Road vehicles -- Human performance and state in the context of automated driving

This document introduces basic common underlying concepts related to driver performance and state in the context of automated driving. The concepts in this document are applicable to all levels of automated driving functions that require a human/driver to be engaged or fallback-ready (SAE level 1, 2 and 3). It can also be used with levels that enable a driver to resume manual control of the vehicle (a compatible feature for SAE levels 1 to 5). Common underlying concepts can be applicable for human factors assessment/evaluations using driving simulators, tests on restricted roadways (e.g. test tracks) or tests on public roads. The information applies to all vehicle categories. This document contains a mixture of information where technical consensus supports such guidance, as well as discussion of those areas where further research is required to support technical consensus. These common underlying concepts can be also useful for product descriptions and owner manuals. The contents in this document are informative, rather than normative, in nature.

Véhicules routiers -- Etat et performance humaine dans le contexte de la conduite automatisée

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08-Jan-2020
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TECHNICAL ISO/TR
REPORT 21959-1
Second edition
2020-01
Road vehicles — Human performance
and state in the context of automated
driving —
Part 1:
Common underlying concepts
Véhicules routiers — Etat et performance humaine dans le contexte
de la conduite automatisée —
Partie 1: Concepts fondamentaux
Reference number
ISO/TR 21959-1:2020(E)
ISO 2020
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ISO/TR 21959-1:2020(E)
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© ISO 2020

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ii © ISO 2020 – All rights reserved
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ISO/TR 21959-1:2020(E)
Contents Page

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

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

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

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

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

4 Purpose .......................................................................................................................................................................................................................... 1

5 Human performance in the context of automated driving ....................................................................................... 2

5.1 General ........................................................................................................................................................................................................... 2

5.2 Transition from manual to automated driving ............................................................................................................ 2

5.2.1 Transition process model ......................................................................................................................................... 2

5.2.2 Definition of related concepts .............................................................................................................................. 3

5.2.3 Measures for human performance in releasing control to automation ........................... 4

5.3 Transition from automated to manual driving ............................................................................................................ 4

5.3.1 Transition process models ...................................................................................................................................... 5

5.3.2 Definition of related concepts .............................................................................................................................. 7

5.3.3 Measures for human performance in regaining control from automation ................... 8

6 Human states in the context of automated driving ........................................................................................................11

6.1 General ........................................................................................................................................................................................................11

6.2 General concepts for mental state related to automated driving .............................................................11

6.3 Concepts corresponding to automation related driver states ....................................................................12

6.4 Concepts corresponding to non-driving related driver states ....................................................................13

6.5 Driving position and posture ...................................................................................................................................................15

7 Driver readiness/availability ...............................................................................................................................................................16

8 Drivers’ experiences and attitudes regarding driving automation system ..........................................16

8.1 Prior system image ...........................................................................................................................................................................17

8.2 Education and training ..................................................................................................................................................................18

8.3 User’s understanding of driving automation system ..........................................................................................18

8.3.1 User’s thought about how driving automation system works ..............................................18

8.3.2 User’s mental attitude to driving automation system ..................................................................18

8.4 User’s use of driving automation system ......................................................................................................................19

8.4.1 User’s behavioural attitude while using driving automation system ..............................19

8.4.2 User’s interaction with driving automation system .......................................................................19

8.4.3 User’s behaviour/manner at driving automation system .........................................................19

Bibliography .............................................................................................................................................................................................................................20

© ISO 2020 – All rights reserved iii
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ISO/TR 21959-1:2020(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

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

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).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/

iso/ foreword .html.

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

Ergonomics.

This second edition cancels and replaces the first edition (ISO/TR 21959-1:2018), which has been

technically revised. The main changes compared to the previous edition are as follows:

— editorial modifications to the format of the figures;
— corrections of the references to clause numbers (Clause 7 is now Clause 8);
— corrections to redundant descriptions.
A list of all parts in the ISO 21959 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 2020 – All rights reserved
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ISO/TR 21959-1:2020(E)
Introduction

Although automation technology is advancing at a very fast pace, the majority of automated driving

levels (as defined by SAE) still require a human to fulfil specific remaining (driving related) tasks while

being in automated driving mode. The basic requirements with respect to the driver strongly depend on

the level of automation and are subject to human factors research all over the world. The SAE standards

[70] [71] [72]

SAE J3016 and SAE J3114 have already introduced working definitions of key concepts in this

field. This document puts an emphasis on common underlying concepts of driver performance and state

in the context of automated driving.

Driver performance includes driver’s activities in transitions both from manual driving to automated

driving and from automated to manual driving, as well as interaction behaviour while using the system.

Driver state here means driver’s internal conditions that may affect performance including knowledge

of and attitudes toward driving automation systems.

Concepts on driver performances in transition from manual to automated driving and from automated

to manual driving are described in Clause 5. Concepts on driver state related to the transition are

described in Clause 6 and a specific concept “readiness/availability” that refers to driver state that

predicts the intervention performance is described in Clause 7. Concepts for driver’s experiences

and attitudes that may affect driver performance and state in the context of automated driving are

described in Clause 8.
© ISO 2020 – All rights reserved v
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TECHNICAL REPORT ISO/TR 21959-1:2020(E)
Road vehicles — Human performance and state in the
context of automated driving —
Part 1:
Common underlying concepts
1 Scope

This document introduces basic common underlying concepts related to driver performance and

state in the context of automated driving. The concepts in this document are applicable to all levels of

automated driving functions that require a human/driver to be engaged or fallback-ready (SAE level 1,

2 and 3). It can also be used with levels that enable a driver to resume manual control of the vehicle (a

compatible feature for SAE levels 1 to 5).

Common underlying concepts can be applicable for human factors assessment/evaluations using driving

simulators, tests on restricted roadways (e.g. test tracks) or tests on public roads. The information

applies to all vehicle categories.

This document contains a mixture of information where technical consensus supports such guidance,

as well as discussion of those areas where further research is required to support technical consensus.

These common underlying concepts can be also useful for product descriptions and owner manuals.

The contents in this document are informative, rather than normative, in nature.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.

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

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4 Purpose

The purpose of this document is to provide common underlying concepts for human performance and

state for the researchers and developers of driving automation systems (more specifically SAE levels

1–5) in order to facilitate the sharing of information and knowledge as these systems are developed

and deployed.

This document does not provide design principles on how a human-machine interface (HMI) for

automated driving should be designed or developed. However, common concepts and measures could

be used during the development phase when different HMI designs are evaluated in terms of usability,

user experience and safety.

It is not intended that anything in this document restricts or provides direction regarding the

technology used to create these systems, or the underlying design of these system.

© ISO 2020 – All rights reserved 1
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ISO/TR 21959-1:2020(E)
5 Human performance in the context of automated driving
5.1 General

Human performance has two aspects—behaviour being the means and its consequence being the

[16]

end . The focus on consequences, and hence on performance, is especially relevant for situations

such as the transition processes from automated to manual control (level 0) and vice versa (see

Figures 1 to 4). The following subclauses give an overview of possible measures for driver- and system-

initiated transitions. For transitions between different automation levels (e.g. 4→2 or 3→1) within one

vehicle appropriate measures can be selected or adapted according to the specific circumstances.

5.2 Transition from manual to automated driving
5.2.1 Transition process model

Figure 1 shows a process model for a prototypical transition from manual to automated control, either

initiated by the driver or by the system.

EXAMPLE After entering the highway the driver is informed about the availability of a “highway pilot

function” . He/she decides to activate automation by a dedicated steering wheel button.

1) See: https:// www .daimler .com/ innovation/ case/ autonomous/ highway -pilot -2 .html, Hunger 2017. Highway

pilot system is an example of a suitable product available commercially. This information is given for the convenience

of users of this document and does not constitute an endorsement by ISO of this product.

2 © ISO 2020 – All rights reserved
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ISO/TR 21959-1:2020(E)
Figure 1 — Driver/system-initiated transition from manual to automated driving
(concepts are further specified in 5.2.2 and 5.2.3)
5.2.2 Definition of related concepts

a) Manual control: Driving phase where a human driver is performing the dynamic driving task

(DDT)—all of the real-time operational and tactical functions required to operate a vehicle in on-

road traffic (see Reference [70] for the definition of level 0 automation). In cases where lower level

automation features are already active, this phase can be regarded as including the remaining

(manual) elements of the DDT required by the driving automation system. For example, driving

with adaptive cruise control requires the driver to perform the lateral control (sub) task as well as

the object and event detection and response (OEDR) subtask.

b) Automation available: If all operational conditions for a driving automation system are fulfilled

the system is ready to be activated by either the user or the system. This system availability may

be signalled to the user via the driver vehicle interface (e.g. screen, tones). However, even if an

automation feature is available, the driver may have to judge whether activation is appropriate

[taking into account the mechanical condition of the vehicle (not detected by the vehicle, e.g. broken

suspension component)] This will be covered in a future planned document (ISO/SAE 22736).

c) Request to engage automation: Event usually initiated by the user through the driver-vehicle

interface of the vehicle to activate the driving automation system. Apart from user-initiated

transitions, system-initiated transitions from manual to automated control may also be possible,

especially after the driver has temporarily overridden the automated mode by manual intervention.

At the end of driver intervention, the system may automatically activate/resume from suspended

to active mode. For example, in some automated steering control systems, after the driver has

transitioned from automated to manual control by manual use of the steering wheel, when the

driver is no longer moving the steering wheel, the system may automatically activate/resume from

manual to automated steering control.

d) Activation of automation: Onset of the driving automation system activation. There may be a

delay between requesting the activation and the activation itself either due to technical reasons or

by intentionally introducing an activation process as an HMI design feature.
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ISO/TR 21959-1:2020(E)

e) Driver state transition (manual to automated): Process where the driver is releasing control to

the driving automation system. The transition includes physical aspects (releasing hands and feet

from primary vehicle controls) as well as cognitive aspects (ensuring that automation has taken

over successfully). The physical transition phase ends when the driver fully releases manual

vehicle control (hands and feet do not have any action on longitudinal or lateral vehicle control).

Behavioural markers for the end of the cognitive transition are less obvious.

f) Automated driving: Driving phase where a level 1 – level 5 (L1 – L5) system is performing specific

aspects of the DDT.

g) Acceptable driver state by automation level: Driver state that is required or activity that is

allowed by the driving automation system. The driver state may or may not be monitored by the

driving automation system. Requirements on acceptable driver states are strongly dependent on

the automation level. Sleep is commonly seen as not acceptable by L2/L3 features or physically

leaving the driver’s seat is not acceptable for L2/L3 features.

h) Non-Driving Related Activity (NDRA): Any activity not related to the monitoring of the driving

automation system and/or the current driving situation is called non-driving related activity. This

can include activities that take up any of visual, auditory, visual-manual, auditory-manual, manual,

or cognitive capabilities.

i) Non-Driving Related Task (NDRT): Any activity related to a dedicated task that is different from

the monitoring of the driving automation system and/or the current driving situation is called non-

driving related task. An activity becomes a task when it has a specific goal, and the task can be

made up of a series of activities leading up to this goal. A NDRT can also be called secondary task,

but only as long as there is a primary task, in this case operating the vehicle. When driving is no

longer the driver’s primary task—such as during automated driving at SAE levels 3 and higher—

the NDRT stops being a secondary task. Under such circumstances the NDRT itself can be regarded

as the primary task.
5.2.3 Measures for human performance in releasing control to automation

a) Time to activate system: It is the time interval between events “automation available” and

“request to engage automation”.

b) Time to release controls: It is the time interval between events “activation of automation” and

“full release of vehicle control”.

c) Time to start/resume NDRA: It is the time interval between events “activation of automation”

and “start of NDRA”.

d) Method used to engage driving automation system: It is the specification of required driver

action to fully release control to driving automation system (e.g. double-pull of stalk at steering

column or simultaneous activation of dedicated steering wheel controls).
5.3 Transition from automated to manual driving

Transitions from automated to manual driving, may have two different “sources”. They may be system

initiated or they may be driver initiated as is presented in the subclauses below.

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ISO/TR 21959-1:2020(E)
5.3.1 Transition process models

Figure 2 shows the process model for a system-initiated transition from automated to manual vehicle

control with definitions of relevant time periods. This transition model assumes the result of a fully

stabilised vehicle.

EXAMPLE 1 While using a highway pilot system the function issues a request to intervene (RtI) due to an

internal system error. After preparing for taking over manual vehicle control the driver deactivates the highway

pilot function and switches to manual driving mode.
Figure 2 — System-initiated transition from automated to manual driving
(concepts are further specified in 5.3.2 and 5.3.3)

In addition to system-initiated transitions, user-initiated transitions without a RtI are covered, as

level 1 to level 3, and some level 4 or level 5 systems may be designed to be deactivated by the user

at any point in time during full operation. There are two types of reasons for a user to deactivate the

automation feature which are described below.

Figure 3 describes the process of regaining manual vehicle control due to the detection of system

performance limitations (mandatory transition). In this case the L1/L2 driving automation system

does not issue a RtI to the driver.

EXAMPLE 2 While using a L2 automation system in a construction zone the driver observes that the system

is following invalid lane markings. He/she decides to immediately take-over control by manually overriding the

lateral steering control (leading to manual driving mode).

2) See: https:// www .daimler .com/ innovation/ case/ autonomous/ highway -pilot -2 .html, Hunger 2017. Highway

pilot system is an example of a suitable product available commercially. This information is given for the convenience

of users of this document and does not constitute an endorsement by ISO of this product.

© ISO 2020 – All rights reserved 5
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ISO/TR 21959-1:2020(E)

Figure 3 — Mandatory human-initiated transition of automated to manual driving due to

detection of system performance limits (concepts are further specified in 5.3.2 and 5.3.3)

On the other hand, the driver may want to deactivate the driving automation system without detecting

system performance limitations (optional limitations). For this case the transition process described

above can be slightly adapted (see Figure 4).

EXAMPLE 3 While using a traffic jam pilot feature in heavy traffic on a city freeway, the driver deactivates all

driving automation features using a designated control for that purpose and switches to manual driving in order

to exit the freeway and find a faster route.
Figure 4 — Optional human-initiated transition of automated to manual driving

(without system performance limit; concepts are further specified in 5.3.2 and 5.3.3)

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ISO/TR 21959-1:2020(E)
5.3.2 Definition of related concepts

a) Request to intervene (RtI): Notification by a driving automation system to a driver/fallback-

ready user indicating that he/she should take over vehicle control (perform all or parts of the DDT).

According to the requirements of SAE level 3+ systems, all system related limits are recognized

and appropriate driver requests to take-over control are triggered. The RtI time stamp is essential

to analyse the subsequent transition process as it is regarded as the beginning of the driver state

transition process. If a request to intervene is not issued by the driving automation system because

of an automation-external vehicle failure (for example, flat tyre, broken steering system and so on),

the (perceivable) failure signal can alternatively be used as a corresponding event. Drivers of level 2

automation may encounter (silent) system failures or performance limits which are not recognized

and communicated by the driving automation system. In this case the driver may notice suspicious

vehicle control due to a potential system limit which triggers the decision to regain manual control.

b) Silent system failure and/or silent system limit: System performance limitation of a driving

automation system that is not recognized and communicated to the user. The performance limitation

may be due to internal failure states or incorrect interpretation of the driving environment.

c) Critical event due to system limit: Situation that can be specified in time and space that the

driving automation system cannot handle safely and that will occur in case the driver does not

intervene. System limits are associated with different time budgets:

— Long-term system limits (e.g. the planned transition of a highway pilot when reaching the exit)

can be communicated well in advance to allow for a sufficient preparation. Driver performances

with respect to early communication of long-term system limits are not within primary interest

of this document.

— Mid-term system limits (e.g. approaching a construction site that cannot be handled by the

system) require the driver/fallback-ready user to take-over within a certain time budget. This

type of system limit is central to the definition of SAE level 3 systems.

— Short-term limits (e.g. due to sensor range) ask the driver for immediate take-over of the DDT.

This type of system limit is central to the definition of SAE level 2 systems.

Although the majority of system limits will be detected and communicated via a RtI for higher

automation levels, there may be system limits for level 2 systems without a RtI (see Figure 4).

d) Take-over mode: The concept refers to the system behaviour after a RtI has been issued. The

remaining performance depends on the automation level and on the design of a particular system.

Level 3-5 systems can be able to continue operation after issuing a RtI for a specified period of

time. Depending on the type of system limit, system operation might be of degraded nature. Level

2 systems may be deactivated immediately after a RtI has been issued. The take-over mode may

already contain measures to reach a minimal risk condition (such as a stopping manoeuvre).

However, details on human performance aspects with respect to a minimal risk manoeuvre will be

[51]
covered in a future, planned document (ISO/TR 21959-2 ).

e) Driver state transition (automated to manual): Process of transforming the actual driver state

(possibly determined by NDRA) to a target driver state suitable to effectively take-over manual

control. This process can be analysed on a sensory, motoric and cognitive level. Relevant time

markers within this phase are: interrupting/finishing a non-driving related task, start of visual

reorientation, gaze on road centre, hands on wheel/feet on pedals, etc.

f) Significant driver intervention: Action initiated by the user of a driving automation system to

request manual control over all or some parts of the DDT. The way manual control can be resumed

by the user depends on the driving automation system design, but usually comprises significant

driver intervention on primary vehicle controls and regular deactivation mechanisms (e.g. based

on buttons or switches). The system reaction towards the driver intervention also depends on

the particular design of the automation function. For higher-level automated driving systems

(ADS) which remain active after a RtI, “significant driver intervention” corresponds to requesting

system deactivation. The subsequent change of the system status may be immediate or delayed

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ISO/TR 21959-1:2020(E)

(e.g. in order to establish safe conditions for manual driving). SAE level 2 functions may deactivate

vehicle control of some or all aspects of the DDT at the time of issuing the RtI. In this case driver

intervention takes place in manual mode and typically corresponds to significant and relevant

actions on primary vehicle controls.

g) Post transition (manual) control: A defined, extended time window to analyse the quality

of manual control driving after a RtI has been issued. The post transition driving phase can be

decomposed in the driver intervention itself and a control stabilisation phase.

h) Completion of driving manoeuvre: The kind of take-over action that is expected by the driver to

successfully handle the system limit. The intervention depends on the demands of the take-over

situation. Examples range from easy tasks such as bringing hands back
...

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