Naturalistic driving studies -- Vocabulary

This document defines terms and definitions commonly used for the annotation of video from NDSs collected during real-world driving in an uncontrolled setting.

Études naturalistiques de conduite -- Vocabulaire

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TECHNICAL ISO/TR
REPORT 21974-1
First edition
2018-11
Naturalistic driving studies —
Vocabulary —
Part 1:
Safety critical events
Études naturalistiques de conduite — Vocabulaire —
Partie 1: Événements critiques de sécurité
Reference number
ISO/TR 21974-1:2018(E)
ISO 2018
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ISO/TR 21974-1:2018(E)
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ISO/TR 21974-1:2018(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

Annex A (informative) Conceptual Framework for Categorizing Safety-critical events (SCEs)

in Naturalistic Driving Data ...................................................................................................................................................................... 5

Annex B (informative) Principles of Conflict Classification .......................................................................................................11

Annex C (informative) Additional Conflict Attributes......................................................................................................................28

Bibliography .............................................................................................................................................................................................................................52

© ISO 2018 – All rights reserved iii
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ISO/TR 21974-1:2018(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

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electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/patents).

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World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following

URL: www .iso .org/iso/foreword .html.

This document was prepared by Technical Committee ISO/TC 22, Road vehicles, Subcommittee SC 39,

Ergonomics.
A list of all parts in the ISO 21974 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/members .html.
iv © ISO 2018 – All rights reserved
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ISO/TR 21974-1:2018(E)
Introduction

A better understanding of driver behaviour is critical to future improvements in transportation safety.

Naturalistic Driving Studies (NDSs), which observe driver behaviour in a “natural” or uncontrolled

driving environment, offer unique insight into drivers’ typical behaviour under both “normal” driving

conditions and during the critical seconds before a crash or other safety-critical event (SCE). Much

of the value and power of NDSs lies in the video that is recorded of the driver and the environment

surrounding the vehicle. Although rich in information, video should be manually reviewed and coded

by trained data coders before it is scientifically analyzed. Given the potential for human error and

interpretation, coding protocols that are well-designed, thoroughly tested, and standardized across

studies are essential.

Naturalistic driving data sets, such as the Second Strategic Highway Research Program (SHRP 2), are

becoming more available to a diverse group of researchers. As a result, it is important to have a common

terminology for monitoring, coding, and analyzing data to allow research protocols to be replicated and

results to be compared across studies. In fact, the research community has called for the development of

fully tested, common coding protocols for use in NDSs. This document addresses that need by providing

a standard vocabulary for coding SCE characteristics in NDSs. The foundation for this document came

from the SHRP 2 naturalistic driving study annotation effort and subsequent revisions to the SHRP 2

[1]

dictionary to accommodate heavy vehicles (trucks and buses) . Substantial improvements have been

made to this document in both content and structure. However, large parts of this document are largely

verbatim with the original foundational documents.

It is recommended that vehicles in these studies be instrumented with at least a forward-looking view

and an in-vehicle view capturing the driver’s face and upper body. Rear- and side-facing views are

often helpful when interpreting conflicts that occur behind or next to the instrumented vehicle. A view

capturing the steering wheel, driver’s hands and/or dashboard can be helpful for additional analyses

(which are outside the scope of this document). However, equipment and labor costs may make these

additional views unfeasible. In addition, basic measurements of the kinematics of the instrumented

vehicle should be available, including at least longitudinal acceleration, lateral acceleration, and vehicle

speed. Other kinematic measurements that help assess conflict situations (if feasible) include brake and

throttle pedal application and/or pressure, proximity to and speed of surrounding non-instrumented

vehicles (e.g., via radar), latitude and longitude, and activation of key vehicle safety systems (e.g., anti-

lock brakes).

The main objective of this document is to define different types of SCEs based on a taxonomy of general

conflict classes and a set of basic variables for characterizing the events. The definitions supplied here

apply to data collected from light and heavy vehicles [i.e., category M and N according to Classification

and definition of power-driven vehicles and trailers: Council Directive 70/156/EEC (as amended by 92/53/

EEC), Annex 2]. However, this does not preclude the definitions specified in this document from, with

caution, being adapted for use with data collected from other vehicle types such as powered two-

wheelers or an infrastructure-based (rather than vehicle-based) data collection system.

This document does not address methods for identifying candidate SCEs (also referred to as triggers),

a complete set of annotations for these SCEs (e.g., driver distraction), or the recommended approach to

analyzing this data. This document also does not address the definition, extraction, or annotation of

controls. These controls, often called baselines in transportation literature, are selected for comparison

to events of interest in epidemiological as well as other analyses. Since the definition, extraction, and

annotation of these controls are highly dependent on the research question of interest, they have been

excluded from the scope of this document. The scope of the present document is graphically illustrated

in Figure 1.
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ISO/TR 21974-1:2018(E)
Figure 1 — Document scope
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TECHNICAL REPORT ISO/TR 21974-1:2018(E)
Naturalistic driving studies — Vocabulary —
Part 1:
Safety critical events
1 Scope

This document defines terms and definitions commonly used for the annotation of video from NDSs

collected during real-world driving in an uncontrolled setting.
2 Normative references
There are no normative references in this document.
3 Terms and definitions

ISO and IEC maintain terminological databases for use in standardization at the following URL

addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
3.1
combined avoidance capacity
total response capacity of all responding conflict partners (3.3)

Note 1 to entry: The capacity of a responding conflict partner takes into account both the abilities and limitations

of the vehicle or road user as well as any environmental or infrastructural constraints, if present.

Note 2 to entry: Conflict partners that do not exhibit an observable response are not included in this construct.

3.2
conflict

situation where the trajectory(ies) of one or more road users or objects (conflict partner; 3.3) led to one

of three results: 1) a crash (3.4) or road departure (3.12), 2) a situation where an evasive manoeuvre(s)

(3.5) was required to avoid a crash or road departure, or 3) an unsafe proximity between the conflict

partners

Note 1 to entry: The key concept underlying the present framework is that of conflict.

Note 2 to entry: Three general classes of traffic conflict are of interest in naturalistic driving analyses: trajectory

conflict (3.2.1), single-vehicle conflict (3.2.2), and proximity conflict (3.2.3).

3.2.1
trajectory conflict
crash course between at least two conflict partners (3.3)
3.2.2
single-vehicle conflict

conflict (3.2) involving loss of vehicle control (e.g., horizontal and/or lateral skidding or rotation) or

proximity to the road edges (e.g., road departure; 3.12) rather than proximity to another entity

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ISO/TR 21974-1:2018(E)
3.2.3
proximity conflict

conflict (3.2) involving two or more entities that are not on a crash course but nevertheless come in

close temporal and/or spatial proximity to a crash (3.4)
3.3
conflict partner
any entity that is part of a conflict (3.2)

Note 1 to entry: This may include other vehicles (3.18), pedestrians (3.10), pedal cyclists (3.9), other non-motorists

(3.8), other road users, animals, and objects (including roadside barriers that exceed the ground clearance of the

affected vehicle).

Note 2 to entry: If a conflict partner (3.3) is present, then the conflict is either a trajectory conflict (3.2.1) or a

proximity conflict (3.2.3).

Note 3 to entry: Low roadside barriers (e.g., curbs) within the ground clearance of the vehicle are not considered

conflict partners.
3.4
crash

situation in which the subject vehicle (i.e., instrumented vehicle; 3.14) has any contact with at least one

other conflict partner (3.3) either on or off the trafficway (3.15), either moving or stationary (fixed or

non-fixed), that is observable or in which kinetic energy is measurably transferred or dissipated

Note 1 to entry: This excludes roadway (3.11) features meant to be driven over such as speed bumps and low

roadside barriers (curbs, medians, etc.) within the ground clearance limitations of the vehicle.

Note 2 to entry: A crash may also be a single-vehicle conflict (3.2.2) that includes at least one of the following

conditions: vehicle rollover, airbag deployment, injury, more than 90° degrees of horizontal vehicle rotation, or

all four tires leaving the trafficway.
3.5
evasive manoeuvre

any action performed by any conflict partner (3.3) to change its trajectory or speed in an attempt to

avoid or reduce the severity of a potential crash (3.4), avoid or reduce the severity of a road departure

(3.12), or regain vehicular control after a loss of control

Note 1 to entry: Examples include steering, braking, accelerating, running, stopping, or a combination of these.

Note 2 to entry: At least one of the manoeuvres exceeds normal vehicle control inputs.

3.6
metadata

information that provides a description about the structural content of its referenced dataset or the

methods with which the data were collected

Note 1 to entry: Examples of metadata include location (e.g., country, region) of data collection, sampling

methods, units of measure, etc.
3.7
naturalistic driving study
NDS

any driving study where research subjects are recruited to drive on public roads (not in a simulator or

on a test track), where there is no in-vehicle experimenter or confederate vehicles, and where driving

conditions are not experimentally controlled or manipulated

Note 1 to entry: Subjects are not instructed to drive differently than they normally would, and the data collection

instrumentation is unobtrusive.

Note 2 to entry: Typically, these studies last a minimum of several weeks for each subject and can go much longer.

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3.8
non-motorist
non-motorized conveyance

person is being transported by a non-motorized conveyance, other than a pedaled cycle (3.9)/a human-

powered device by which a non-motorist may move or may move another non-motorist

EXAMPLE Baby carriages, coasters, wagons, ice skates, roller skates, push carts, push scooters, skateboards,

skis, sleds, non-motorized wheel chairs, rickshaws, etc.
3.9
pedal cyclist

person on any type of manually propelled pedaled cycle, as either the driver or the passenger, including

bicycles, tricycles, and unicycles

Note 1 to entry: This category includes pedal cyclists holding on to a motorized vehicle.

Note 2 to entry: This category also includes power-assisted pedaled cycles that have limitations on speed (i.e.,

pedelecs). Pedaled cycles that are capable of propelling themselves and do not require pedaling for propulsion

are not included in this category (see “Motorcycle or moped”).
3.10
pedestrian

any person who is on or near a roadway (3.11) or a sidewalk, path, or other space that is contiguous

with a roadway, or on areas beside the roadway into which a vehicle can travel, and who is not in or on

either a motorized or a non-motorized conveyance

Note 1 to entry: This includes persons who are in contact with the ground, roadway, etc., but who are holding on

to a vehicle.
3.11
roadway

portion of a trafficway (3.15) that is designed and ordinarily used for vehicular travel, including all

designated or implied travel lanes (through lanes, turn lanes, acceleration and deceleration lanes), but

not shoulders, painted (whether usable or not), medians of any type, roadsides, gore areas, etc., that are

of a similar road surface to the parking lanes, parking areas, or driveways
3.12
road departure

conflict (3.2) in which the subject vehicle (3.14) is on a path toward and crosses, or is at risk of crossing,

the road edge

Note 1 to entry: The road edge is the outside edge of the shoulder (if present) or a physical raised curb or median

on the left or right side of the trafficway, including low barriers such as curbs and curb-style medians that are

within the ground clearance of the vehicle.
3.13
safety-critical event
SCE

conflict (3.2) or series of related conflicts that involves the subject vehicle (3.14) either alone or in

combination with another vehicle (3.18), pedal cyclist (3.9), pedestrian (3.10), object, or road edge

Note 1 to entry: This document describes the range of conflict types that may comprise an SCE, and an SCE may

be composed of a single conflict type or multiple simultaneous or sequential conflict types. Conflicts should be

non-intentional and non-premeditated (unplanned) by at least one conflict partner (3.3).

3.14
subject vehicle
vehicle (3.18) that has been instrumented and collects data

Note 1 to entry: Available videos will capture images from the subject vehicle’s perspective.

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ISO/TR 21974-1:2018(E)

Note 2 to entry: Kinematic data are typically available only from the subject vehicle or (in the case of radar data)

the subject vehicle’s perspective.
3.15
trafficway

any right-of-way designated for moving persons or property from one place to another, including the

surface on which vehicles normally travel (i.e., the roadway; 3.11), plus the shoulders, painted medians,

and painted gore areas at grade with the roadway

Note 1 to entry: The trafficway also includes parking lanes and parking areas (e.g., parking lots, driveways).

Note 2 to entry: The trafficway is bound by the outer edges of the shoulder or by raised roadside barriers (e.g.,

curb, guardrail, pylon) and thus does not include raised medians, grassy medians, sidewalks, etc.

3.16
trigger

data analysis method that includes the application of algorithms (e.g., acceleration thresholds) for

searching time series driving data for key situational characteristics or thresholds in order to identify

potential events of interest (e.g., kinematic threshold for potential SCEs; 3.13)

3.17
urgent response
urgency of response

situation in which the required evasive manoeuvre (3.5) approaches the combined avoidance capacity

(3.1) of the responding conflict partners (3.3) required to prevent a crash

Note 1 to entry: Both the time available for response and the required response magnitude (e.g., level of

deceleration) should be considered.

Note 2 to entry: While it may be possible to quantify the required urgency of the evasive manoeuvre (see Annex A

for more discussion), the methods for doing so are not within the scope of this document. Quantification of the

urgency is not required in order to use the concept as described in this document.

Note 3 to entry: The urgency of the response is independent of any evasive manoeuvres that are actually

performed.
3.18
vehicle

any motorized means of transportation, excluding pedelecs, which are included in the pedal cyclist

definition (3.9)
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ISO/TR 21974-1:2018(E)
Annex A
(informative)
Conceptual Framework for Categorizing Safety-critical events
(SCEs) in Naturalistic Driving Data
A.1 Introduction

Safety-critical events (SCEs) of interest in naturalistic driving studies (NDSs) traditionally include

crashes and near-crashes, but also less-severe but still safety-relevant non-crash events, such as crash-

relevant conflicts and proximity conflicts. The main objective of this document is to define and rank

different types of SCEs based on a taxonomy of general conflict classes and a set of basic variables

for characterizing the events. The objective of this Annex is to outline the conceptual basis for these

definitions.

The key concept underlying the present framework is that of traffic conflict. There are at least three

general classes of traffic conflict that are of interest in naturalistic driving analyses. First, a trajectory

[1]

conflict, which is the main conflict class traditionally studied (e.g., Reference ), is defined in terms of a

crash course between at least two conflict partners. Second, a single-vehicle conflict is defined in terms

of loss of control or proximity to the road edges rather than by proximity to another conflict partner.

Third, a proximity conflict involves two or more conflict partners that are not on a crash course but

nevertheless come in close temporal and/or spatial proximity to a crash. Importantly, these general

conflict types are not mutually exclusive and may coexist in a single SCE. Thus, an SCE may include

several conflicts occurring in a series.

Furthermore, these conflict classes are characterized by two types of criteria. First, the outcome criteria

state that conflicts result in some level of criticality or severity, and that conflicts within each class

may be graded based on these severity levels. However, the definitions of these severity levels differ

between the conflict classes, as further described below. Second, the intentionality criterion states that

conflicts are always non-intended by at least one of the conflict partners. The following section further

describes these two criteria, and the remainder of the Annex describes how they apply to each of the

three conflict classes.
A.2 General conflict criteria
A.2.1 Outcome criteria

The basic criterion for an event to count as a crash is that it involves contact with another entity that

extends above the ground clearance of the subject vehicle that is observable and in which kinetic energy

is measurably transferred or dissipated. In the case of a single vehicle crash, the vehicle departs the

road with all four tires, rolls over, results in an air bag deployment, or involves occupant injury. The

crash can then be further characterized in terms of outcome criteria such as delta-V (the change in

velocity during impact) and/or other observable outcomes such as injuries or property damage. In the

context of naturalistic driving analysis, the choice of crash-severity criteria to be applied to a given

conflict depends mainly on practical rather than conceptual considerations, in particular the types of

outcomes that can be reliably observed in naturalistic data.

For non-crashes, the specific outcome criteria differ between the main conflict classes, but the common

characteristic is that there is some degree of kinematic proximity to crash impact, or, in the case of

single-vehicle conflicts, some degree of loss of control or proximity to the road edges. The specific

criteria for each conflict class are further discussed below.
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A.2.2 Intentionality criterion

The intentionality criterion requires that the conflict (i.e., crash impact or kinematic proximity) was

not intended by at least one of the conflict partners. This criterion is important to exclude situations

where a driver (or, in the case of trajectory and proximity conflicts, all conflict partners) intentionally

produces a situation that satisfies the outcome criteria. For example, this may involve a driver leaving

the road on purpose to take a shortcut. Importantly, it should be noted that the action leading up to

a true conflict (e.g., initiating a pass on a blind corner) may have been intentional, but as long as the

outcome [e.g., a small time-to-collision (TTC) to an oncoming vehicle] was not intended by at least one

of the conflict partners, the event should be considered a conflict.
A.3 Trajectory conflict

In traditional conflict theory, a conflict is defined as “a situation where two or more conflict partners

approach each other in time and space to such extent that a crash is imminent if their movements

[2]

remain unchanged ”. Thus, the defining features of traditional conflicts are (1) that two or more

conflict partners (which may be vehicles, other road users, or stationary objects) enter into a collision

course with each other where (2) collision is imminent. This general situation is referred to as a trajectory

conflict, which includes the main types of SCEs traditionally analyzed in NDSs, such as crashes, near-

crashes, and crash-relevant conflicts.

Given this conceptualization of a trajectory conflict, a key further issue is to characterize the degree

of imminence of a crash. Here, imminence is conceptualized in terms of the urgency of the conflict

situation. Urgency is here defined as given the point of time of the avoidance manoeuvre, what is the

required magnitude of the avoidance response relative to the possible (or available) avoidance response

magnitude of the involved responding conflict partners. More specifically, a response is urgent when

the magnitude of the evasive manoeuvre required to avoid a crash approaches the combined avoidance

capacity of all responding conflict partners in the given situation.

There are two key concepts here: the required evasive manoeuvre magnitude, and the combined

avoidance capacity of all responding conflict partners. The required evasive manoeuvre magnitude is

discussed in greater detail in the paragraphs that follow, but in short it represents the “minimum” that

should be done to avoid a crash. It is dependent on the relative speed of the conflict partners and the

angle of their approach. It is independent of the actual response(s) performed and may be of greater (in

the case of crashes) or lesser (in the case of non-crash conflicts) magnitude than the actual response(s).

Avoidance capacity is here defined as the ability of a conflict partner to execute an evasive manoeuvre,

where an evasive manoeuvre is any action performed to avoid a potential crash by changing the

trajectory or speed, such as steering, braking, accelerating, running (pedestrian and animals), or

stopping. The combined avoidance capacity refers to the combined ability of all responding conflict

partners to change their trajectories to avoid a crash. It is also independent of the actual response(s)

performed, but dependent on the characteristics both of the responding conflict partners themselves

(e.g., brake condition, tire tread, vehicle specifications, braking and steering capabilities, pedestrian

mobility, etc.), and the environment in which the conflict takes place (e.g., road surface condition,

presence of other road users or obstacles). For example, the combined avoidance capacity of a vehicle

that brakes and/or swerves as it heads toward a pole is less than the combined avoidance capacity

in a situation where the same vehicle approaches a pedestrian and both conflict partners respond to

the situation, everything else being equal (since the pedestrian but not the pole can move). Finally, a

responding conflict partner is defined as a conflict partner exhibiting a visible response to the conflict.

If a conflict partner does not exhibit such a response, it is excluded from the conceptualization of

“combined avoidance capacity” for the given conflict. Thus, if the pedestrian in the example above does

not move, then the combined avoidance capacity is the same as in the situation with the pole.

A useful way to think about the urgency of a situation is in terms of the proximity to the point-of-

no-return (PONR). The PONR refers to the point in time during the conflict envelope when the crash

becomes unavoidable given the maximum avoidance capacities of the responding conflict partners.

Thus, for example, for a given set of kinematics (locations, speeds, and accelerations) of the involved

conflict partners, the urgency of the situation increases with reduced road friction (thus reducing

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the avoidance capacities of the conflict partners). Consequently, the PONR will be reached earlier,

and the situation will be more urgent, when the road is wet or icy compared to when it is dry. This

conceptualization of urgency may be used as the basis for characterizing different types of non-crash

conflicts. For example, near-crashes (NCs) may be defined as situations with high urgency. That is,

the magnitude of the required evasive manoeuvre approaches the maximum combined avoidance

capability of the responding conflict partners (see B.2.1.4 and B.2.1.5). In other words, a near crash

corresponds to a situation with a small margin to the PONR. By contrast, a crash-re

...

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