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(Motor Authority) — Self-driving cars are part of an uncertain future, but automakers have signaled they are constantly moving to a day when at least some of us won’t drive at all.

Along the way, automakers will offer various levels of self-driving capability. Just what are those levels and how are they defined? We are here to help.

While the levels of self-driving sophistication can vary significantly from one brand to another, the defined bounds of those levels are relatively set.

In 2014, the Society of Automotive Engineers adopted a common taxonomy for self-driving cars that defined six levels—from Level 0 to Level 5—of automated driving. The boundaries are pretty obvious. Level 0 was no assistance and Level 5 was fully autonomous. The SAE updated their tiers in 2021, and while Level 5 remains fully autonomous, Level 0 has changed to include some active safety features that have become in the last few years.

It’s important to note that automakers have described some self-driving functions as being “fully autonomous” or “Level 5” (notably Tesla), but fully self-driving cars are outlawed on virtually every road in the U.S., and the terms and levels aren’t interchangeable. Most self-driving functions on cars hover around Level 2 or Level 3, but the road to Level 4 and Level 5 remains unclear. Fully autonomous, Level 5 cars are decade away at least—and it’s entirely possible that they won’t be initially available to consumers when they arrive, if ever.

So what do these levels mean?

Level 0: No self-driving functions at all. Drivers are responsible for controlling steering, throttle, and braking. They’re also responsible for monitoring everything around the car. Level 0 has been updated to include features that provide warnings and temporary assistance, including automatic emergency braking, blind-spot monitors, and lane-departure warnings. The vast majority of cars on the road today fall into this category.

SAE levels of driving automation, from none to fully self-driving

Level 1: Some driver assistance is allowed. In cars equipped with Level 1 automation, the car can take control of the steering or the throttle/brake in certain situations, but it relies on the driver to immediately take over if those systems fail. Many newer cars are equipped with adaptive cruise control that can slow the car to a stop, which falls into this category. Some cars are equipped with active lane control (also known as lane-departure prevention) systems that allow for limited periods of hands-off driving.

Cadillac’s Super Cruise system undergoing testing.

Level 2: More driver assistance. Level 2 automation differs from Level 1 automation in how many systems are used to assist drivers, but the two levels are identical in requiring drivers to immediately take over if those systems fail or stop operating. Level 2 allows for the use of adaptive cruise control and active lane control at the same time. Several automakers offer systems that will take control of the throttle/brake and steering for a limited time, but require driver interaction or those systems stop, usually with several warnings to the driver. (What happens when they shut off is important in distinguishing between Level 2 automation and higher levels.) Some of today’s most sophisticated systems can handle all the controls at full speed on the highway but still require the driver to monitor the road ahead. The best of them is GM’s Super Cruise. Ford’s Bluecruise and Tesla’s Full Self-Driving also qualify here.

Volvo Drive Me autonomous car pilot project in Gothenburg, Sweden

Level 3: Level 3 remains theoretical. It consists of conditional automation. Many automakers have said that they’ll skip Level 3 automation because it may be dangerous to immediately hand all driving functions back to a human that isn’t required to pay attention to the road. Level 3 automation can handle all driving situations in certain situations and constantly monitors the road, unlike Level 2 cars. Limited testing has shown that these systems may not be safer than no automation at all, but most experts stop short of saying Level 3 cars should be prohibited. While SAE’s chart says “traffic jam chauffeur” qualifies for this level, no car on the road has it, even cars with systems labeled traffic-jam assist. Those systems, which typically handle the controls in low-speed traffic-jam situations, still require the driver to monitor the system.

Level 4: Nearly self-driving. Most automakers are targeting Level 4 automation for several reasons. First, it’s likely to be cheaper because Level 4 self-driving cars may not require driver controls such as a steering wheel, throttle, or brake pedals—building a car with redundant controls for both the driver and autonomous systems would be costly and complicated. Second, Level 4 differs from Level 3 primarily because it doesn’t require human intervention if self-driving systems fail. GM’s Cruise Automation is one of several entities that operates limited local fleets of driverless cars that fall into this category.

Level 5: Fully self-driving. This may seem like a logical small step from Level 4, but for most automakers, Level 5 autonomy is a giant leap for self-driving cars. Level 5 cars would be self-driving, all the time, everywhere. Considering most of the nation’s roadways aren’t pencil-straight interstates on a sunny day, the final SAE level would require extensive testing for sensors that can’t yet read road lines in bad weather, low light, on dirt roads, or countless other variable circumstances. Although having a steering wheel and pedals wouldn’t preclude a car from being Level 5, to achieve the designation those controls would be useless: Level 5 cars aren’t driven by humans in any circumstance.