An illustration of a vehicle using a car automation system

As more partially automated driving technology becomes available on new cars, automakers, enthusiasts, and other experts are likely to speak about the capabilities of these systems in terms of levels of vehicle autonomy.

These levels were developed by SAE International (originally known as the Society of Automotive Engineers) as common vocabulary to use when discussing this emerging technology.

But their use is spreading. The National Highway Traffic Safety Administration—the U.S. auto-safety regulator—has adopted them, and they’ve also crept into auto advertising and marketing. For example, Nissan advertises its ProPilot Assist partially automated driving system as a “Level 2 driver assistance system,” and ride-hailing company Lyft calls its self-driving research program “Level 5.”

More on Advanced Car Safety Systems

Consumer Reports believes that the SAE levels can give drivers a false sense of understanding of what their cars can and cannot do. Focusing too much on how to classify a new technology can be a distraction for consumers. Instead, CR believes there’s only one important question that can determine whether one day a mainstream vehicle is truly autonomous: Are you the driver, or are you a passenger in a self-driving car?

There are no self-driving cars on the road today that consumers can buy. The latest safety and convenience systems, such as adaptive cruise control (ACC) and lane keeping assist (LKA), still require an attentive driver, and it’s important that drivers understand that. “For now, these features are there to help drivers, not to do the driving for them,” says Kelly Funkhouser, CR’s program manager for vehicle interface and automation.

Below are the SAE levels and their definitions.

Level 0: No Automation

This level is called “No Automation,” but some partially automated safety and driver-assist systems—including automatic emergency braking, cruise control, and electronic stability control—are included here. Most warning systems such as forward collision warning, blind spot warning, and lane departure warning are also part of this level. 

Level 1: Driver Assistance

This describes many of today’s new cars. The human driver is responsible for the safety and operation at all times, but the car can take over at least one vital function: steering or speed control. ACC is the best example of existing technology at this level.

Level 2: Partial Automation

Today’s more advanced cars qualify as Level 2. The driver is still responsible for the safe operation of the vehicle, but it can take over steering, braking, and acceleration under certain conditions. The driver is expected to do everything else and monitor road conditions. Tesla’s Autopilot and similar systems from Mercedes-Benz, BMW, and Volvo are examples of partial automation.

Level 3: Conditional Automation

The car has the ability to control acceleration, braking, and steering. It can stop at stop signs, obey traffic signals and signs, and navigate complex traffic situations without any driver intervention. Although the car can drive itself, the human driver must still pay attention and take over, instantly, at any time.

CR feels that this level may be the most difficult for drivers to manage, and could be setting drivers up to fail. That’s because the driver “is not required to monitor the environment,” according to SAE, but still “must be ready to take control of the vehicle at all times with notice,” such as in an emergency, if the system fails, or if it encounters a situation it doesn’t understand.

This level still requires drivers to be “fallback-ready” whether or not the car requests attention from the driver, and it ultimately holds the driver responsible if a crash takes place. That's even though experiments have shown that users of partially automated systems tend to put too much trust in technology and therefore stop paying attention.

Level 4: High Automation

This level essentially means a car is capable of full automation, but it can also give drivers a warning and time to take over controlling the car. Level 4 systems must have the ability to maintain full control of the vehicle if the driver does not intervene, even when prompted. This level is fully autonomous, thereby allowing drivers to do something other than pay attention to the road.

Level 5: Full Automation

It is expected at this level that there is no human driver—only passengers. The vehicle is capable of performing all aspects of driving and will not allow humans to drive the car. Automakers envision these cars without a steering wheel, an accelerator, or a brake pedal, because they won’t be necessary. The only difference between Level 4 and Level 5 is that drivers will not have the ability to take control of the vehicle at any time.

Where Automation Stands Today

Almost every new car sold in the U.S. today falls into a wide gray area from Level 0 to Level 2. SAE Level 4 and Level 5 cars aren’t yet available for the public to buy, but many of today’s vehicles offer some automated control of the steering wheel, brakes, and/or accelerator, as seen with ACC, LKA, and/or automatic lane changing. Whether those features mean a car is Level 1 or Level 2 depends on how well the systems work, and whether or not they are turned on or off.

But CR feels that what level a car is matters less than how safe it is for its occupants and others on the road. Just because a car is at a higher SAE level doesn’t make it safer. In fact, some of the so-called better systems are the ones that give CR’s testers the most pause. Funkhouser says that when systems perform well most of the time, it’s human nature to want to pick up your phone or do something to keep your brain active, trusting the systems to nudge the car back into its lane if it should wander. Systems at Levels 1, 2, or 3 are intended to assist the driver, not take over responsibility. They always have limitations and can fail unexpectedly.

CR’s view: It’s more important for drivers to know the limitations of new automotive technology than how that technology is categorized.