An overview of taxonomy, legislation, regulations, and standards for automated mobility
Updated: Feb 14
We are regularly updating this article. The changelog is at the bottom of the article.
I have been working on autonomous vehicles and driver assistance systems for 23 years. During this time, I have had many touchpoints with legal and regulatory aspects as well as with functional safety and the respective certification. I helped write the SAE levels of automated driving, and I cover many aspects in the class I am teaching at Stanford University. Discussion and publication often do not address these topics holistically. This post is an attempt to summarize all aspects in one article. Comments, additions, suggestions are welcome. Contact firstname.lastname@example.org. Thanks.
Why are legislation, standards, and taxonomy for vehicle automation needed or at least useful in the first place? Well, obviously, for the same reasons that apply to other technology fields as well:
A taxonomy provides a common set of definitions and language to describe the technology. Terms are defined to mean the same thing when used accordingly.
Standards extend definitions and language to technical implementations and enable performance and safety minimums. In the course of progressing maturity of a technology, a standard is typically approved through expert consensus by a recognized standardization body. It provides for repeated and common use, rules, guidelines, or characteristics for products or related processes and production methods, with which compliance is not mandatory. Standards enable interoperability.
Legislation and regulations are issued by governments and define product characteristics or their related processes or production methods, including the applicable administrative provisions, with which compliance is mandatory. Regulations often make use of standards. Laws and regulations enable operability.
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The purpose of a taxonomy is to provide a standardized classification system to define terms and terminology. Unfortunately, the attempt to establish common terminology has so far been only semi-successful. It seems that the world now has agreed on one taxonomy (the SAE levels, see below), but the world, including some carmakers, has not been able to agree to use those consistently.
The German Federal Highway Research Institute (BASt) was the first in 2010 to publish a taxonomy for vehicle automation. The result of the activity was rather short-sighted as an equivalent to SAE level 5 does not exist in the BASt taxonomy—remember this was written three years after the DARPA Urban Challenge and one year after Google launched the project that is now known as Waymo. BASt defines five levels as follow:
Level 0: driver only (equivalent to SAE level 0).
Level 1: assisted (SAE level 1).
Level 2: partially automated (SAE level 2).
Level 3: highly automated (SAE level 3).
Level 4: completely automated (more or less equivalent to SAE level 4).
NHTSA issued in 2013 a preliminary statement of policy concerning automated vehicles, in which five levels of automation are defined:
Level 0: No automation (equivalent to SAE level 0).
Level 1: Function-specific automation (SAE level 1).
Level 2: Combined function automation (SAE level 2).
Level 3: Limited self-driving automation (SAE level 3).
Level 4: Full self-driving automation (SAE level 4 and 5 combined).
NHTSA deprecated their levels in 2016 to adopt the SAE levels.
SAE launched their first Taxonomy and Definitions for Terms Related to Driving Automation Systems for On-Road Motor Vehicles in 2014, with updates in 2016 and 2018. While the levels themselves did not change since 2014, the descriptive language in the standard has become significantly richer and more clarifying over time.
Level 0: No Driving Automation—The performance by the driver of the entire Dynamic Driving Task (DDT), even when enhanced by active safety systems.
Level 1: Driver Assistance—The sustained and Operational Design Domain (ODD)-specific execution by a driving automation system of either the lateral or the longitudinal vehicle motion control subtask of the DDT (but not both simultaneously) with the expectation that the driver performs the remainder of the DDT.
Level 2: Partial Driving Automation—The sustained and ODD-specific execution by a driving automation system of both the lateral and longitudinal vehicle motion control subtasks of the DDT with the expectation that the driver completes the Object and Event Detection and Response (OEDR) subtask and supervises the driving automation system.
Level 3: Conditional Driving Automation—The sustained and ODD-specific performance by an Automated Driving System (ADS) of the entire DDT with the expectation that the DDT fallback-ready user is receptive to ADS-issued requests to intervene, as well as to DDT performance-relevant system failures in other vehicle systems, and will respond appropriately.
Level 4: High Driving Automation—The sustained and ODD-specific performance by an ADS of the entire DDT and DDT fallback without any expectation that a user will respond to a request to intervene.
Level 5: Full Automation—The sustained and unconditional (i.e., not ODD- specific) performance by an ADS of the entire DDT and DDT fallback without any expectation that a user will respond to a request to intervene.
Assisted driving features
Level 0: You're driving.
Level 1: You're driving, but you're assisted with either steering or speed.
Level 2: You're driving, but you're assisted with both steering and speed.
Automated driving features
Level 3: You're not driving, but you will need to drive if prompted in order to maintain safety.
Level 4: You're not driving, but either a) you will need to drive if prompted in order to reach your destination (in a vehicle you can drive) or b) you will not be able to reach every destination (in a vehicle you can't drive).
Level 5: You're not driving, and you can reach any destination.
This table, also created by Bryant, summaries the levels and compares SAE, BASt, and NHTSA levels: