ITARDA 26th Work Shop Report2023
26th Presentation Session for Traffic Accident Investigations, Analysis, and Research

Analysis on the influence of road structure on Autonomous Emergency Brake (AEB)
Daisuke Yamaguchi
Researcher, Autonomous Driving Section, Autonomous Driving Group1

ITARDA 26th Work Shop Report

1. Introduction
1-1.

Background and purpose of this research

Since the formulation of “Public Private ITS Initiative/Roadmaps” in 2014, Japanese government strategies forautonomous driving technology have been revised annually in order to realize autonomous driving as quickly as
possible through cooperation between relevant government ministries and agencies and private enterprise. InSeptember 2021, the Japanese government established its “Digital Agency”, which in August 2022, announced thegovernment strategy “Utilizing Digital Technology for the Future of Transportation Society 2022” which expanded
upon and succeeded “Public Private ITS Initiative/Roadmaps”. The revised Road Traffic Law also came into effectin April 2023, enabling “Level 4” autonomous driving to be performed on public roads. Although the situation
surrounding autonomous driving technology in Japan is constantly changing in manners such as these, in order torealize a digital society that utilizes autonomous driving technology, there is a significant need for research that bothsupports and promotes advanced safe vehicles and autonomous driving initiatives.
Recent years have seen the popularization of advanced safety vehicles equipped with driving assistance functions,such as autonomous emergency brake (hereinafter, “AEB”) and lane departure warning systems (LDWS). AEB is
one of the main driving assistance functions of advanced safety vehicles, and since November 2021, has been
gradually started to be required for new passenger vehicles. Understanding the effects of AEB, which is expected
to become more widespread in the future, is essential not only for future vehicle development, but also forformulating road policies and constructing traffic safety infrastructure.
Although prior research on AEB exists, such as that conducted by Kinoshita(1)(2), Kondo,(3) and Yoshida(4)regarding AEB in different vehicle types, differences in AEB generations, and AEB use on expressways, there are
few instances of research analyzing the impact of road structures and traffic characteristics on the effectiveness ofAEB, specifically targeting national highways.
Therefore, the purpose of this study is to assess what effects driver assistance functions (including AEB) equippedon advanced safety vehicles have on reducing rear-end collisions under various road traffic environments andconditions on national highways.
1-2.

Overview of AEB

AEB uses various sensors such as on-board cameras and millimeter-wave radar to detect vehicles and pedestrians
in front of the vehicle. When it senses the risk of a collision, AEB not only alerts the driver to brake, but also
activates the brakes automatically in order avoid the collision and reduce damage when the system determines thatthe collision is unavoidable.
Since November 2021, AEB has become mandatory on new passenger cars, and is also scheduled to become
mandatory for current production models of passenger cars from December 2025. According to the Survey on theStatus of ASV Technology Adoption(5) conducted by the Ministry of Land, Infrastructure, Transport and Tourism(MLIT) as part of its General Vehicle Safety Information project, the percentage of new vehicles equipped with
AEB has been increasing yearly, reaching 91% in 2020 (Fig. 1).

2

ITARDA 26th Work Shop Report2.

Data used for this research
The analysis performed over the course of this research utilized the Traffic Accident/Vehicle Integrated Databaseowned by the Institute for Traffic Accident Research and Data Analysis (ITARDA). The Traffic Accident/Vehicle
Integrated Database was created by combining traffic accident statistics (e.g., accident-related information such as
types of accidents and registration numbers) provided by Japan’s National Police Agency, vehicle-relatedinformation (e.g., vehicle identification numbers and registration numbers) provided by MLIT, and AEB-related
information (e.g., vehicle identification numbers, manufacturer information, and vehicle functions, including AEB)
provided in cooperation with the Japan Automobile Manufacturers Association (JAMA).
3.

Macro analysis focusing on traffic accidents involving vehicles equipped with AEB and road traffic
environments, etc.

3-1.

Overview of analysis

This analysis compared AEB-equipped vehicles with non-AEB-equipped vehicles by looking at rear-end
accidents that occurred on national highways and in which the primary party was a passenger car. Totals were thencalculated using “number of accidents per vehicles owned”. The specific method for calculating “number ofaccidents per vehicles owned” involves dividing the total number of accidents that involving AEB-equipped
vehicles and non-AEB-equipped vehicles during the analysis period (2017 to 2021) by the total number of registeredand reported vehicles during the same period. Here, “primary party” refers to the party deemed to be the mostnegligent among the parties involved in the traffic accident.
During each stage of the analysis, a chi-square test was performed to determine whether a significant relationshipexists between whether or not a vehicle is equipped with AEB and the number of accidents. In the graphs below,"**" is used when the p-value is less than 0.01 (P < 0.01).
3-2.

Horizontal alignment

To confirm whether the horizontal alignmentof roads affects accident numbers and whether it
has any impact on AEB’s ability to reduce(Thousand vehicles)
(千台)
5,000
87%
58%
3,000alignments were divided into intersections and

2,000

non-intersections in order to calculate numbers

1,000

of rear-end collision accidents involving AEB-0equipped vehicles and non-AEB-equippedvehicles per number of vehicles owned, as well
as reduction rates for rear-end collision

100%

74%

4,000

accidents in AEB-equipped vehicles, horizontal

91%

43%

80%
4,010
3,701
60%
3,414

2,672
16% 1,832

1%
37

1%
47

12%
2% 5%
100 197 521 705

40%
20%
0%

‛10 ‛11 ‛12 ‛13 ‛14 ‛15 ‛16 ‛17 ‛18 ‛19 ‛20
Numberof AEBPercentage of AEBAEB装着台数
AEB装着率(%)
equipped vehicles

equipped vehicles

Fig. 1 - Trends in numbers and percentages of new vehicles(passenger cars) equipped with AEB

accidents caused by AEB-equipped vehicles(Fig. 2).
As shown by the graphs, in the case of both intersections and non-intersections, rear-end collision accident
reduction rates were lower for curved and bending sections of roads than for straight sections of roads. In the caseof curves, accident reduction effects are thought to be lower due to limitations with the detection range of in-vehicle
cameras or due to delays in or failures to detect vehicles ahead when steering, etc.
3Intersections
Number of accidents per vehicles
owned (accidents/million vehicles)

保有台数あたり追突事
故件数（件／百万台）

40.2

40

**
47%

3020

**

100% 単 路 500

67%

80%
60%
13.2

12.8
6.8

1040%
20%

00%
Curves/bends
カーブ・屈折Non-AEBAEB非搭載
equipped

Straightroads
直線
Reduction
AEBAEB搭載
削減率
rateequipped

保有台数あたり追突事
故件数（件／百万台）

交差点 50Non-intersections
Number of accidents per vehicles
owned (accidents/million vehicles)

ITARDA 26th Work Shop Report
**
422.2

40072%

300
200
100100%
80%
60%

57%
**
91.8

116.3

40%
20%

39.2

00%
Curves/bends
カーブ・屈折Non-AEBAEB非搭載

Straightroads
直線Reduction
AEBAEB搭載
削減率rateequippedequipped

Fig. 2 - Number of rear-end collision accidents involving AEB-equipped and non-AEB-equipped vehicles pervehicles owned / Rear-end collision accident reduction rate in AEB-equipped vehicles (horizontal alignment)
3-3.

Vertical gradients

To confirm whether the vertical gradient of roads affects accident numbers and whether it has any impact on
AEB’s ability to reduce accidents in AEB-equipped vehicles, vertical gradients were divided into intersections andnon-intersections in order to calculate numbers of rear-end collision accidents involving AEB-equipped vehicles
and non-AEB-equipped vehicles per number of vehicles owned, as well as reduction rates for rear-end collision
accidents caused by AEB-equipped vehicles (Fig. 3).
As shown by the graphs, in the case of both intersections and non-intersections, rear-end collision accident
reduction rates were lower for uphill and downhill sections of roads than for straight sections of roads. On uphillsections of roads, accident reduction effects are thought to be lower due to drivers often overriding the system bydepressing the accelerator pedal, while on downhill sections of roads, accident reduction effects are thought to belower due gravitational acceleration being applied in the forward direction, which generally causes stopping

Intersections
Number of accidents per vehicles
owned (accidents/million vehicles)

保有台数あたり追突事
故件数（件／百万台）

4030
2010

**
48%

**38.367%

100% 単 路 500
58%

60%
40%

12.7

11.1

80%

5.7

3.6

**
1.5

20%
0%

0Uphill上り
Non-AEBAEB非搭載
equipped

Flat
平坦

Downhill
下り
Reduction
AEBAEB搭載
削減率
rateequipped

保有台数あたり追突事
故件数（件／百万台）

交差点 50Non-intersections
Number of accidents per vehicles
owned (accidents/million vehicles)

distances to be extended.

400

**
384.3
59%

100%
73%

69%

300
200
10080%
60%

**
100.2

105.3
41.1

040%
**
29.5 9.0

20%0%Uphill上り
AEB非搭載Non-AEBequipped

Flat
平坦
AEBAEB搭載

Downhill
下り
Reduction
削減率
rateequippedFig. 3 - Number of rear-end collision accidents involving AEB-equipped and non-AEB-equipped vehicles pervehicles owned / Rear-end collision accident reduction rate in AEB-equipped vehicles (vertical gradients)
3-4.

Speed limits

To confirm whether speed limits affect accident numbers and whether it has any impact on AEB’s ability to reduce
accidents in AEB-equipped vehicles, speed limits were divided into intersections and non-intersections in order tocalculate numbers of rear-end collision accidents involving AEB-equipped vehicles and non-AEB-equipped
vehicles per number of vehicles owned, as well as reduction rates for rear-end collision accidents caused by AEBequipped vehicles (Fig. 4).
As shown by the graphs, in the case of both intersections and non-intersections, rear-end collision accident
reduction rates were lower for sections of roads in which the speed limit was 30 km/h or less and or more than 604

ITARDA 26th Work Shop Report

km/h. When the speed limit is more than 60 km/h, accident reduction effects are thought to be lower due to therelationship between requirements for AEB performance certification systems and AEB safety criteria. Furthermore,
it was observed that speed limits of 30 km/h or less are used on special sections of national highways, such as where

Intersections
Number of accidents per vehicles
owned (accidents/million vehicles)

保有台数あたり追突事
故件数（件／百万台）

20

**
68%

**
54%

**
27.7
61%
10.7

8.0

10**

80%

13.9

46%

60%

5.0

2.5

1.7 0.8

100% 単 路 300**
64%6.3 3.4

040%
20%
0%

30km/h
30km/hor以下less

40km/h
40km/hor以下
less

50km/h
50km/hor以下
less

Non-AEBAEB非搭載
equipped

60km/h
60km/h
60km/h Morethanor以下
less
60超過
km/h
Reduction
AEBAEB搭載
削減率
rateequipped

保有台数あたり追突事
故件数（件／百万台）

交差点 30Non-intersections
Number of accidents per vehicles
owned (accidents/million vehicles)

the road alignment is poor.

200
10075%

**
56%

**
272.6
68%

**
70%

100%
59%

171.3

**
66.9
16.6
13.15.8

86.4

80%
60%

**
51.5 58.7
24.0

040%
20%
0%

30km/h
30km/hor以下
less

40km/h
50km/h
40km/h 50km/h
or以下
lessor以下
less

Non-AEBAEB非搭載
equipped

60km/h
60km/hor
less
以下

AEBAEB搭載

equipped

60km/hMorethan60km/h
超過
Reduction
削減率
rate

Fig. 4 - Number of rear-end collision accidents involving AEB-equipped and non-AEB-equipped vehicles pervehicles owned / Rear-end collision accident reduction rate in AEB-equipped vehicles (speed limits)
4.

Detailed analysis focusing on traffic accidents involving vehicles equipped with AEB and road traffic
environments, etc.

4-1.

Overview of analysis

Using data from 2017 to 2021, we identified areas (500 m × 500 m) where rear-end collision accidents involving
AEB-equipped vehicles frequently occur in Shizuoka Prefecture. Then, by overlaying rear-end collision accidents
involving AEB-equipped vehicles with ETC2.0 probe information, we examined road structures and traffic
environments where rear-end collision accidents often occur.
The area that was examined was the fourth mesh number, 523833703, located in the Aoi Ward of Shizuoka Cityin Shizuoka Prefecture. This area was selected due to the large number of rear-end collisions involving AEBequipped vehicles that occur there. (This area ranked first in the order of difference between rear-end collisions
involving AEB-equipped vehicles and rear-end collisions involving non-AEB-equipped vehicles).
4-2.

Enlarged view ofintersection
Approx.
Approx.
8m
3m

Accident sites

We graphed accident sites in order to understand thecharacteristics of areas where accidents involving AEB-Approx.
Approx.
3m
3m

equipped vehicles often occur (Fig. 5).
This enabled us to see that of the areas analyzed, rear-endcollision accidents involving AEB-equipped vehicles wereFig. 5 - Rear-end collision accident sites

concentrated in the northwest merging section of the Abecho intersection. Furthermore, as can be seen from theenlarged view, the distance between the pedestrian crossing

Position ofstop line

and the stop line at this location is extremely large.
4-3.

Rapid deceleration conditions

We graphed rapid deceleration sites in order tounderstand the traffic conditions of areas where accidents
5Fig. 6 - Rapid deceleration sites

ITARDA 26th Work Shop Reportinvolving AEB-equipped vehicles are concentrated (Fig. 6).
Rapid deceleration also occurs at the point beyond the stopline at the northwest merging section of the Abe-cho

Position ofstop line

intersection. From this, we can infer that drivers decelerate
rapidly when the traffic light changes from green to yellow as
they feel it will not be possible to cross the intersection intime.
4-4.

Acceleration conditions

We graphed acceleration sites in order to understand theFig. 7 Acceleration sites

traffic conditions of areas where accidents involving AEB-equipped vehicles are concentrated (Fig. 7).
From this, one can see that drivers are often accelerating at the point where rear-end collision accidents areconcentrated. This suggests that some drivers accelerate to cross the intersection at the moment the traffic lightchanges from green to yellow.
5.

Conclusion
Over the course of this research, we analyzed how road traffic characteristics such as road structures and traffic

environment affect reductions in rear-end collision accidents involving AEB-equipped vehicles. As a result, thefollowing findings were obtained.
・ In the case of curves, accident reduction effects are lower due to limitations with the detection range of invehicle cameras or due to delays in or failures to detect vehicles ahead when steering, etc.
・ On uphill sections of roads, accident reduction effects are lower due to drivers often overriding the system
by depressing the accelerator pedal. Also, on downhill sections of roads, accident reduction effects are lower
due gravitational acceleration being applied in the forward direction, which generally causes stopping
distances to be extended.
・ In sections where the speed limit is more than 60 km/h, accident reduction effects are lower due torequirements for AEB performance certification systems and AEB safety criteria not taking suchrequirements into consideration.
・ Rear-end collision accidents involving AEB-equipped vehicles have occurred in sections where vehicles are
rapidly decelerating and accelerating simultaneously (such as where stop lines are set a far distance away).
In the future, it should be possible to deepen our analysis by focusing on the damage mitigation effects of AEB
by collecting and utilizing information on accidents involving property damage. Furthermore, in anticipation of thefuture proliferation of autonomous vehicles, we must consider analysis and evaluation methods that focus on driving
assistance functions such as Adaptive Cruise Control. However, because such functions can be activated anddeactivated, exploring new analysis and evaluation methods remains a challenge.
<Acknowledgment>This study was supported by the Intelligent Transport System (ITS) Promotion Office of the Road Traffic
Management Division of the Road Bureau at the Ministry of Land, Infrastructure, Transport and Tourism (MLIT).
For this, we would like to express our sincere gratitude.

6ITARDA 26th Work Shop Report<References/Sources>(1) Kinoshita, Yoshihiko (2018) Analysis of the results of reducing rear-end collision accidents via AEB, 21st
Presentation Session for Traffic Accident Investigations, Analysis, and Research(2) Kinoshita, Yoshihiko (2019) Kinoshita, Yoshihiko (2019), Supplementary analysis of the results of collision
damage mitigation brakes (Automatic Emergency Brakes: AEB) in reducing rear-end accidents, 22nd
Presentation Session for Traffic Accident Investigations, Analysis, and Research(3) Kondo, Naoya (2019) Analysis of the effects of collision damage mitigation brakes (Automatic Emergency
Brakes (AEB)) by generation, 22nd Presentation Session for Traffic Accident Investigations, Analysis, andResearch(4) Yoshida, Shinpei (2022) Analysis of Situation Regarding Occurrences of Rear-End Collision Accidents atExpressways and Effects of AEB, 25th Presentation Session for Traffic Accident Investigations, Analysis,and Research(5) Ministry of Land, Infrastructure, Transport and Tourism (MLIT): General Vehicle Safety Information- Survey on the Status of ASV Technology Adoption, https://www. MLIT.go.jp/jidosha/anzen/01asv/data/r3souchakudaisu.PDF(End)

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