Self-driving cars are a popular topic in both the auto industry and general media coverage these days, with several makers testing vehicles and promising fully autonomous cars at some future point.

But when it comes to self-driving cars, different automakers may not be talking about the same thing.

Some vehicles on sale today offer driver-assistance systems that take over certain driving functions on a limited basis.

Those are a far cry from systems that can pilot a vehicle from point to point with no human intervention at all.

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They represent a transitional stage between fully manual driving and a time many in the auto industry insist will come: when at least some people will not drive themselves at all.

To help clarify things, this guide is based on the levels of autonomy—Level 0 to Level 5—that was defined by the Society of Automotive Engineers in 2014.

Keep in mind that while automakers may discuss "fully-autonomous" or "Level 5" features, vehicles with that degree of autonomy are not legal on most public roads in the U.S.

Moreover, laws in the areas where they are allowed on public roads explicitly permit only tests, not consumer sales.

SAE levels of autonomy

SAE levels of autonomy

With that, here is our primer on the levels of autonomy.

Level 0: No self-driving capabilities. Drivers are responsible for controlling steering, throttle, and braking, as well as monitoring their surroundings.

Most cars on the road today fall into this category, even those equipped with some assistance features like blind-spot monitoring and forward-collision warning.

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Level 1: Some driver assistance. Cars can take control of the steering or the throttle/brake in certain situations, but rely on the driver to immediately take over if those systems fail.

Vehicles equipped with adaptive cruise control, autonomous emergency braking, and lane-departure prevention fall into this category.

Level 2: More driver assistance. Like Level 1, cars rely on a human driver to take over if autonomous systems fail or are out of their depth.

However, the extent of autonomous functionality is greater, including being able to control steering and the throttle and brakes simultaneously.

These systems cannot perform specific tasks, such as highway merging or navigating stop-and-go traffic.

Second-generation Ford Fusion Hybrid automated driving research vehicle

Second-generation Ford Fusion Hybrid automated driving research vehicle

Level 3: Conditional autonomy. Cars can handle all driving situations and constantly monitor the environment, but still require human intervention in case of system failure.

Many automakers may skip this level over concerns that it may be too dangerous to hand control back to a human driver after a long period of inactivity.

Limited testing has indicated drivers may not be prepared to take over in those situations.

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Level 4: Nearly autonomous. Unlike Level 3, cars can bring themselves to a stop without any human intervention if a technical problem occurs.

That, combined with the ability to handle all types driving situations and circumstances, makes manual controls redundant. Certain high-profile autonomous-car projects announced by automakers fall into this category.

Ford plans to launch a production self-driving car with no manual controls for ride-sharing services by 2021, and Volvo will have an autonomous version of its XC90 SUV by then.

The Swedish automaker has said it will include manual controls because its customers prefer the option to drive themselves at times.

Waymo self-driving prototype

Waymo self-driving prototype

Level 5: Fully autonomous. Level 5 cars operate autonomously at all times, and are not intended to be controlled by human drivers at all.

Note that this represents a major technological leap from Level 4.

To achieve Level 5, automakers will have to conduct extensive testing to cover every possible road and weather condition.

No automaker has laid out a firm timeline for Level 5 cars to hit the road, but many have said that it's at least a decade or longer away.

It's also unclear whether Level 5 cars will be sold to the public at all; some analysts suggest they may be restricted only to ride-sharing programs or other services.


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