle-vehicle accidents" and perform an analysis of factors from the perspectiveof vehicles and persons.
4.1 Factors of vehicles (for the tendency that the rollover ratio of all vehicle models increased from old tonew data)
4.1.1 Change on the other vehicle side
At first, we studied a possibility that the rollover ratios increase due to the effects of the other vehicle side in the3

ITARDA 24th Work Shop Reportcase of vehicle-vehicle accidents (increase of new vehicle models such as SUV and minivan). The component
rates of vehicles involved in vehicle-vehicle accidents in the case of old data and new data respectively are shownin Figs.4 and 5. The figures show that there is a difference of component rate between all vehicles and rollovervehicles while the component rates between the other vehicles and rollover vehicles has a similar distribution.
Therefore, it can be considered that an increase in rollover ratio is not the problem on the other vehicle side but
the problem on the rollover vehicle side.
Component rates of vehicles involved in vehiclevehicle accidents (old data)

Similardistribution

The other vehicle ofrollover vehicle

Component rates of vehicles involved in vehiclevehicle accidents (new data)

Similardistribution

Difference

All vehicles

The other vehicle ofrollover vehicle

Rollover vehicles

Unknown

Difference

All vehicles

Rollover vehiclesUnknown

Truck, etc.

Truck, etc.

Fig.5 Component rates of vehicles involvedin vehicle-vehicle accidents (new data)

Fig.4 Component rates of vehicles involvedin vehicle-vehicle accidents (old data)

4.1.2 Change on the rollover vehicle side
Next, to examine the features of rollover vehicle side, we focused attention on SSF (Static Stability Factor),
which is the rollover evaluation index of U.S. NCAP (Fig.6). SSF is a value which divides 1/2 of tread width byvehicle gravity center height. However, since the data of gravity center height is difficult to obtain, we madecalculation as simple SSF by approximating gravity center height as vehicle height x 0.38 (Euro NCAP, 2011).
The relation of rollover ratio in the case of all vehicle

The smaller, the easier to overturnmodels which added up old and new data, and simple SSF isshown in Fig.7. The component rate of vehicle models in thecase of rollover vehicles is shown in Fig.8. Fig.7 shows thatThe larger, the harder to overturnthere is a strong correlation (R=-0.72) between simple SSF

Tread widthand rollover ratio and the lower simple SSF becomes, the2・Gravity center
heighthigher rollover ratio becomes. Also, Fig.8 shows an increasein the component rate of vehicle models with low SSF.

The larger, the easier to overturn

SimpleSSF

Tread width
2・Vehicle height x 0.38
*General value

Fig.6 SSF and simple SSF

Therefore, such change of vehicle model constitution is considered as a cause of increase in the rollover rate.

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Vehicle model constitution rate ofrollover vehicles

New and old data:
Relations between rollover ratio and SSF

Truck
SUV

MinivanRollover ratio

Super tall light wagon
Light Van

Tall lightwagon

Van

MinivanCommercial van
Ordinary sedancar

Ordinary
sedan car

Small sizedvehicle

Super talllight wagon

SUV
MinivanLight truck

Ordinary
sedan car

Tall lightwagon
Light sedan car

Small sizedvehicle

Light vanLight truck

N=187

Simple SSF

Fig.7 Relations between rollover ratio andsimple SSF by vehicle model

N=189

Fig.8 Vehicle model constitution ofrollover vehicles

4.2 Factors of vehicles (for the tendency that an increase in rollover ratio of single vehicle accidents islower than that of vehicle-vehicle accidents)
4.2.1 Relations between ESC and single vehicle rollover
As a factor that an increase in the rollover ratio of single vehicle accidents is low, we focused attention on in
ESC (Electronic Stability Control). The rollover ratios of vehicle-vehicle accidents and single vehicle accidents
classified according to the presence of ESC is shown in Fig.9. Although effects of ESC are not seen in vehiclevehicle accidents, the rollover rate decreased in the case of

Total of old and new data, small andmedium sized vehicles

vehicles equipped with ESC, indicating that ESC contributes tosuppress an occurrence of rollover.

Without ESC

With ESC
Vehiclevehicle
accidents

Singlevehicleaccidents

Fig.9 Presence of ESC and rollover ratio ofaccidents

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4.2.2 Change of the number of units equipped with ESC
Since vehicles made in Japan in 2002 and before were not

Presence of ESC and the numberof units of rollover accidents

equipped with ESC, the number of ESC-equipped vehicles

Occurrenceof rollover
No rollover

was 0 in old data. In new data, the ratio of ESC-equippedvehicles reaches 15% (small sized vehicle) and 38%(medium sized vehicle) of ESC-unequipped vehicles (Fig.6)

medium sized vehicles is lower than that of small sizedvehicles, which matches the ratio of ESC equipment
mentioned above.

Medium sized vehicle(new) without ESC

to increase the rollover ratio of single vehicle accidents for

Medium sized vehicle(new) with ESCof single vehicle accidents. As shown in Fig.3, the tendency

Light (new) withoutESC

Light (new) with ESC

and it can be considered that ESC affects the rollover ratio

Fig.10 Presence of ESC and the number of units ofrollover accidents (new data)

4.3 Factor of persons (for the tendency that the rollover ratio of all vehicle models increased from old tonew data)
4.3.1 Change of age composition of drivers
To analyze factors of persons, we examined the component rate of drivers by age. (Fig.11). Age groups wereclassified into young group (24 years old or younger), middle-aged group (25 to 64 years old), and elderly group(65 years old or older). The ratio of elderly drivers in rollover accidents is higher for small sized vehicles
compared with medium sized vehicles, and the ratio increased from old data to new data.
Small sized vehicle (rollover)Medium sized vehicle (rollover)

Old New

Old New

Old New

Old New

Old New

Old New

Light van

Other light vehicles

Light tall wagon
ElderlygroupLight super tallwagon

Others andmedium sizedvehicles

Van
Younggroup

YounggroupMiddleaged group

Old NewOld New

MinivanMiddleElderly
aged group
group

SUV

Fig.11 Variation with time of the component rate of drivers by age(rollover vehicles)
4.3.2 Relations between rollover ratio and elderly drivers
Next, we examined the relations between elderly drivers and rollover ratio. In Figs.12 and 13, the component
rate and rollover ratio of elderly drivers are plotted by vehicle model on the graphs. Although the correlation
between elderly drivers and rollover ratio was weak in old data, strong positive correlation (R=0.81) was found in
new data. There is a possibility that, with the change of times, behavioral characteristics of elderly people hassome kind of effect on rollover of vehicles.

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ITARDA 24th Work Shop Report

Old data

New data

Rollover ratio (%)

Rollover ratio (%)

Super tall light wagon

Minivan
SUVVanLightsedan car

Tall lightwagon

Light vanLight vanTall lightwagon
MinivanLight sedan car

SUVSuper tall light wagon

Ordinary sedan car
Van

Ordinary sedan car

Component rate of elderly drivers (%)

Component rate of elderly drivers (%)

Fig.12 Relations between elderly
drivers and rollover ratio (old data)

Fig.13 Relations between elderly drivers
and rollover ratio (new data)

5．Summary
As a result of examining the change of rollover accidents using new and old micro data, the following was
found in the examples used this time.
・ The vehicle rollover ratio increased. The change of vehicle model constitution can be considered as main
factor for such increase.
・ The tendency to increase the rollover ratio is smaller in single vehicle accidents than that of vehicle-vehicleaccidents. There is a possibility that the obligation to equip vehicles with ESC will be more effective forsuppressing the occurrence of rollover accidents of single vehicles.
・ Simple "SSF”, which is rollover evaluation index of U.S. NCAP, has strong correlation with rollover ratio andcan be used for evaluating easiness of rollover of vehicles.
・ There is a possibility that the aging of drivers has affected an increase in the rollover ratio.
<References/Sources>
1) ITARDA(交通事故総合分析センター)，2006 年，予防安全に関する調査研究車両の転覆及び走行レーンに関する事故実態
研究，引用 116 ページ-引用 127 ページ
2) Euro NCAP，2011 年，THE DYNAMIC TEST OF CAR ELECTRONIC STABILITY CONTROL (ESC) SYSTEMSPROTOCOL，引用 1 ページ-引用 3 ページ
3) 本宮 嘉弘，高塚 尚和，2017 年，軽微な事故で横転する軽ハイトワゴンの危険性， 日本交通科学学会誌，第 17 巻（第 2号），引用 34 ページ-引用 41 ページ
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Analysis for SUVs and MPVs in the ASEAN Market，自動車技術会マレーシア支部誌，第 2 巻（第 3 号），引用 279 ページ288 ページ
5) Jia-Ern Pai，2017年， Trends and Rollover-Reduction Effectiveness of Static Stability Factor in Passenger Vehicles，DOT HS
182 444，引用62ページ-引用132ページ

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