Levels of vehicle automation
When most people think of an automated vehicle, they’re likely imagining a vehicle that can drive itself without any assistance or backup from a human in (at least) all the situations that a human is capable of driving in. That might be the most charismatic kind of automated vehicle, but until the technology reaches that level of advancement, we will need to consider more limited technologies.
The Society of Automotive Engineers (SAE) categorizes vehicle automation in levels from 0 to 5. This post briefly summarizes the SAE taxonomy — for full detail, see the original SAE document here.
Level 0 technologies simply provide warnings and other information to a human driver — no automation to see here.
Level 1 automation technologies control either steering or speed, but not both simultaneously: lane-keeping technologies adjust steering as needed to stay in the lane, and adaptive cruise control maintains the vehicle’s speed, adapting it as necessary to stay a safe distance away from vehicles ahead. As the driver, you have to control either the speed or the steering.
In a Level 2 automated vehicle, the system both steers the car and controls its speed — but you, the human driver, must continuously keep watch over the system and must be available to take over control with no advance warning if the system screws up. This is the highest level of automation currently on the market, available from companies like Mercedes and Tesla. Though you might be tempted, you should not let your attention stray from the road, much less take a nap — you must be ready to take over instantly when needed.
At Level 3, you won’t need to keep watch over the system while it’s driving — but you do have to be available to take control within a short time when requested by the system. Just how long that takeover time should be is an unresolved question: so far, the SAE document tells us it would be “several seconds”. At this level of automation, you can watch a movie or read, for example, but napping is still out of the question — you need to be able to take over full driving duties within those “several seconds”.
A Level 4 vehicle can drive itself without any need for intervention from a human driver — provided risk is limited in some way. One example would be an automated car that chauffeurs you while it’s in the tightly regulated driving environment of the freeway, but asks you to take over when it’s approaching the messier, more dynamic environment of urban arterials and streets — but if you’ve fallen asleep or for some other reason fail to take over, no problem. Before it gets itself into a situation where it’s in over its head, the automated system is capable of pulling over to a safe location where it will wait patiently for you to collect your bearings and prepare to drive on, manually. (This is in stark contrast with Level 3, where if you fail to take control in time, the vehicle wouldn’t necessarily be able to pull over safely before it’s embroiled in a situation that overwhelms its capabilities.)
Alternatively, a Level 4 vehicle could be restricted to less challenging situations, altogether avoiding conditions that the automated system can’t handle on its own, thus eliminating any need to hand off control to a human driver (and eliminating the need to pull over if the human doesn’t respond in time). Limiting the risk of driving in this way means that a less sophisticated system can drive with no need for human driver monitoring or backup. Such a Level 4 system could stick to simplified, controlled environments, such as exclusive roads or lanes protected by barriers; it could drive slowly and with special caution; or it could combine these approaches to minimize risk — for example, by driving slowly in private areas with light vehicle and pedestrian traffic and low speed limits.
In short, you can achieve full automation with quite primitive technology, provided you dumb the problem down enough. (This is how automated trains like Vancouver’s SkyTrain have been able to run without human drivers or attendants since the mid-1980s — trains don’t need to steer, of course, and they can drive safely at speed thanks to grade separation or barriers along the tracks that keep people out of the way.)
Finally, a Level 5 vehicle would be able to drive itself in any situation without any need for special measures to reduce risk, and without any need for a human to monitor or be available to take over.
Before the SAE laid out these levels of automation, other groups had drafted their own taxonomies; the best known was developed by the US National Highway Traffic Safety Administration (NHTSA). That taxonomy was fundamentally quite similar to the SAE’s, with SAE Level 1 corresponding to NHTSA Level 1, and so on. The main differences are that SAE’s Level 5 is equivalent to NHTSA’s Level 4, while SAE’s Level 4 has no equivalent in NHTSA’s scheme. The fact that the SAE’s taxonomy includes this category for fully driverless operation in restricted situations makes their scheme much more useful than the one NHTSA developed. NHTSA has since adopted the SAE taxonomy; unfortunately, some people still follow NHTSA’s old system, so if you hear someone talking about Level 4, it’s a good idea to check whether they’re referring to the SAE levels.