Autonomous driving is a term we're slowly becoming accustomed to, along with electric and hybrid power, as the vehicles of the future take over the roads. We're still a fair way off completely driverless cars, but the technology that makes driving that little bit easier - and safer - is appearing in cars up and down the highways.

But you may have noticed there are different levels of autonomous driving, from 0 to 5, and we'd wager that you actually don't know what they mean - only the boffins in the car industry will have the slightest clue as to what they actually mean.

So allow us to explain exactly what each level of autonomy means, and if there are any examples currently available on the road.

Who sets out these autonomous vehicle levels?

SAE International (Society of Automotive Engineers) published the classification system for the six different levels in 2014, titled "Taxonomy and Definitions for Terms Related to On-Road Motor Vehicle Automated Driving Systems". Each level is classified by how much a driver is required to intervene and how attentive they need to be when behind the wheel of an autonomous vehicle.

More and more, these levels are being referred to around the discussion of autonomous features in cars.

What are the autonomous driving levels?

There are six levels of autonomy in the SAE's classification system, ranging from level 0 to level 5:

Level 0: Level 0 vehicles are ones that are probably sitting on your driveway right now. They have no autonomous or self-driving controls at all, with all aspects of driving needing to be taken care of by the driver, including reacting to hazards.

Level 1: Level 1 autonomy is the most common on roads. It refers to cars that have systems that allow the car and driver to share control of the vehicle. Adaptive cruise control, which controls speed and distance compared to the vehicle in front is a good example, as the driver still has to take care of steering.  

A park assist feature is an example of level 1 too, as the driver has to control the speed of the vehicle while the car takes care of the steering. There are a range of safety features across most manufacturers which offer some level 1 autonomy.

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Level 2: Level 2 cars have internal systems that take care of all aspects of driving: steering, acceleration and braking. However, the driver must be able to intervene if any part of the system fails. Level 2 is also referred to as "hands-off", however on the contrary, the driver is required to keep their hands on the wheel at all times.

Tesla's Autopilot can be considered level 2, as it can automatically keep you in the right lane on the road and keep you at a safe distance from the car in front when in a traffic jam. Nissan's ProPilot is also level 2, as found in the new Nissan Leaf.

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Level 3: Level 3 vehicles are ones that can be truly considered autonomous. Often referred to as "eyes-off" vehicles, those that fall into level 3 can allow the driver to sit back and relax as the car can take care of everything while driving along the road. Drivers are allowed to safely use their phone or watch movies, although they are still required to be on-hand to intervene if necessary, so falling asleep isn't an option.

Audi's A8L was the first car to claim level 3 autonomy, as it can take over all aspects of driving, in slow-moving traffic up to 60kph. The same tech has since found its way into the regular A8 and Audi is saying that the new Audi A7 will achieve level 3 autonomy in its lifetime. Of course, whether it's legal to drive in an "eyes-off" state is a different matter: currently you still have to be in control of the vehicle when driving on public highways.

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Level 4: Level 4 cars are referred to as "mind-off", because they're so capable that the driver isn't required to intervene at all, so you can go to sleep if you want. However, there are some restrictions, as the full self-driving mode can only be activated in certain, geofenced areas or in traffic jams. If the car isn't in a specified area or in a traffic jam, then it must be able to get itself to safety if the driver isn't able to take control in an emergency.

The best example of a level 4 vehicle is Google's Waymo project. Waymo vehicles have been operating driver free for some time in the US, although a test driver is on hand just in case anything goes wrong. 

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Level 5: Level 5 cars are ones that require no human interaction whatsoever, they are fully autonomous vehicles. Examples of level 5 vehicles are robotic taxis. We're still a long way off putting level 5 vehicles into mass production.

Audi's Aicon concept is an example of a level 5 autonomous vehicle. Experts believe it will be some time before we ever see cars like it on the road, but they certainly offer an exciting glimpse into a driverless future. 

How are autonomous vehicles tested?

Because autonomous vehicle technology is still in its infancy, testing it on public roads obviously comes with safety concerns. Therefore it is required by law for a driver to be in the car, ready to take over if needed. Car manufacturers need to obtain a special license allowing them to test on public roads, too. 

Testing in closed road conditions is much safer and manufacturers can even test their systems in virtual environments, before putting their cars on the road where they could potentially crash. 

Apple is currently testing self-driving vehicles on public roads, with some 27 vehicles in operation. 

Will there be autonomous taxis?

Hopefully one day, yes. Ride-sharing giant Uber has signed a deal with Volvo to develop self-driving vehicles together. Volvo will build the car - and Volvo is known for its high levels of safety - and Uber will then take it, tinker with it and put it on the roads in the form of a self-driving taxi. 

Nissan is another car manufacturer looking into self-driving cabs, and has begun trialling its Easy Ride service in Yokohama, Japan. The plan is to have a fully-fledged autonomous taxi service up and running in time for the 2020 Tokyo Olympics.