ISO/SAE PAS 22736:2021
(Main)Taxonomy and definitions for terms related to driving automation systems for on-road motor vehicles
Taxonomy and definitions for terms related to driving automation systems for on-road motor vehicles
This document describes [motor] vehicle driving automation systems that perform part or all of the dynamic driving task (DDT) on a sustained basis. It provides a taxonomy with detailed definitions for six levels of driving automation, ranging from no driving automation (Level 0) to full driving automation (Level 5), in the context of [motor] vehicles (hereafter also referred to as “vehicle” or “vehicles”) and their operation on roadways: Level 0: No Driving Automation Level 1: Driver Assistance Level 2: Partial Driving Automation Level 3: Conditional Driving Automation Level 4: High Driving Automation Level 5: Full Driving Automation These level definitions, along with additional supporting terms and definitions provided herein, can be used to describe the full range of driving automation features equipped on [motor] vehicles in a functionally consistent and coherent manner. “On‑road” refers to publicly accessible roadways (including parking areas and private campuses that permit public access) that collectively serve all road users, including cyclists, pedestrians, and users of vehicles with and without driving automation features. The levels apply to the driving automation feature(s) that are engaged in any given instance of on-road operation of an equipped vehicle. As such, although a given vehicle may be equipped with a driving automation system that is capable of delivering multiple driving automation features that perform at different levels, the level of driving automation exhibited in any given instance is determined by the feature(s) that are engaged. This document also refers to three primary actors in driving: the (human) user, the driving automation system, and other vehicle systems and components. These other vehicle systems and components (or the vehicle in general terms) do not include the driving automation system in this model, even though as a practical matter a driving automation system may actually share hardware and software components with other vehicle systems, such as a processing module(s) or operating code. The levels of driving automation are defined by reference to the specific role played by each of the three primary actors in performance of the DDT and/or DDT fallback. “Role” in this context refers to the expected role of a given primary actor, based on the design of the driving automation system in question and not necessarily to the actual performance of a given primary actor. For example, a driver who fails to monitor the roadway during engagement of a Level 1 adaptive cruise control (ACC) system still has the role of driver, even while s/he is neglecting it. Active safety systems, such as electronic stability control (ESC) and automatic emergency braking (AEB), and certain types of driver assistance systems, such as lane keeping assistance (LKA), are excluded from the scope of this driving automation taxonomy because they do not perform part or all of the DDT on a sustained basis, but rather provide momentary intervention during potentially hazardous situations. Due to the momentary nature of the actions of active safety systems, their intervention does not change or eliminate the role of the driver in performing part or all of the DDT, and thus are not considered to be driving automation, even though they perform automated functions. In addition, systems that inform, alert, or warn the driver about hazards in the driving environment are also outside the scope of this driving automation taxonomy, as they neither automate part or all of the DDT, nor change the driver’s role in performance of the DDT (see 8.13). It should be noted, however, that crash avoidance features, including intervention-type active safety systems, may be included in vehicles equipped with driving automation systems at any level. For automated driving system (ADS) features (i.e., Levels 3 to 5) that perform the complete DDT, crash mitigation and avoidance capability is part of ADS functionality (see also 8.13).
Taxonomie et définitions des termes relatifs aux systèmes de conduite automatisée des véhicules routiers à moteur
General Information
Standards Content (Sample)
PUBLICLY ISO/SAE
AVAILABLE PAS
SPECIFICATION 22736
First edition
2021-08
Taxonomy and definitions for terms
related to driving automation systems
for on-road motor vehicles
Taxonomie et définitions des termes relatifs aux systèmes de conduite
automatisée des véhicules routiers à moteur
Reference number
©
ISO/SAE International 2021
© ISO/SAE International 2021
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Foreword
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© ISO/SAE International 2021 – All rights reserved iii
TABLE OF CONTENTS
1. SCOPE . 1
2. REFERENCES . 2
2.1 Applicable Documents . 2
2.1.1 SAE Publications . 2
2.1.2 ANSI Accredited Publications . 2
2.1.3 Other Publications . 2
2.2 List of Abbreviations . 2
3. DEFINITIONS . 3
4. TAXONOMY OF DRIVING AUTOMATION . 22
5. LEVELS OR CATEGORIES OF DRIVING AUTOMATION . 28
5.1 Level or Category 0 - No Driving Automation . 28
5.2 Level or Category 1 - Driver Assistance . 28
5.3 Level or Category 2 - Partial Driving Automation . 29
5.4 Level or Category 3 - Conditional Driving Automation. 29
5.5 Level or Category 4 - High Driving Automation . 29
5.6 Level or Category 5 - Full Driving Automation . 30
6. SIGNIFICANCE OF OPERATIONAL DESIGN DOMAIN (ODD) . 30
7. DEPRECATED TERMS . 32
7.1 Autonomous, Driving Modes(s), Self-Driving, Unmanned, Robotic . 33
7.1.1 Autonomous . 33
7.1.2 Driving Mode(s). 33
7.1.3 Self-Driving . 33
7.1.4 Unmanned . 33
7.1.5 Robotic . 33
7.2 Automated or Autonomous Vehicle . 34
7.3 Control . 34
8. ADDITIONAL DISCUSSION . 34
9. NOTES . 40
Figure 1 Examples of driving automation system features/types that could be available during a given
trip . 6
Figure 2 Schematic (not a control diagram) view of driving task showing DDT portion . 7
Figure 3 . 9
Figure 4 . 9
Figure 5 . 10
Figure 6 . 10
Figure 7 . 10
Figure 8 . 11
Figure 9 Diagram showing vehicle axes of motion (SAE J670) . 12
Figure 10 Simplified logic flow diagram for assigning driving automation level to a feature . 25
Figure 11 ODD relative to driving automation levels . 32
Figure 12 ODD relative to driving automation levels . 32
Figure 13 Use case sequence for a Level 3 feature showing ADS engaged, occurrence of a failure or
out-of-ODD condition, and the fallback-ready user performing the fallback, or, if the fallback-
ready user fails to do so, a failure mitigation strategy, such as stop-in-lane . 37
Figure 14 Use case sequence at Level 4 showing ADS engaged, a catastrophic event (e.g., complete
power failure) and the system achieving a minimal risk condition . 37
Table 1 Summary of levels of driving automation . 23
Table 2 Roles of human user and driving automation system by level of driving automation . 26
Table 3 User roles while a driving automation system is engaged . 28
iv © ISO/SAE International 2021 – All rights reserved
PUBLICLY AVAILABLE SPECIFICATION ISO/SAE PAS 22736:2021(E)
Taxonomy and definitions for terms related to driving
automation systems for on-road motor vehicles
1. SCOPE
This document describes [motor] vehicle driving automation systems that perform part or all of the dynamic
driving task (DDT) on a sustained basis. It provides a taxonomy with detailed definitions for six levels of driving
automation, ranging from no driving automation (Level 0) to full driving automation (Level 5), in the context of
[motor] vehicles (hereafter also referred to as “vehicle” or “vehicles”) and their operation on roadways:
Level 0: No Driving Automation
Level 1: Driver Assistance
Level 2: Partial Driving Automation
Level 3: Conditional Driving Automation
Level 4: High Driving Automation
Level 5: Full Driving Automation
These level definitions, along with additional supporting terms and definitions provided herein, can be used to
describe the full range of driving automation features equipped on [motor] vehicles in a functionally consistent
and coherent manner. “On-road” refers to publicly accessible roadways (including parking areas and private
campuses that permit public access) that collectively serve all road users, including cyclists, pedestrians, and
users of vehicles with and without driving automation features.
The levels apply to the driving automation feature(s) that are engaged in any given instance of on-road operation
of an equipped vehicle. As such, although a given vehicle may be equipped with a driving automation system
that is capable of delivering multiple driving automation features that perform at different levels, the level of
driving automation exhibited in any given instance is determined by the feature(s) that are engaged.
This document also refers to three primary actors in driving: the (human) user, the driving automation system,
and other vehicle systems and components. These other vehicle systems and components (or the vehicle in
general terms) do not include the driving automation system in this model, even though as a practical matter a
driving automation system may actually share hardware and software components with other vehicle systems,
such as a processing module(s) or operating code.
The levels of driving automation are defined by reference to the specific role played by each of the three primary
actors in performance of the DDT and/or DDT fallback. “Role” in this context refers to the expected role of a
given primary actor, based on the design of the driving automation system in question and not necessarily to
the actual performance of a given primary actor. For example, a driver who fails to monitor the roadway during
engagement of a Level 1 adaptive cruise control (ACC) system still has the role of driver, even while s/he is
neglecting it.
Active safety systems, such as electronic stability control (ESC) and automatic emergency braking (AEB), and
certain types of driver assistance systems, such as lane keeping assistance (LKA), are excluded from the scope
of this driving automation taxonomy because they do not perform part or all of the DDT on a sustained basis,
but rather provide momentary intervention during potentially hazardous situations. Due to the momentary nature
of the actions of active safety systems, their intervention does not change or eliminate the role of the driver in
performing part or all of the DDT, and thus are not considered to be driving automation, even though they
perform automated functions. In addition, systems that inform, alert, or warn the driver about hazards in the
driving environment are also outside the scope of this driving automation taxonomy, as they neither automate
part or all of the DDT, nor change the driver’s role in performance of the DDT (see 8.13).
© ISO/SAE International 2021 – All rights reserved 1
It should be noted, however, that crash avoidance features, including intervention-type active safety systems,
may be included in vehicles equipped with driving automation systems at any level. For automated driving
system (ADS) features (i.e., Levels 3 to 5) that perform the complete DDT, crash mitigation and avoidance
capability is part of ADS functionality (see also 8.13).
2. REFERENCES
2.1 Applicable Documents
The following publications form a part of this specification to the extent specified herein. Unless otherwise
indicated, the latest issue of SAE publications shall apply.
2.1.1 SAE Publications
Available from SAE International, 400 Commonwealth Drive, Warrendale, PA 15096-0001, Tel: 877-606-7323
(inside USA and Canada) or +1 724-776-4970 (outside USA), www.sae.org.
SAE J670 Vehicle Dynamics Terminology
SAE J3063 Active Safety Systems Terms and Definitions
Shi, E., Gasser, T., Seeck, A., and Auerswald, R., “The Principles of Operation Framework: A Comprehensive
Classification Concept for Automated Driving Functions,” SAE Intl. J CAV 3(1):27-37, 2020,
https://doi.org/10.4271/12-03-01-0003.
2.1.2 ANSI Accredited Publications
Copies of these documents are available online at http://webstore.ansi.org/.
ANSI D16.1-2007 Manual on Classification of Motor Vehicle Traffic Accidents
2.1.3 Other Publications
49 U.S.C. § 30102(a)(6) (definition of [motor] vehicle)
Crash Avoidance Metrics Partnership - Automated Vehicle Research Consortium, “Automated Vehicle
Research for Enhanced Safety - Final Report,” available at https://www.regulations.gov/document?D=NHTSA-
2014-0070-0003.
Gasser, T. et al., “Legal Consequences of an Increase in Vehicle Automation,” July 23, 2013, available at
http://bast.opus.hbz-
nrw.de/volltexte/2013/723/pdf/Legal_consequences_of_an_increase_in_vehicle_automation.pdf.
Michon, J.A., 1985, “A Critical View of Driver Behavior Models: What Do We Know, What Should We Do?” In
Evans, L. and Schwing, R.C. (Eds.). Human behavior and traffic safety (pp. 485-520). New York: Plenum Press,
1985.
Smith, B.W., “Engineers and Lawyers Should Speak the Same Robot Language,” in ROBOT LAW (2015),
available at https://newlypossible.org.
2.2 List of Abbreviations
ACC Adaptive cruise control
ADAS Advanced driver assistance system
ADS Automated driving system
ADS-DV Automated driving system-dedicated vehicle
2 © ISO/SAE International 2021 – All rights reserved
AEB Automatic emergency braking
DDT Dynamic driving task
DSRC Dedicated short range communications
ESC Electronic stability control
LKA Lane keeping assistance
ODD Operational design domain
OEDR Object and event detection and response
3. DEFINITIONS
3.1 ACTIVE SAFETY SYSTEM (SAE J3063)
Active safety systems are vehicle systems that sense and monitor conditions inside and outside the vehicle for
the purpose of identifying perceived present and potential dangers to the vehicle, occupants, and/or other road
users, and automatically intervene to help avoid or mitigate potential collisions via various methods, including
alerts to the driver, vehicle system adjustments, and/or active control of the vehicle subsystems (brakes, throttle,
suspension, etc.).
NOTE: For purposes of this report, systems that meet the definition of active safety systems are considered to
have a design purpose that is primarily focused on improving safety rather than comfort, convenience,
or general driver assistance. Active safety systems warn or intervene during a high-risk event or
maneuver.
3.2 AUTOMATED DRIVING SYSTEM (ADS)
The hardware and software that are collectively capable of performing the entire DDT on a sustained basis,
regardless of whether it is limited to a specific operational design domain (ODD); this term is used specifically
to describe a Level 3, 4, or 5 driving automation system.
NOTE: In contrast to ADS, the generic term “driving automation system” (see 3.6) refers to any Level 1 to 5
system or feature that performs part or all of the DDT on a sustained basis. Given the similarity between
the generic term, “driving automation system,” and the Level 3 to 5 specific term, “automated driving
system,” the latter term should be capitalized when spelled out and reduced to its abbreviation, ADS,
as much as possible, while the former term should not be.
3.3 [DRIVERLESS OPERATION] DISPATCHING ENTITY
An entity that dispatches an ADS-equipped vehicle(s) in driverless operation.
NOTE: The functions carried out by a dispatching entity may be divided among one or several agents,
depending on the usage specification for the ADS-equipped vehicle(s) in question.
EXAMPLE: A fleet of Level 4 closed campus ADS-dedicated vehicles is placed into service by a driverless
operation dispatching entity, which engages the ADS for each vehicle after verifying its operational
readiness and disengages the ADS when each vehicle is taken out of service.
3.4 DISPATCH [IN DRIVERLESS OPERATION]
To place an ADS-equipped vehicle into service in driverless operation by engaging the ADS.
NOTE 1: The term “dispatch,” as used outside of the context of ADS-equipped vehicles, is generally understood
to mean sending a particular vehicle to a particular pick-up or drop-off location for purposes of
providing a transportation service. In the context of ADS-equipped vehicles, and as used herein, this
© ISO/SAE International 2021 – All rights reserved 3
term includes software-enabled dispatch of multiple ADS-equipped vehicles in driverless operation
that may complete multiple trips involving pick-up and drop-off of passengers or goods throughout a
day or other pre-defined period of service, and which may involve multiple agents performing various
tasks related to the dispatch function. In order to highlight this specialized use of the term dispatch,
the term is modified and conditioned by the stipulation that it refers exclusively to dispatching vehicles
in driverless operation.
NOTE 2: Only ADS-equipped vehicles capable of driverless operation (namely, an ADS-DV or a dual-mode
vehicle) are potentially subject to being dispatched.
3.5 DRIVING AUTOMATION
The performance by hardware/software systems of part or all of the DDT on a sustained basis.
3.6 DRIVING AUTOMATION SYSTEM OR TECHNOLOGY
The hardware and software that are collectively capable of performing part or all of the DDT on a sustained
basis; this term is used generically to describe any system capable of Level 1 to 5 driving automation.
NOTE: In contrast to this generic term for any Level 1 to 5 system, the specific term for a Level 3 to 5 system
is “automated driving system (ADS).” Given the similarity between the generic term, “driving automation
system,” and the Level 3 to 5 specific term, “Automated Driving System,” the latter term should be
capitalized when spelled out and reduced to its abbreviation, ADS, as much as possible, while the
former term should not be (see 3.2).
3.7 [DRIVING AUTOMATION SYSTEM] FEATURE
A Level 1-5 driving automation system’s design-specific functionality at a given level of driving automation within
a particular ODD, if applicable.
NOTE 1: Because the term “driving automation system” subsumes both driver support features and ADS
features, it is also acceptable to refer to them as such.
NOTE 2: A given driving automation system may have multiple features, each associated with a particular level
of driving automation and ODD.
NOTE 3: Each feature satisfies a usage specification.
NOTE 4: Features may be referred to by generic names (e.g., automated parking) or by proprietary names.
EXAMPLE 1: A Level 3 ADS feature that performs the DDT, excluding DDT fallback, in high-volume traffic on
fully access-controlled freeways.
EXAMPLE 2: A Level 4 ADS feature that performs the DDT, including DDT fallback, in a specified geo-fenced
urban center.
4 © ISO/SAE International 2021 – All rights reserved
3.7.1 MANEUVER-BASED FEATURE
A driving automation system feature equipped on a conventional vehicle that either:
1. Supports the driver by executing a limited set of lateral and/or longitudinal vehicle motion control actions
sufficient to fulfil a specific, narrowly defined use case (e.g., parking maneuver), while the driver performs
the rest of the DDT and supervises the Level 1 or Level 2 feature’s performance (i.e., Level 1 or Level 2
driver support features);
or
2. Executes a limited set of lateral and longitudinal vehicle motion control actions, as well as associated object
and event detection and response (OEDR) and all other elements of the complete DDT in order to fulfil a
specific, narrowly defined use case without human supervision (Level 3 or 4 ADS features).
EXAMPLE 1: A Level 1 parking assistance feature automatically performs the lateral vehicle motion control
actions necessary to parallel park a vehicle, while the driver performs the longitudinal vehicle
motion control actions and supervises the feature.
EXAMPLE 2: A Level 2 parking assistance feature automatically performs the lateral and longitudinal vehicle
motion control actions necessary to parallel park a vehicle under the supervision of the driver.
EXAMPLE 3: A Level 3 highway overtaking assistance feature automatically performs the lateral and
longitudinal vehicle motion control actions, as well as associated OEDR, necessary to pass a
slower-moving vehicle on a multi-lane highway when activated by the driver or fallback-ready
user.
3.7.2 SUB-TRIP FEATURE
A driving automation system feature equipped on a conventional vehicle that requires a human driver to perform
the complete DDT for at least part of every trip.
NOTE: Sub-trip features require a human driver to operate the vehicle between the point-of-origin and the
boundary of the feature’s ODD and/or after leaving the feature’s ODD until the destination is reached
(i.e., trip completion).
EXAMPLE 1: A Level 1 adaptive cruise control (ACC) feature performs longitudinal vehicle motion control
functions to support the driver in maintaining consistent headway to a lead vehicle in its lane when
travelling at higher speeds.
EXAMPLE 2: A Level 2 highway feature performs lateral and longitudinal vehicle motion control functions to
support the driver in maintaining position within its lane of travel, as well as consistent headway
to a lead vehicle in its lane when travelling at higher speeds.
EXAMPLE 3: A Level 3 traffic jam feature performs the complete DDT on a fully access-controlled freeway in
dense traffic, but requires a human driver to operate the vehicle upon ODD exit (e.g., when traffic
clears, as well as before entering the congested freeway, and again upon exiting it).
EXAMPLE 4: During a given vehicle trip, a user with a Level 4 automated parking feature dispatches the vehicle
in driverless operation for the purpose of finding a parking space in a nearby designated parking
facility. Following a period of shopping, the user retrieves the vehicle via dispatch in order to begin
his/her trip home.
3.7.3 FULL-TRIP FEATURE
ADS features that operate a vehicle throughout complete trips.
EXAMPLE 1: A Level 4 ADS-DV is dispatched in driverless operation for purposes of providing ride-hailing
services to customers located within its geo-fenced area of operation.
© ISO/SAE International 2021 – All rights reserved 5
EXAMPLE 2: A Level 5 dual-mode vehicle is dispatched in driverless operation by its owner to go to a
designated airport, pick up several family members, and bring them home. All vehicle occupants
remain passengers throughout the return trip.
Figure 1 illustrates how a trip could be completed by use of various combinations of driving automation features
engaged at different levels of driving automation.
Figure 1 — Examples of driving automation system features/types that could be available during a
given trip
3.8 DRIVER SUPPORT [DRIVING AUTOMATION SYSTEM] FEATURE
A general term for Level 1 and Level 2 driving automation system features.
NOTE: Level 1 (driver assistance) and Level 2 (partial automation) features are capable of performing only part
of the DDT, and thus require a driver to perform the remainder of the DDT, as well as to supervise the
feature’s performance while engaged. As such, these features, when engaged, support—but do not
replace—a driver in performing the DDT.
3.9 DRIVERLESS OPERATION [OF AN ADS-EQUIPPED VEHICLE]
On-road operation of an ADS-equipped vehicle that is unoccupied, or in which on-board users are not drivers
or in-vehicle fallback-ready users.
NOTE 1: ADS-DVs are always dispatched in driverless operation (subject to NOTE 3 in 3.33.3).
NOTE 2: ADS-equipped dual-mode vehicles may be dispatched in driverless operation.
NOTE 3: On-board passengers are neither drivers nor fallback-ready users.
EXAMPLE: A Level 4 ADS-DV is dispatched in driverless operation for purposes of providing transportation
service.
3.10 DYNAMIC DRIVING TASK (DDT)
All of the real-time operational and tactical functions required to operate a vehicle in on-road traffic, excluding
the strategic functions such as trip scheduling and selection of destinations and waypoints, and including,
without limitation, the following subtasks:
6 © ISO/SAE International 2021 – All rights reserved
1. Lateral vehicle motion control via steering (operational).
2. Longitudinal vehicle motion control via acceleration and deceleration (operational).
3. Monitoring the driving environment via object and event detection, recognition, classification, and response
preparation (operational and tactical).
4. Object and event response execution (operational and tactical).
5. Maneuver planning (tactical).
6. Enhancing conspicuity via lighting, sounding the horn, signaling, gesturing, etc. (tactical).
NOTE 1: Some driving automation systems (or the vehicles equipped with them) may have a means to change
longitudinal vehicle motion control between forward and reverse.
NOTE 2: For simplification and to provide a useful shorthand term, subtasks (3) and (4) are referred to
collectively as object and event detection and response (OEDR) (see 3.19).
NOTE 3: In this document, reference is made to “complete(ing) the DDT.” This means fully performing all of
the subtasks of the DDT, whether that role is fulfilled by the (human) driver, by the driving automation
system, or by a combination of both.
NOTE 4: Figure 2 displays a schematic view of the driving task. For more information on the differences
between operational, tactical, and strategic functions of driving, see 8.11.
Figure 2 — Schematic (not a control diagram) view of driving task showing DDT portion
For purposes of DDT performance, Level 1 driving automation encompasses automation of part of the innermost
loop (i.e., either lateral vehicle motion control functionality or longitudinal vehicle motion control functionality and
limited OEDR associated with the given axis of vehicle motion control); Level 2 driving automation encompasses
automation of the innermost loop (lateral and longitudinal vehicle motion control and limited OEDR associated
with vehicle motion control), and Level 3 to 5 driving automation encompasses automation of both inner loops
(lateral and longitudinal vehicle motion control and complete OEDR). Note that DDT performance does not
include strategic aspects of driving (e.g., determining whether, when, and where to travel).
© ISO/SAE International 2021 – All rights reserved 7
3.11 FAILURE MITIGATION STRATEGY
A vehicle function (not an ADS function) designed to automatically bring an ADS-equipped vehicle to a controlled
stop in path following either: (1) prolonged failure of the fallback-ready user of a Level 3 ADS feature to perform
the fallback after the ADS has issued a request to intervene, or (2) occurrence of a system failure or external
event so catastrophic that it incapacitates the ADS, which can no longer perform vehicle motion control in order
to perform the fallback and achieve a minimal risk condition. (See 8.6.)
NOTE: Some vehicles equipped with Level 2 driver support features may be designed to brake a vehicle to a
full stop if the driver fails to indicate his/her continued supervision of feature performance during
engagement. Although that is similar in function to a failure mitigation strategy as defined above, the
term “failure mitigation strategy” is reserved for ADS features that do not require driver supervision.
3.12 [DYNAMIC DRIVING TASK (DDT)] FALLBACK
The response by the user to either perform the DDT or achieve a minimal risk condition (1) after occurrence of
a DDT performance-relevant system failure(s), or (2) upon operational design domain (ODD) exit, or the
response by an ADS to achieve minimal risk condition, given the same circumstances.
NOTE 1: The DDT and the DDT fallback are distinct functions, and the capability to perform one does not
necessarily entail the ability to perform the other. Thus, a Level 3 ADS, which is capable of performing
the entire DDT within its ODD, may not be capable of performing the DDT fallback in all situations
that require it and thus will issue a request to intervene to the DDT fallback-ready user when
necessary (see Figures 3 to 6).
NOTE 2: Some Level 3 features may be designed to automatically perform the fallback and achieve a minimal
risk condition in some circumstances, such as when an obstacle-free, adjacent shoulder is present,
but not in others, such as when no such road shoulder is available. The assignment of Level 3
therefore does not restrict the ADS from automatically achieving the minimal risk condition, but it
cannot guarantee automated achievement of minimal risk condition in all cases within its ODD.
Moreover, automated minimal risk condition achievement in some, but not all, circumstances that
demand it does not constitute Level 4 functionality.
NOTE 3: At Level 3, an ADS is capable of continuing to perform the DDT for at least several seconds after
providing the fallback-ready user with a request to intervene. The DDT fallback-ready user is then
expected to resume manual vehicle operation, or to achieve a minimal risk condition if s/he
determines it to be necessary.
NOTE 4: At Levels 4 and 5, the ADS must be capable of performing the DDT fallback and achieving a minimal
risk condition. Level 4 and 5 ADS-equipped vehicles that are designed to also accommodate
operation by a driver (whether in-vehicle or remote) may allow a user to perform the DDT fallback,
when circumstances allow this to be done safely, if s/he chooses to do so (see Figures 7 and 8).
However, a Level 4 or 5 ADS need not be designed to allow a user to perform DDT fallback and,
indeed, may be designed to disallow it in order to reduce crash risk (see 8.9).
NOTE 5: While a Level 4 or 5 ADS is performing the DDT fallback, it may be limited by design in speed and/or
range of lateral and/or longitudinal vehicle motion control (i.e., it may enter so-called “limp-home
mode”).
NOTE 6: While performing DDT fallback, an ADS may operate temporarily outside of its ODD (see 3.21 NOTE
1).
EXAMPLE 1: A Level 1 adaptive cruise control (ACC) feature experiences a system failure that causes the
feature to stop performing its intended function. The human driver performs the DDT fallback by
resuming performance of the complete DDT.
8 © ISO/SAE International 2021 – All rights reserved
EXAMPLE 2: A Level 3 ADS feature that performs the entire DDT during traffic jams on freeways is not able to
do so when it encounters a crash scene and therefore issues a request to intervene to the DDT
fallback-ready user. S/he responds by taking over performance of the entire DDT in order to
maneuver around the crash scene (see Figure 4). (Note that in this example, a minimal risk
condition is not needed or achieved.)
EXAMPLE 3: A Level 4 ADS-dedicated vehicle (ADS-DV) that performs the entire DDT within a geo-fenced city
center experiences a DDT performance-relevant system failure. In response, the ADS-DV
performs the DDT fallback by turning on the hazard flashers, maneuvering the vehicle to the road
shoulder and parking it, before automatically summoning emergency assistance (see Figure 7).
(Note that in this example, the ADS-DV automatically achieves a minimal risk condition.)
The following Figures 3 through 8 illustrate DDT fallback at various levels of driving automation.
Figure 3
Sample use case sequence at Level 3 showing ADS engaged and occurrence of a vehicle system failure that
prevents continued DDT performance. User performs fallback and achieves a minimal risk condition.
Figure 4
Sample use case sequence at Level 3 showing ADS engaged and occurrence of an ADS system failure that
does not prevent continued DDT performance. User performs the fallback and resumes DDT performance.
© ISO/SAE International 2021 – All rights reserved 9
Figure 5
Sample use case sequence at Level 3 showing ADS engaged and occurrence of exiting the ODD that does not
prevent continued DDT performance. User performs the fallback and resumes DDT performance.
Figure 6
Sample use case sequence at Level 4 showing ADS engaged and occurrence of a vehicle system failure that
prevents continued DDT performance. ADS performs the fallback and achieves a minimal risk condition.
Figure 7
10 © ISO/SAE International 2021 – All rights reserved
Sample use case sequence at Level 4 showing ADS engaged and occurrence of an ADS failure that does not
prevent continued DDT performance by an available human user. The ADS feature may prompt a passenger
seated in the driver’s seat (if available) to resume DDT performance; if no driver’s seat with receptive passenger,
the ADS automatically achieves a minimal risk condition.
Figure 8
Use case sequence at Level 4 showing ADS engaged with ODD exit, which does not prevent continued DDT
performance by an available human user. The ADS feature may prompt a passenger seated in the driver’s seat
(if available) to resume DDT performance; if no driver’s seat with receptive passenger, the ADS automatically
achieves a minimal risk condition.
3.13 FLEET OPERATIONS [FUNCTIONS]
The activities that support the management of a fleet of ADS-equipped vehicles in driverless operation, which
may include, without limitation:
• Ensuring operational readiness.
• Dispatching ADS-equipped vehicles in driverless operation (i.e., engaging the ADSs prior to placing the
vehicles in service on public roads).
• Authorizing each trip (e.g., payment, trip route selection).
• Providing fleet asset management services to vehicles while in-use (e.g., managing emergencies,
summoning or providing remote assistance as needed, responding to customer requests and break-downs).
• Serving as the responsible agent vis-a-vis law enforcement, emergency responders and other authorities
for vehicles while in use.
• Disengaging the ADS at the end of service.
• Performing vehicle repair and maintenance as needed.
3.14 LATERAL VEHICLE MOTION CONTROL
The DDT subtask comprising the activities necessary for the real-time, sustained regulation of the y-axis
component of vehicle motion (see Figure 9).
NOTE: Lateral vehicle motion control includes the detection of the vehicle positioning relative to lane
boundaries and application of steering and/or differential braking inputs to maintain appropriate lateral
positioning.
© ISO/SAE International 2021 – All rights reserved 11
3.15 LONGITUDINAL VEHICLE MOTION CONTROL
The DDT subtask comprising the activities necessary for the real-time, sustained regulation of the x-axis
component of vehicle motion (see Figure 9).
NOTE: Longitudinal vehicle motion control may include forward and reverse directionality depending on the
usage specification.
Figure 9 - Diagram showing vehicle axes of motion (SAE J670)
3.16 MINIMAL RISK CONDITION
A stable, stopped condition to which a user or an ADS may bring a vehicle after performing the DDT fallback in
order to reduce the risk of a crash when a given trip cannot or should not be continued.
NOTE 1: At Levels 1 and 2, the in-vehicle driver is expected to achieve a minimal risk condition as needed.
NOTE 2: At Level 3, given a DDT performance-relevant system failure in the ADS or vehicle, the DDT fallback-
ready user is expected to achieve a minimal risk condition when s/he determines that it is necessary,
or to otherwise perform the DDT if the vehicle is operable.
NOTE 3: At Levels 4 and 5, the ADS is capable of automatically achieving a minimal risk condition when
necessary (i.e., due to ODD exit, if applicable, or due to a DDT performance-relevant system failure
in the ADS or vehicle). The characteristics of automated achievement of a minimal risk condition at
Levels 4 and 5 will vary according to the type and extent of the system failure, the ODD (if any) for
the ADS feature in question, and the particular operating conditions when the system failure or ODD
exit occurs. It may entail automatically bringing the vehicle to a stop within its current travel path, or it
may entail a more extensive maneuver designed to remove the vehicle from an active lane of traffic
and/or to automatically return the vehicle to a dispatching facility.
EXAMPLE 1: A Level 4 ADS feature designed to operate a vehicle at high speeds on freeways experiences a
DDT performance-relevant system failure and automatically removes the vehicle from active
lanes of traffic before coming to a stop.
EXAMPLE 2: A vehicle in which a Level 4 ADS is installed experiences a DDT performance-relevant system
failure in its primary electrical power system. The ADS utilizes a backup power source in order to
achieve a minimal risk condition.
12 © ISO/SAE International 2021 – All rights reserved
3.17 [DDT PERFORMANCE-RELEVANT] SYSTEM FAILURE
A malfunction in a driving automation system and/or other vehicle system that prevents the driving automation
system from reliably performing its portion of the DDT on a sustained basis, including the complete DDT, that it
would otherwise perform.
NOTE 1: This definition applies to vehicle fault conditions and driving automation system failures that prevent
a driving automation system from performing at full capability according to design intention.
NOTE 2: This term does not apply to transient lapses in performance by a Level 1 or 2 driver support feature
that are due to inherent design limitations and that do not otherwise prevent the system from
performing its part of the DDT on a sustained basis.
EXAMPLE 1: A Level 1 driver support feature that performs the lateral vehicle motion control subtask of the
DDT experiences a DDT performance-relevant system failure in one of its cameras, which
prevents it from reliably detecting lane markings. The feature causes a malfunction indication
message to be displayed in the center console at the same time that the feature automatically
disengages, requiring the driver to immediately resume performing the lateral vehicle motion
control subtask of the DDT.
EXAMPLE 2: A Level 3 ADS experiences a DDT performance-relevant system failure in one of its radar
sensors, which prevents it from reliably detecting objects in the vehicle’s pathway. The ADS
responds by issuing a request to intervene to the DDT fallback-ready user. The ADS continues
to perform the DDT, while reducing vehicle speed, for several seconds to allow time for the DDT
fallback-ready user to resume operation of the vehicle in an orderly manner.
EXAMPLE 3: A vehicle with an engaged Level 3 ADS experiences a sudden tire blow-out, which causes the
vehicle to handle very poorly, giving the fallback-ready user ample kinesthetic feedback indicating
a vehicle malfunction necessitating intervention. The fallback-ready user responds by resuming
the DDT, turning on the hazard
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