ITARDA 22th Work Shop Report2019
22nd Presentation Session for Traffic Accident Investigations, Analysis, and Research

Characteristics of lane deviation accidents seen from micro investigations
Kenshi Torikai
Investigator, Tsukuba Traffic Accident Investigation Office, Research Division

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ITARDA 22th Work Shop Report

1.

Introduction

In recent years, the number of fatal traffic accidents in Japan has been declining, yet the number still remains highat 3,449 such accidents that occurred in 2018. Of these, lane deviation accidents in which a driver deviated fromtheir own lane that resulted in a collision with an oncoming vehicle, roadside structure, or other object, accounted
for half of all fatal accidents while riding in a four-wheel vehicle. The hope is that these can be reduced in the future.
ITARDA has reported on the results of analyses on lane deviation accidents in aiming to reduce them by focusing
attention on them (2012, 2016). In 2012, macro data and micro data were used to analyze the characteristics of lane
deviation incidents, and matters like analyses of the causal factors and patterns and types of vehicle deviation
behavior were reported on. Moreover, in 2016 a deeper analysis of lane deviation characteristics was undertaken,
and countermeasures to avoid lane deviations and estimates for the reduction in the number of casualties achievablefrom these were reported.
For this study, an analysis of the characteristics of lane deviation accidents was performed by using data like theresults of reconstructing accidents through the use of ITARDA's micro case studies. The objective with this was toaggregate basic data conducive to advancing preventive safety systems, particularly lane deviation prevention
systems, which are expected to become more widespread in the future.
2.

Background and objectives

2-1. Number of lane deviation accidents and their fatality rate
The number of fatal accidents by type of accident while riding in a four-wheel vehicle are shown in Fig. 1. Here,accidents while riding in a four-wheel vehicle refer to single vehicle accidents and vehicle-vehicle accidents where
the primary party was driving a four-wheel vehicle. The number of accidents represents the total number between
the years 2014 and 2018. What is more, lane deviation accidents were aggregated as a total of the number of single
vehicle accidents (excluding collisions with parked vehicles and overturn accidents) and head-on collision accidents.
0Lane deviation
Crossing collision
Rear-end
When turning right
When turning left
Vehicle-vehicle, etc.
Single vehicle, etc.

1000

2000

3000

4000

5000
4576

1657
841
792
241
599

150

Fig. 1. Number of fatal accidents by type of accident while riding in a four-wheel vehicle
This reveals that lane deviation accidents account for the greatest share of the number of fatal accidents while riding
in a four-wheel vehicle. The recognition obtained from this is that reducing lane deviation accidents will pose amajor challenge moving forward.
In addition, a comparison of the fatality rates by type of accident for fatal accidents while riding in a four-wheelvehicle is shown in Fig. 2. Here, the fatality rate indicates the ratio of the number of fatal accidents to the total
number of casualty accidents for each type of accident.

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0%

1%

2%

3%

4%

5%

Lane deviationCrossing collision
Rear-end
When turning right

6%
5.2%

0.3%
0.1%
0.4%

When turning left

0.2%

Other

0.3%

Fig. 2. Fatality rate by type of accident while riding in a four-wheel vehicle
From this, we can see that the fatality rate from lane deviation accidents is the highest by far compared with othertypes of accidents. In other words, the large number of fatal accidents indicated in Fig. 1 is estimated to be acontributing factor to the elevated risk of death from lane deviation accidents, and ideally such accidents should be
reduced in the future.
2-2. Lane deviation prevention systems
In recent years, lane deviation prevention systems have become widespread as a type of preventive safety systemdesigned to prevent lane deviation accidents by vehicles. These have been added as an evaluation item by the JNCAP conducted by NASVA as well as by Euro-NCAP which is carried out in Europe, with high hopes for boththeir effectiveness and further advancement in the future.
These systems can be broadly categorized into two types: one type that sounds a warning to alert the driver of danger,
and another type where the system takes control of the steering wheel to prevent the vehicle from deviating from itslane.
Moreover, these systems are not constantly active, but rather their activation / non-activation is governed byconditions such as the running speed, weather, and so forth. Regarding the running speed, it is generally assumed
that they will be used on expressways, and while there are differences based on the car manufacturer and model,
they are designed to activate at around appr oximately 50 - 100km/h (Fig. 3; Source: Various company homepages).

Side collision
Company ACompany B
Company C
Company D
Company E
Company FWarningSteering assist

Company GFig. 3. Activation speed ranges for lane deviation prevention systems

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Analytical method

3-1. Selecting the micro data
The micro data used for the analysis consisted of data from the five-year period from 2013 - 2017.
This included investigations that had been performed into 181 lane deviation accidents, the breakdown for which
included 125 single vehicle accidents and 56 head-on collision accidents.
3-2. Reconstructing lane deviation accidents through the use of micro dataITARDA creates detailed road diagrams for all of the accidents that it investigates. In addition to these road diagrams,
it also uses a combination of interviews with the parties to the accident, the extent of the damage to the vehicles,
and other factors to make it possible to estimate the running trajectory and trends in the speed of the vehicles in theaccident from before and after the collision.
With this study, accident reconstructions were conducted on 81 of the 181 lane deviation accidents (46 single vehicle
accidents and 35 head-on collision accidents). This was done in order to be able to obtain the information needed
to estimate the running trajectory of the vehicles, as well as to limit this to accident types involving gentle deviation
rather than types where the vehicle deviated due to slips or abrupt steering.
The following two points were verified based on the results of these accident reconstructions.
(1) The time from the lane deviation to the collision(2) The deviation angle (the angle formed by the vehicle trajectory and the lane when the vehicle veered from itslane)
With respect to (1), the distance between the point of the lane deviation and the point of the collision was found by
dividing by the collision speed. For (2), the angle was found by tracing the vehicle trajectory over top the road
diagram. The lane deviation point was defined as the point where the exterior of the deviating vehicle intersectedwith the lane.
4.

Analyzing the characteristics of lane deviation accidents

4-1. Types of road lanes
Based on the results of investigations of the scenes of accidents from the micro investigations, the number of lane
deviation accidents by type of road lane are shown in Fig. 4 and Fig. 5. According to this, accidents on straightsegments account for the greatest share of single vehicle accidents, while many head-on collision accidents occur
on both straight segments and left-hand curves.
9030
2265

2320

60
33

1025

30

1012Fig. 4. Number of accidents by type of road lane(single vehicle)

Intersection

Right curve

Straight line

Intersection

Right curve

Left curve

Straight line

0

Left curve

0Fig. 5. Number of accidents by type of road lane(head-on collisions)
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4-2. By type of deviation
Based on the results of investigations at the scene of accidents, interviews with the parties to the accidents, and soforth from the micro investigations, the number of lane deviation accidents by type of deviation are shown in Fig.
6 and Fig. 7. These reveal that accidents due to gentle deviations accounted for the majority of both single vehicle
accidents and head-on collision accidents.
90

85

60
4640

60
2130

20

127

40

420

Other / Unclear

Affected by roadsurface

Loss of control

Gentle deviation

Other / Unclear

Affected by roadsurface

Loss of control

Sudden steering

Gentle deviation

Fig. 6. Number of accidents by type of deviation(single vehicle)

Sudden steering

00Fig. 7. Number of accidents by type of deviation(head-on collision)

4-3. By human factor
When it comes to the "gentle deviation" accidents from “4.2. By type of deviation,” the number of lane deviation
accidents by human factor are shown in Fig. 8 and Fig. 9 based on the interviews with the accident parties (excludingcases where this was unknown). This reveals that the majority of accidents were caused by dozing off, absentminded driving, and distracted driving, in that order. Absent-minded driving and distracted driving accounted forthe greatest share of head-on collision accidents, followed next by dozing off. When lane deviation accidents are
considered as a whole, the three factors of dozing off, absent-minded driving, and distracted driving can beconsidered to be the main human factors at play.
2024121820

101110118300
Drunk driving

6Illness30

Distracted driving

Dozing off

Drunk driving

Illness

Distracted driving

Absent-mindeddriving

Dozing off

Fig. 8. Number of accidents by human factors(single vehicle)

Absent-mindeddriving

00Fig. 9. Number of accidents by human factors(head-on collisions)
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4-4. By danger perception timing
When it comes to the "gentle deviation" accidents from “4.2. By type of deviation,” the number of lane deviation
accidents by danger perception timing are shown in Fig. 10 and Fig. 11 based on the interviews with the accident
parties (excluding cases where this was unknown). This reveals that the majority of single vehicle accidents
consisted of accidents where the driver was unaware of the other object at the time of the collision, or in other words,
up until the collision occurred. The same holds true for head-on collision accidents, in that most accidents involved
the driver only perceiving danger at the time of the collision.
6020
143640

81020

6100

10Fig. 10. Number of accidents by danger perception
timing (single vehicle)

Over 2 seconds

1 - 2 seconds

Time of collision

Over 2 seconds

1 - 2 seconds

Time of collision

Within 1 second0
Within 1 second0Fig. 11. Number of accidents by danger perception
timing (head-on collisions)4-5. By danger perception speed
Based on the macro data from the five-year period of 2014 - 2018, the number of lane deviation accidents by danger
perception speed of the primary party are shown in Fig. 12 and Fig. 13 (excluding cases where this was unknown).
This indicates that the majority of accidents occurred at a danger perception speed of between 30km/h and 40km/h
for both single vehicle accidents and head-on collision accidents. The trend observed from this is that a large shareof such accidents occur at speeds of no more than 50km/h.
10000

15000
10000

5000
5000

Fig. 12. Number of accidents by danger perception
speed (single vehicle)

Over 100km/h

100km/h or less

70km/h or less

60km/h or less

50km/h or less

40km/h or less

30km/h or less

20km/h or less

10km/h or lessOver 100km/h

100km/h or less

70km/h or less

60km/h or less

50km/h or less

40km/h or less

30km/h or less

20km/h or less

010km/h or less

0Fig. 13. Number of accidents by danger perception
speed (head-on collisions)

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4-6. Time from the lane deviation until the collision
For the 35 head-on collision accidents out of the 81 cases of accident reconstructions performed that are shown in
3-2., the number of lane deviation accidents by time from the lane deviation until the collision are shown in Fig. 14(excluding cases where this was unknown). With single vehicle accidents, most collisions occur with roadsidestructures such as guardrails to the side of the road, and so due to their short interval of time from the deviation until
the collision, they were excluded from this verification. According to this, most head-on collision accidents had atime of two seconds or less from the lane deviation until the collision occurred.
201610765

1Over 4 seconds

4 second or less3 second or less2 second or less1 second or less

0Fig. 14. Number of accidents by time from thedeviation until the collision (head-on collisions)4-7. Lane deviation angleOf the 81 cases of accident reconstructions performed that are shown in 3-2., the number of lane deviation accidents
by lane deviation angle are shown in Fig. 15 and Fig. 16 (excluding cases where this was unknown). From this, atrend was observed whereby most accidents occurred between angles of 1° and 4° for both single vehicle accidents
and head-on collision accidents.
2020
1313

1210
70

20

3
1

42

3

10000Fig. 15. Number of accidents by deviation angle(single vehicle)

7° or less6° or less5° or less4° or less3° or less2° or less1° or lessOver 10°10° or less9° or less8° or less7° or less6° or less5° or less4° or less3° or less2° or less

01° or less

02
Over 10°310° or less369° or less

68° or less

10Fig. 16. Number of accidents by deviation angle(head-on collision)

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5.

Considerations and recommendations

5-1. Speed range in which lane deviation prevention systems activate
The actual conditions of accidents by speed shown in 4-5 are indicated in Fig. 17 as a share of accidents by speed.
As was mentioned in 2-2., current lane deviation prevention systems activate at approximately 50 - 100km/h. When
this is factored into consideration, we see that the accident coverage rate by such systems would be 18% for single
vehicle accidents and 14% for head-on collision accidents, for 16% for lane deviation accidents as a whole. Yet
conversely, 52% of single vehicle accidents and 68% of head-on collision accidents, for a total of 61% of all lane
deviation accidents, occur within a speed range of 21 - 50km/h which accounts for the largest share of accidents.
For the future, being able to expand the speed range in which lane deviation prevention systems activate to 20km/h
or more would lead to coverage of roughly 77% of all lane deviation accidents, which would presumably contributeenormously to reducing accidents.

0%Single vehicle
Head-on collision
Lane deviation
accidents as a whole

10km/h or less

25%
17%

7%

50%

12%

52%

11%

12%

12%

11 - 20km/h

75%

21 - 50km/h

100%
18%

1%

68%

14% 0%

61%

16% 0%

51 - 100km/h

Over 100km/h

Fig. 17. Share of lane deviation accidents by speed
5-2. Lane deviation prevention systems (warning)
As indicated in 4-6, most accidents had a time from when the deviation occurred until the collision of two seconds
or less. For this, if we consider a person's reaction time (time from when they notice the warning until they can
actually set their body in motion) to be approximately one second, then even when a warning sounds when thedriver deviates from their lane prior to a collision, they only have a brief window of time in which to avoid said
collision.
Warnings are a useful way to prevent inattentive or distracted driving. However, when the verification results fromthis study are taken into consideration, such warnings should not be overly relied upon. Drivers must still drive in astate of readiness whereby they are prepared to take action at a moment's notice.
5-3. Lane deviation prevention systems (steering assist)
As indicated in 4-7, the most commonly seen deviation angle when vehicles deviated from their lane was between
1° and 4° versus the lane. The number of accidents and their total share for lane deviation accidents as a whole areshown in Fig. 18. This reveals that accidents with a deviation angle between 0° and 5° account for approximately
80% of the total.
If in the future lane deviation prevention systems can cover lane deviations with an angle of 5° or less, then thiswould cover approximately 80% of lane deviation accidents as a whole, and would presumably contributeenormously to reducing accidents.

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4030
20
1068%

80%

88%

90%

93%

94%

98% 100%

44%
26%1%

0

100%
75%
50%
25%
0%

Over 10°10° or less9° or less8° or less7° or less6° or less5° or less4° or less3° or less2° or less1° or less

１０度超

１０度以下

９度以下

８度以下

７度以下

６度以下

５度以下

４度以下

３度以下

２度以下

１度以下

Fig. 18. Number of accidents by deviation angle and their total share
6.

Conclusion

The following lessons were learned from this study as a result of analyzing lane deviation accidents through the useof information like micro data from ITARDA.


Accidents frequently occur along straight segments, with "gentle deviation" a common type of deviation.
The most common human factors included dozing off, absent-minded driving, and distracted driving, with manypeople only becoming aware they were in danger of causing an accident once the collision occurred.

Most accidents occurred within a danger perception speed of 21 - 50km/h, with this range accounting for

approximately 60% of all accidents.
The most common time from when the vehicle deviated from its lane until it collided with an oncoming vehicle
was approximately one - two seconds.

The most common lane deviation angle was between 1° and 4°, with accidents at an angle of 5° or less accounting
for approximately 80% of the total.

Based on these characteristics, the hope is that further advances will be achieved in lane deviation prevention
systems. Yet even so, there will still be a need for drivers to concentrate on safe driving without becoming overly
reliant on such systems.
＜引用・参考文献＞
1.
2.

公益財団法人交通事故総合分析センター、自主研究報告書、2012 年、「自動車における車線逸脱の要因分析」、公益財団法人交通事故総合分析センター
公益財団法人交通事故総合分析センター、自主研究報告書、2016 年、「車線逸脱事故の死傷者数低減について」、公益財団法人交通事故総合分析センター

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