Analysis of the results of reducing rear-end collision accidents via AEB
Yoshihiko Kinoshita, Senior Researcher, Research Division

PrefaceIn recent years, vehicles equipped with a variety of preventive safety technologies, such as Automatic
Emergency Brakes, Lane Departure Warning devices, rear-view cameras, and more have become widely prevalent.
Moreover, there has recently been a rise in the number of vehicles equipped with new devices such as those thatprevent the vehicle from starting to move when the pedals have been misapplied. As vehicles equipped with thelatest safety devices such as these become widespread, determining what sort of changes they bring about in theoccurrence of traffic accidents as a result will be crucial when it comes to considering traffic safety measures in the
future. Conventionally, public databases concerning vehicles equipped with such preventive safety technologies
have been lacking, and so it has not been possible to analyze their performance. Recently, with the cooperation of
the Japan Automobile Manufacturers Association, Inc. and Japan Automobile Importers Association, since FY2016
it has been possible to aggregate and analyze traffic accident data by using information on whether or not eachindividual vehicle is equipped with preventive safety equipment. Therefore, this study will focus on Automatic
Emergency Brakes (hereinafter referred to as “AEB”) to analyze their results in reducing rear-end collision
accidents with four-wheeled vehicles (hereinafter referred to as “rear-end collision accidents”), while also
considering scenarios in which they fail to perform satisfactorily in conjunction with this.
Prerequisites for the analysis are indicated below.
 The vehicles causing the rear-end collision (hereinafter referred to as the “primary party”) were restricted tofour-wheeled, private passenger cars, with the number of medium- / small-sized passenger cars initiallyregistered and the number of kei-passenger cars (small passenger cars in Japan with an engine displacement
upto 660cc) initially reported between April 2015 and December 2017 used. However, data in which it was
unclear whether or not the vehicle was equipped with AEB was excluded, and certain imported vehicles
were also not included in this.
 A macro database was used for the aggregated accident data, with the focus placed on rear-end collision
accidents that occurred in 2016 and 2017 in which the abovementioned vehicles were involved as theprimary party. The vehicles on the receiving end of the rear-end collision (hereinafter referred to as the“secondary party”) were restricted to four-wheeled vehicles, and whether or not they were equipped with
AEB was not taken into consideration.
 The AEB’s function and grade were not taken into consideration, only whether or not the vehicle was
equipped with such a system at the time of the completion inspection was considered.
The results of reducing rear-end collision accidents were analyzed by determining the extent to which thepresence of AEB reduced the number of rear-end collision accidents that occurred per 100,000 vehicles sorted bythose with and those without AEB.
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1. Explanation of how Automatic Emergency Brakes (AEB) operate
The mechanism by which AEB operates is shown in Fig. 1. First off, while the user’s vehicle is in motion,
radar and cameras are used to detect the presence of any leading vehicles. Subsequently, when the distance between
vehicles shrinks below a pre-set distance, the device determines that there is the possibility of a rear-end collisionand issues a warning to alert the driver of the danger. If the driver still fails to apply the brakes and the distance
between vehicles grows shorter still, the device will pressurize the brakes as a precaution to enable them to be
applied immediately. If this distance shrinks further still and the device determines that there is an extremely highpossibility of a rear-end collision, then it will automatically apply the brakes. However, just because the brakes
activate automatically does not necessarily mean that an accident can be avoided, and there are cases where thevehicle decelerates but a collision still occurs. Therefore, this device is also called a Collision Mitigation Braking
System.

Possibility ofcollision

Possibility of Possibility ofcollisioncollision

Low

Detection

WarningMediumBrake pressurizingHigh

Automatic braking

Fig. 1. Mechanism by which Automatic Emergency Brakes operate

The results obtained from AEB are expected to come in two types: (1) Avoiding accidents by stopping thevehicle prior to a collision and (2) Mitigating damage when a collision does occur by decelerating the vehicle prior
to said collision. Since most of the bodily harm due to rear-end collision accidents only amounts to slight injuries,
the expectation is that the results of mitigating damage from (2) will result in no injuries. However, there is no dataon accidents resulting in no injuries (which is to say accidents only involving property damage) among the statistical
data on traffic accidents in Japan, so this study was unable to explore the results of mitigating damage from (2).
Due to the aforementioned reason, this study performed an analysis by focusing on the results of using AEB toavoid rear-end collision accidents from (1).

2. Occurrence status for recent rear-end collision accidents
Before analyzing the results from AEB, we will take a look at the occurrence status for recent rear-end collision
accidents. Fig. 2 shows trends in all rear-end collision accidents and rear-end collision accidents involving private
passenger cars between the years of 2008 and 2017. Such accidents were gradually trending upwards up through2013 solely for kei-passenger cars, but since 2014 the number of accidents has been on a downward trajectory forsmall- and medium-sized passenger cars, and likewise for all parties.
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×103
Small-sized passenger cars
Kei-passenger cars

50
Medium-sized passenger cars

H29
2017

2016
H28

2015
H27

2014
H26

2013
H25

2012
H24

2011
H23

2010
H22

2009
H21

0
2008
H20

No. of accidents

100

Fig. 2. Number of rear-end collision casualty accidents by primary party

Fig. 3 shows trends in the number of vehicles owned as of the end of each year for each party, with the numberof medium-sized and kei-passenger cars owned continuing to increase. Yet despite this, the downward trend in thenumber of rear-end collision accidents for all parties has continued since 2014, as can be seen from Fig. 2. This isthought to be a manifestation of the results from the dissemination of AEB. Fig. 4 shows the results found by
dividing the number of accidents from Fig. 2 by the number of vehicles owned from Fig. 3, while Fig. 5 shows theresults obtained by standardizing the number of accidents from Fig. 4 using the year 2008 as the baseline. According
to Fig. 4, while the number of accidents in and of themselves differ for each of the parties, they all exhibit similar
trends. Moreover, Fig. 5 indicates a number of commonalities shared among them, like the fact that the downward
trend in rear-end collision accidents has remained largely the same since 2008, and that the downward trend has
become more pronounced since 2013 / 2014. From this, it can be surmised that the results of AEB began to become

×105
Small-sized passenger cars
Kei-passenger cars

2016

H28

2015
H27

2014
H26

2013
H25

2012
H24

2011
H23

2010
H22

2009
H21

Medium-sized passenger cars

2008
H20

300
250
200
150
100
50
0

2007
H19

No. of vehicles owned

manifest starting from around 2013 / 2014 in relation to each of the parties.

Fig. 3. Trends in the number of vehicles owned as of the end of each year3

400
No. of accidents

Small-sized passenger cars

Kei-passenger cars

300
Medium-sized passenger cars

200
100

2015
H27

2014
H26

2013
H25

2012
H24

2011
H23

2010
H22

2009
H21

2008

H20

0

Fig. 4. Number of rear-end collision casualty accidents per 100,000 vehicles owned

Kei-passenger cars
Small-sized passenger cars

2015
H27

2014
H26

2013
H25

H24
2012

H23
2011

H22
2010

H21
2009

Medium-sized passenger cars

H20
2008

1.0
0.8
0.6
0.4
0.2
0.0

Fig. 5. Number of rear-end collision casualty accidents per the numberof vehicles owned standardized with the year 2000 as the baseline

3. Analysis of the reduction results for rear-end collision casualty accidents via AEB(1) Number of registered vehicles by whether or not they are equipped with AEB
The number of registered vehicles as of the end of each year between 2015 – 2017 by whether or not they are
equipped with AEB is shown in Table 1, while a graph of the trends in this number is shown in Fig. 6. For thesubsequent analysis, the number of accidents per 100,000 vehicles will be calculated by whether or not they are
equipped with AEB, but in doing so the median number of vehicles from the beginning and end of the years will
be used instead of the number of vehicles from just the end of the year. The reason for this is that, if we take thenumber of vehicles being driven in February as an example, then the figure for this that should be used would be
one found by adding the number of vehicles newly registered in January to the number from the end of the previous
year, and so therefore it is no longer appropriate to use the number from the end of the year. The annual median
numbers of vehicles from 2016 and 2017 are displayed in Fig. 6.

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Table 1. Number of vehicles registered at year-end by whether or not they
are equipped with AEB and by party

Without AEB

With AEB

Medium-sized passenger cars
Small-sized passenger cars
Kei-passenger cars
Medium-sized passenger cars
Small-sized passenger cars
Kei-passenger cars

End of 2015
End of 2016
150,220
371,207
181,198
432,929
311,322676,274
241,631
898,707
188,673
628,198
576,631
1,341,345

End of 2017
526,412625,244949,862
1,806,397
1,417,246
2,287,164

2,500,000
Kei-cars

Converted to 2017
annual median

2,000,000
1,500,000

Medium cars

With AEB

Small cars
Converted to 2016
annual medianKei-cars

1,000,000

Small cars

Without AEB

500,000
Medium cars
0
End of 2015

End of 2016

End of 2017

Fig. 6. Trends in the number of vehicles owned by whether or not they are equipped with AEB
and by party, and the number converted to an annual median

Table 2 shows the aggregate values for the annual median number of vehicles for 2016 and 2017 calculated
from Table 1. These numbers will be used for the subsequent analysis.
Table 2. Number of registered vehicles from 2016 and 2017 used for the analysis
AEB
Equipped

Not equipped

Medium-sized passenger cars

709,523

1,922,721

Small-sized passenger cars

836,150

1,431,158

Kei-passenger cars

1,306,866

2,773,243

Total

2,852,539

6,127,122(2) Results of reducing rear-end collision accidents by primary party
The results of calculating the number of rear-end collision casualty accidents per 100,000 vehicles by mediumsized / small-sized / kei- passenger cars and by whether or not they have AEB are shown in Fig. 7. The “**” symbols
listed within the figure indicate results for which a “significance test” was performed to see whether the conclusionthat having AEB equipped reduced accidents can be trusted in a statistical sense. The “*” symbol indicates5significance via a significance level of 5%, while “**” indicates significance via a significance level of 1%. By
way of example, significance via a significance level of 1% means that reliable results can be obtained 99 times
out of 100. However, caution is required regarding the fact that this does not mean that the figures for the results ofreducing accidents are reliable The actual reduction rate could potentially be larger or smaller than the calculated
figures, but this proves that the results indicating that AEB reduces rear-end collision accidents are reliable.

No. of rear-end collision casualty accidents
involving four-wheeled vehicles per 100,000
vehicles

250

200

Without

AEB無し
AEB

With AEB
AEB有り

**

**

**
47.3％

62.1％

150
51.3％
100

50

0
Medium cars
普通乗用

Small cars
小型乗用

Kei-cars
軽乗用Fig. 7. Reduction results for rear-end collision accidents by party

We can see from Fig. 7 that results have been achieved whereby roughly half of the rear-end collision accidents
could be avoided by equipping vehicles with AEB. What is more, while the reduction results differ by party, since
the figures themselves do not serve as suitably reliable indicators (as was touched on in the statistics listed above),
it would not be appropriate to discuss this difference. However, the parties can be thought of as being affected bydifferences in the driving characteristics of each user group. As such, the results found from surveying the
distribution of danger perception speeds and driver ages by party are shown in Fig. 8 and Fig. 9 as reference. From
the results we can see that for danger perception speeds there is a bulge in the medium speed range in the order of
medium-sized passenger cars ⇒ small-sized passenger cars ⇒ kei-passenger cars, while similarly for driver age
this bulges among the young age range in the same order. In general, an increase in speed proves a disadvantage
for applying the brakes, and the risk of accidents is thought to be high among younger drivers due to their relativelack of driving experience. Therefore, the conclusion could be drawn that it would be more difficult to obtain the
results of AEB with kei-passenger cars, with their broad medium speed range for danger perception speed and large
number of young drivers, given their elevated risk of rear-end collision accidents. Yet in actuality, results wereobtained for these that compare favorably to those from medium- / small-sized passenger cars, so presumably the6results are impacted by a number of other factors aside from speed and driver age.
Over 70 km/h
70km/h超
50km/h以下
50km/h or less
20km/h以下
20km/h or less

70 km/h or less
70km/h以下
40km/h以下
40 km/h or less
10km/h以下
10 km/h or less

60km/h以下
60
km/h or less
30km/h以下
30km/h or lessSmall cars
小型乗用(n=85699)

Kei-cars
軽乗用(n=96867)

100%
Over 60km/h
80%30km/h or less
60%
40%
20%
0%
Mediumcars
普通乗用(n=68029)

Fig. 8. Distribution by danger perception speed by party (disregarding secondary party classifications)

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75歳～
75 or older
30～49歳30 – 49 years

65～74歳65 – 74 years～29歳
29 or younger

50 – 64 years
50～64歳

Small cars
小型乗用(n=85764)

Kei-cars
軽乗用(n=96968)

100%
65 or older
80%

60%

40%
29 or younger
20%0%Mediumcars
普通乗用(n=68105)

Fig. 9. Distribution of driver age groups by party (disregarding secondary party classifications)(3)

Rear-end collision accident reduction results by the danger perception speeds of the primary party
Next, the results from AEB by the danger perception speeds of the primary party are shown in Fig. 10. The

reason for the large number of accidents per number of vehicles when danger perception speeds are low is because
rear-end collision accidents often occur at low speeds. This figure reveals that rear-end collision accident reduction
results can be obtained through the use of AEB in the low – medium speed ranges. However, in many cases
significant results could not be obtained for the high speed range in excess of 60km/h. This is believed to be because
as the traveling speed reaches high speeds, the number of cases increase in which AEB is effective at decelerating,but collisions still occur because it is unable to bring the vehicle to a stop immediately ahead of the other vehicle.
Presumably AEB could offer damage mitigation results in such cases, but as was explained in the beginning, thisstudy is unable to explore such results.

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61.6％
Small cars
小型乗用

14.7％
38.1％
10.0％
70km/h<≦70km/h≦50km/h≦40km/h≦30km/h≦20km/h≦10km/h

70km/h<≦70km/h≦60km/h≦50km/h≦40km/h≦30km/h≦20km/h≦10km/h

70km/h<≦70km/h≦60km/h≦50km/h≦40km/h≦30km/h***

0≦20km/h*****≦10km/h**≦60km/h

57.1％
41.6％
-16.0％
17.7％

**

普通乗用
Medium
cars

58.8％
43.8％
30.8％
29.2％

55.4％
63.9％
62.3％

20

**

**

**

**

**

** **

**

40

**

64.8％
65.2％
57.9％
53.3％
61.1％
64.0％
27.0％

60

64.6％

42.0％
**

**

No. of rear-end collision casualty accidents involving
four-wheeled vehicles per 100,000 vehicles

With
AEB
AEB有り

**

80

Without
AEB
AEB無し

Kei-cars
軽乗用Fig. 10. Rear-end collision accident reduction results by danger perception speed(4)

Rear-end collision accident reduction results by daytime / nighttime
Since it is anticipated that AEB’s performance in detecting leading vehicles will be affected by the surrounding

environment, the results of analyses for both daytime and nighttime are shown in Fig. 11. The results indicate that
AEB is effective at reducing rear-end collision accidents in both the daytime and nighttime, but conversely affirmeda trend whereby said reduction results are smaller at nighttime compared with during the daytime. Since thesurrounding environment is dark at nighttime, this affects the AEB’s detection performance when it comes toleading vehicles, on top of which traveling speeds are higher at nighttime than in the daytime. As a result, this isthought to be affected by the fact that there are numerous cases for which the results of AEB cannot be adequately
obtained, as affirmed in Fig. 10. The reason for the lower number of accidents per 100,000 vehicles at nighttime
versus the daytime is because the number of rear-end collision accidents itself that occur at night is lower.

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Without

200
No. of rear-end collision casualty accidents involving
four-wheeled vehicles per 100,000 vehicles

With

AEB
AEB無し

**

AEB
AEB有り

**

160
**
120

49.9%

63.5%
53.8%

80

**

**
**
44.8%

40

40.7%

58.2%

0
DaytimeNighttime

DaytimeNighttime

Small cars

Medium cars

DaytimeNighttime

Kei-carsFig. 11. Rear-end collision accident reduction results during the daytime and nighttime(5)

Rear-end collision accidents reduction results by age of the primary party

The latest analysis results have been announced at the 22nd ITARDA Seminar held in 2019, so
please refer to the following address.

https://www.itarda.or.jp/presentation/22

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(6) Estimated results for a scenario in which the AEB has achieved 100% penetration
The rear-end collision accident reduction results were estimated for a hypothetical scenario in which it wasenvisioned that AEB had achieved 100% penetration in 2017. The following equation was used to perform thecalculation.
Estimated number of residual rear-end collision accidents when AEB is equipped on 100% of vehicles =(Total number of rear-end collision accidents from 2017 － Number of rear-end collision accidents involving
primary parties driving AEB-equipped vehicles from 2017)
× (1－AEB reduction results) + (Number of rear-end collision accidents involving primary parties driving
AEB-equipped vehicles from 2017)
The number of rear-end collision accidents that occurred in 2017, the number of these that involved vehicles
equipped with AEB, and the calculated figures for the AEB reduction results recently calculated are enumerated inTable 5.
Table 5. Number of rear-end collision accidents from 2017 and calculations of the reduction results from AEB
No. of accidents where theNo. of rear-end
primary party had an AEBcollision accidents
equipped vehicle

2017 casualty accidents

Accident reduction
results from AEB

Medium-sized passenger cars

32,633

1,102

51.3%

Small-sized passenger cars

40,242864

62.1%

Kei-passenger cars

46,043

2,064

47.3%

The number of rear-end collision accidents reduced and residual accidents found from Table 5 and thecalculation equation are shown in Fig. 13.

46043

Kei-cars
軽乗用

Reduction＝20795 accidents

Residual accidents＝25248

40242

小型乗用
Small
cars

Residual accidents＝15782

Reduction＝24460 accidents

32633

Mediumcars
普通乗用

Residual accidents＝16471
0

5000

10000

15000

Reduction＝16162 accidents
20000

25000

30000

35000

40000

45000

Residual rear-end collision accidents if all

Rear-end collision accidents

対四輪車追突事故
involving four-wheeled vehicles

全車AEB搭載時の残追突事故
vehicles are equipped with AEB

Fig. 13. Estimated results for the rear-end collision accident reduction results for 2017
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50000

It has been estimated that 61,417 rear-end collision accidents were reduced for medium-, small-sized and keipassenger cars combined. This is equivalent to a 52% reduction in rear-end collision accidents, and a 13% reductionin the total number of 472,165 casualty accidents from 2017. Yet on the other hand, 57,501 rear-end collision
accidents with four-wheeled vehicles still occurred, which were presumably accidents that occurred underconditions where AEB tends to not be fully effective.

4. Conditions where the AEB tends to not be effective
Through the analysis thus far, it has been confirmed that there are conditions under which AEB is effective at
reducing rear-end collision accidents. Yet at the same time the fact was cleared that it is still unable to prevent many
rear-end collision accidents via current AEB systems.. In a questionnaire on AEB carried out by the Japan
Automobile Federation, responses indicating an awareness that AEB entailed “automatic braking” and “collisionfree vehicles” accounted for 97.3%. Unfortunately, on the other hand this also laid bare the fact that nearly one outof every two people (45.2%) were under the misapprehension that the device was “a device that would brakeautomatically to avoid collisions,” which is to say they placed too much reliance in its abilities. Presumably,
designations such as “automatic braking” and “collision-free vehicles” fostered this sense of overconfidence in thedevice. For this reason, the Automobile Fair Trade Council decided to apply guidelines banning the use of thedesignation “automatic braking” in commercials and internet videos by auto makers and dealerships starting fromJanuary 1, 2019. The hope is that this impression that AEB is a device that automatically stops the vehicle is one
that will be dispelled in the future. But it is not enough to just dispel this impression. Drivers must be provided withan accurate understanding of AEB’s function. As such, it will also be important to carry out awareness-raising
activities that inform people of the conditions in which rear-end collision accidents occur even when AEB is
equipped. Based on this perspective, on April 20, 2018 the Ministry of Land, Infrastructure, Transport and Tourism
issued a press release cautioning people not to be overconfident in the device’s abilities, and also released a video
to raise awareness of the matter over the internet.1) This video introduced examples involving slippery road surfaces
and steep downhills as cases where the AEB tends to not be effective.

Fig. 14. Scenarios where the AEB tends to not be effective(Left: Slippery road surface; Right: Steep downhill)
In addition, as part of an explanation on AEB the National Agency for Automotive Safety & Victims’ Aidraised the following examples as cases where the device does not fully fulfill its functions.2)(1)

At night or when it is raining(2)

When the windows are dirty(3)

When objects placed on top of the dashboard are reflected in the window

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(4)

When there are objects in front of the detection unit blocking it(5)When maintenance has not been adequately carried out by specialty shops to maintain its accuracy

These various conditions can be broadly classified into the following three categories.


Factors leading to a deterioration in the braking force(tire wear; dampness, snow cover, and frozen sections; steep downhills; a drop in tire pressure, etc.)



Factors leading to a deterioration in the device’s detection performance(darkness, reflected light, dense fog, dirt on the cameras / receivers, misalignment of the sensors, etc.)Objects that are difficult to confirm visually(trucks with protruding load-carrying trays, extremely small objects, etc.)

Extensive awareness-raising should be carried out in order to inform people that various conditions like thosementioned above can produce situations in which the AEB tends to not be effective, and therefore it is importantthat drivers personally ensure safe driving without relying on such devices.

5. Conclusion
This study was able to confirm that AEB offer extensive reduction results for rear-end collision accidents. Yetat the same time, it also affirmed that it is still difficult even for AEB-equipped vehicles to prevent nearly half ofthese rear-end collision accidents. It has also brought to light the fact that the misapprehension (overreliance) thatjust having an AEB allows one to avoid accidents is widespread among users. When cases where the devices areunable to function satisfactorily are factored in, this points to a need for awareness-raising activities designed toprovide users with accurate knowledge regarding the devices. Though this is not just limited to AEB, but could besaid for the full range of other advanced safety devices that will hopefully continue to achieve widespread
popularity in the future. As such, the hope is that the interested parties will undertake further awareness-raising
activities.

References
1） 国 土交通 省自動 車局審 査・ リコー ル課 「衝 突被害 軽減ブ レーキ は万能 では ありま せん 」
https://www.youtube.com/channel/UCwFJ6KstdbqM9P91828lu2g2） （独法）自動車事故対策機構 予防安全性能アセスメント－衝突被害軽減制動制御装置（被害軽減ブレーキ）
http://www.nasva.go.jp/mamoru/active_safety_search/collision_avoidance_system.html

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