Just three years ago, autonomous cars seemed like a pipe dream relegated to a far-off decade in the future. But then, last week, Ford announced that in 2021 it's planning to release an autonomous car without a steering wheel, brake or throttle pedals, designed for ride-sharing.

Just a few days later, Volvo and Uber made public their partnership to develop a driverless car.

Now, it seems autonomous cars aren't just a fanciful future prospect but rather something tangible -- and a real part of mobility in the near future.

The Ford and Volvo/Uber announcements, though, highlight the different approaches each company working on autonomous driving technology is taking. For example, some, like Audi, are introducing autonomous systems slowly into their products to indoctrinate their buyers to the tech. Others, like Ford, are jumping straight to driverless cars.

Accordingly, we've compiled an alphabetical autonomy timeline encompassing the announcements and plans of the leaders in the space. What's more, we've underlined the technology that each company aims to utilize to drive its driverless cars.

SAE automated driving levels

Before we get into the timeline, we need to understand the levels of autonomous driving technology, 0 to 5, as defined by the Society of Automotive Engineers (SAE).

Level 0 is no assistance. Level 1 is defined by systems like collision mitigation braking that automatically brake if a collision is imminent.

Level 2 is the level at which all semi-autonomous systems on the road are at today. These include acceleration, braking and steering assistance.

Level 3 extends those to that the SAE describes as "some driving modes." By that, it means the car can handle elevated levels of automated driving in given parameters -- like on a freeway during the daylight. However, it's expected that the human driver can take over driving duties, if asked.

Level 4 extends the autonomous capabilities and can handle all driving responsibilities, even if a human driver cannot intervene. Still, Level 4 is limited to "some driving modes."

Level 5 takes full autonomy to "all driving modes." That means the car is fully capable of driving itself anywhere in any condition, from a snowy, moonlit road to an unmapped desert. It should be noted that, at this point, Level 5 is theoretical. One Audi representative went so far as to describe it as "mythical." It's unlikely we'll see Level 5 autonomous driving in our lifetimes.

Audi

2016 -- Level 2:Audi's Level 2 “Traffic Jam Assist” system is available on the 2017 Audi A4 and Q7. The system uses steering input to keep the car in its lane. What's more, it allows the driver to take his or her hands off the wheel in slow-moving traffic for 15 seconds at a time. However, the driver can't check out or turn attention away from the road, as he or she must intervene immediately if prompted by the car.

2018-- Level 3:Audi pledges to be the first carmaker to introduce Level 3 autonomy to the road with its next generation A8 full-size sedan. The A8 will debut Audi's Traffic Jam Pilot system. It will handle accelerating, braking and steering up to 35 mph. However, there are many parameters and limitations to engaging Traffic Jam Pilot. It will need to have no fewer than two cars ahead of it and it will need to recognize, based upon information GPS and other outward-facing sensors, that it is on a freeway. Its cameras will also detect whether the road is clear enough for ideal operation. It won't be activated on a snow-covered road, for example.

The car will also require specific driver conditions before Traffic Jam Pilot can be engaged, too. It will need to detect a driver in the driver's seat. What's more, the car will include a camera-based driver awareness system that will look to see if the driver is awake and not incapacitated in some way (suffering a pulmonary embolism, for example).

2020/2021-- Level 3 Plus:Two or three years after the first application Level 3, Audi will roll out what it's roughly defining as "higher" Level 3. That means it will take the capability of Traffic Jam Pilot and extend it up to full freeway speeds. It will require the same freeway verification from GPS and other sensors before it is engaged.

Additionally, with Level 3, Audi will include in each car an event data recorder (think an airplane's black box). It will record the driving data directly preceding a collision, should one occur. Don't worry, Audi won't be recording all your driving data. The event data recorder will be constantly deleting the data it collects, if a collision doesn't take place.

Late 2020s -- Level 4:Full highway and urban autonomy. These functions will only be allowed in pre-mapped and geofenced areas. That means these cars will not be able to drive autonomously everywhere. You couldn't ask the car to autonomously drive from Los Angeles to New York, for example, unless the route were mapped and programmed.

Tech: GPS, LIDAR (Light Imaging, Detection And Ranging), short- and long-range radar, and no fewer than two digital cameras will be needed for Audi's autonomous cars. In total, Audi's autonomous cars will have around 24 sensors with almost 360-degree redundancy. In addition, the cars will include redundant steering and braking systems.

BMW

2016-- Level 2: With the all-new 7 Series, BMW introduced several new Level 2 technologies: Traffic jam Assistant and driverless automated parking. Traffic jam Assistant can follow the car ahead on the highway and can read lane markings and add corrective steering to keep the car in its lane. This system works from 0 to 43 mph.

European versions of the new BMW 7 Series feature a self-parking function that doesn't require a driver inside the car. With the car's key fob, a driver can exit the vehicle and direct the car into a parking spot. However, that function is not available here in the U.S.

Tech: Radar, digital cameras.

2021-- Level 4:Marking its 100th birthday earlier this year, BMW announced that it has also pegged 2021 as the year during which it will unveil a fully autonomous car. It's called iNext and not only will it be autonomous, it will also be intelligent, lightweight and the "next generation of electro-mobility," according to BMW chairman of the board Harald Krüger.

The flagship iNext autonomous car will be created in collaboration with Intel and MobilEye. Although BMW has not yet specified any other dates for unveiling autonomous driving systems for its cars, the trio (BMW, Intel and MobilEye) have said they will create platform-based "future-proof" test cars by 2017 in order to hit the stated 2021 goal. This platform will tackle, as BMW puts it, "'eyes off' (Level 3), 'mind off' level (Level 4)" 'driver off' (Level 5)."

Tech: BMW hasn't indicated specifically which tech will drive drive its autonomous cars, except Intel Atom and Xeo processors.

Ford

2019-- Level 2: In 2019 (or within three years of 2016), Ford plans to introduce two Level 2 autonomous systems: Traffic Jam Assist and Fully Automated Parking. Confusingly, Ford's Level 2 system will carry the same name as Audi's Level 3. Traffic Jam Assist, like other Level 2 systems on the road today, will follow a car ahead in traffic and control braking and acceleration as well as steering to keep the car in its lane.

Tech: Radar, digital cameras and ultrasonic sensors.

2021-- Level 4:Ford announced in August that in 2021, it will have driverless ride-sharing cars without a steering wheel, or brake or accelerator pedals. Unlike Audi and Nissan, which are slowly ramping up autonomy step by step from Level 2 to 3 to 4, Ford is jumping several steps and going right to 4. While there could be many reasons for this, presumably price is the leading factor.

Since it's five years out, Ford doesn't yet know what platform these cars will ride on, what they'll look like or what will power them (gasoline or electrified powertrains). Ford did divulge that these cars will not be initially available for purchase by customers. That could change, however, as the cost of the onboard tech goes down. Likely, at first, the cars will be prohibitively expensive to offer up in Ford showrooms. And because Ford is keen to make autonomy available to the masses as quickly as possible, it doesn't want to keep the tech relegated to extremely expensive luxury vehicles like Audi's flagship A8.

It's important we understand why these Ford steering wheel-less cars are Level 4 and notLevel 5. As we discussed earlier, Level 5 is defined by the SAE as a car's ability to drive autonomous in 'all driving modes.' The Ford cars simply won't be able to do that. They'll only be allowed to operate in 3D-mapped, geofenced locations. Imagine, then, that these will operate in a few heavily mapped urban environments. So, unless you live in one of the first markets for the Ford autonomous cars, it's unlikely they'll be a regular part of your life in the near future.

Tech: LIDAR, radar, digital cameras, 3D maps, 4G connectivity and, according to a Ford representative, possible vehicle-to-infrastructure (V2X) communication technology.

Honda

2016 -- Level 2: Unlike the luxury brands on this list, Honda has pushed its Honda Sensing suite of autonomous driving technologies down into its most affordable models, including the $20,440 Civic Sedan. Honda Sensing includes Adaptive Cruise Control (ACC) that follows the vehicle ahead, and Lane Keep Assist (LKAS), which uses a forward-facing camera that watches lane markings to keep the vehicle in its lane. Honda Sensing also includes Forward Collision Warning, Collision Mitigation Braking, Lane Departure Warning and Road Departure Mitigation

Further bucking the trend of other systems on this list, Honda Sensing can be activated at any speed. Although it's not locked to highways and freeways by GPS- or camera-based data, it's recommended for use on freeways and not backroads or urban environments.

2020-- Level 3:By 2020, Honda aims to have basic automated highway driving from "on ramp to off ramp." Honda is being very conservative with its promised automated timelines beyond that, however. As such, it is hesitant to give specific dates for fear of missing them due to unforeseen technical or legislative hurdles. That said, Honda contends it is at the forefront of automated vehicle research.

Honda has divulged, though, that by 2020 it also wants to integrate Wi-Fi-based vehicle-to-infrastructure (V2X) and vehicle-to-vehicle (V2V) communication technology into its vehicles. Allowing cars and infrastructure to communicate with one another will add an extra level of automation.

2040: Ultimately, no matter where its automated tech is, be it Level 3 or 4, Honda aims to have no crashes in Honda or Acura vehicles by 2040.

Tech: Radar, digital cameras, Wi-Fi-based V2V and V2X.

Kia

2020-- Level 3: Kia announced at CES 2016 that it is launching an autonomous driving sub-brand called "DRIVE WISE." Additionally, the Korean carmaker benchmarked 2020 as the date by which it would introduce its first semi-autonomous tech. Although Kia is in the fledgling stages of developing its semi- and full-autonomy tech, it's already named some of the systems. They include Highway Autonomous Driving (HAD), Preceding Vehicle Following (PVF), Emergency Stop System (ESS) and Traffic Jam Assist (TJA).

2030-- Level 4:Kia also announced that by 2030 it would be ready to introduce full autonomy. These include systems it's calling Urban Autonomous Driving (UAD) and Autonomous Valet Parking.

Tech: HAD relies on radar and cameras. UAD utilizes GPS and exterior sensors to identify the car’s position on the road. It also will rely on live traffic updates. Like Audi, Kia’s Driver Status Monitoring (DSM) system in its autonomous cars will monitor the driver’s face and if it detects he or she isn't paying attention, the car will move to the side of the road and come to a stop.

Mercedes-Benz

2016-- Level 2: In 2013, Mercedes released its first Level 2 automated driving system, called DISTRONC PLUS with Steering Assist, which we tested on the Mercedes-AMG C63 S last year. Since then, the German automaker has released its newest and more robust Level 2 system, DRIVE PILOT, which debuted on the 2017 E-Class.

DRIVE PILOT is fairly robust for a Level 2 autonomous system. Like Honda Sensing, it can control braking and acceleration at or above regular highway speeds. It also allows the driver to go hands-free for as long as 60 seconds at a time up to speeds of 81 mph. Distinctively, DRIVE PILOT can autonomously change lanes, if a driver indicates a lane change with the turn signals. Of course, if it detects a car in the other lane, it won't carry out the maneuver.

The 2017 E-Class is the only production car that has been granted an autonomous driving license in Nevada. That means, DRIVE PILOT is far more capable than Mercedes allows it to be in its current form. However, Mercedes engineers have retarded the capabilities of DRIVE PILOT for the E-Class to ensure drivers understand that they are ultimately responsible for the safe operation of the car.

Additionally, the all-new E-Class is also the first production car to include V2V technology. At first, this means all new E-Classes will be able to communicate important road and driving information to one another. In the future, as other cars receive the tech, this means E-Classes will be able to communicate with other cars as well -- not just its brand mates.

In addition to cars, Mercedes is testing autonomous semi trucks here in the U.S. and in Europe. In fact, it has been sending "platooning" fleets of self-driving semis across Europe this year with the intention of integrating driverless tech into commercial trucking in the near future.

Tech: Radar, digital cameras, and V2V.

Nissan

2016-- Level 2:Nissan announced earlier this year that by 2020, it will have not just one but 10 fully autonomous models on sale. It also laid out its roadmap for how it will get there, adding a new level of autonomy every two years. This year, the Japanese automaker will introduce its first Level 2 system called ProPILOT. This system will be first introduced to the Japanese market in the new Serena minivan. Nissan will then introduce it to the European marked in 2017 in the Qashqai crossover. Nissan hasn't yet said when the tech will be debuted for the U.S. market. At this level of ProPILOT, the car will have what Nissan is calling "single-lane control" in heavy, stop-and-go traffic on highways.

2018-- Level 3:Two years later, in 2018, Nissan will introduce ProPILOT 2.0 with "multiple-lane control." ProPILOT 2.0 will add autonomous lane changes to the ProPILOT capabilities.

2020 -- Level 4: By 2020, Nissan plans to reveal ProPILOT 3.0 with "intersection autonomy." This will take Level 4 autonomy into the urban centers. Although Nissan hasn't specifically divulged it, it's likely that -- as with Audi and Ford -- these cars won't be able to go anywhere. Instead, ProPILOT 3.0 activation will be limited to heavily mapped areas.

Tech: Similar to other carmakers, ProPILOT and its variants will rely on radar, digital cameras and hi-def mapping technology. A Nissan representative admitted detailed maps will be required for ProPILOT 3.0, which indicates it, too, will be limited in its operational availability.

Tesla

2015 -- Level 2: Late last year, Tesla unveiled the now infamous Level 2 Autopilot system, which is part of a suite of driver assistance systems. These include Autosteer, Auto Lane Change, Traffic-Aware Cruise Control, Side Collision Warning and Autopark.

Autopilot has been implicated in several crashes around the globe since its debut. Most notably, a Tesla driver died in a crash in Florida in early May with Autopilot engaged. He was reportedly watching a Harry Potter DVD while his Model S was operating in Autopilot mode.

Until Mercedes released its DRIVE PILOT, Tesla's Autopilot was the only semi-autonomous system on the market that allowed for autonomous lane changes. This is not its only distinction, however. Autopilot is, from the driver's standpoint, the most robust system currently on the market. It's that robustness that's gotten Tesla in trouble, though. Specifically, Consumer Reportsrecently called on Tesla to disable and rename Autopilot until it was made safer.

Notably, Autopilot is in public beta-testing. In fact, Tesla is the only carmaker beta-testing autonomous tech on the public. All other carmakers relegate testing to its engineers and employees.

Tech: Radar and a digital camera

2018 -- Level 4:Tesla won't confirm the exact timing of Level 4 autonomous driving or the tech sensors that will drive higher levels of autonomy. However, Elon Musk said in late 2015 that Tesla was two years away from full autonomy.

Tech: It's unclear what tech Tesla plans to add to its cars to enable higher levels of autonomy. That said, during the launch of Autopilot, Elon Musk revealed the company was gathering hi-def digital maps with its cars. Likely, this information will be utilized to implement Level 4 autonomy in future products.

Volvo

2016 -- Level 2: For the 2017 model year of its vehicles, like the S90 sedan, Volvo introduced Pilot Assist, a semi-autonomous driving system. Like other Level 2 systems, it follows traffic ahead and keeps the car within its lane.

Though, that's not the only news Volvo has made in the autonomous driving realm for 2016. This summer, Volvo Cars and ride-sharing service Uber announced they were joining forces to develop autonomous driving cars.

2017 -- Level 4: Intriguingly, Volvo is skipping Level 3 and going straight to Level 4. However, it won't be a broad-scale rollout -- at first. The Swedish carmaker has said it will have 100 autonomous cars testing on the roads Beijing in 2017, as a part of its Drive-Me program. Volvo plans to lease autonomous cars to customers in Sweden that year as well. These cars will be Level 4. Like so many other brands, the first autonomous Volvo cars will only be able to drive themselves on highways known to the GPS system. This is more than an estimation, though. Volvo has already detailed what its self-driving car interface will look like.

What's more, Volvo has even announced that it is working with Ericsson to stream your favorite shows in the dashboard of your autonomous car in the future. Think its Concept 26 autonomous car interior, but in real life.

2020-- Though Volvo will be achieving Level 4 before 2020, it has made two projections for that year. First it aims to offer autonomous cars to the public -- likely on a broader scale than in 2017. Secondly, and perhaps more importantly, it has long promised that no one will be killed or seriously injured in a Volvo by 2020. Of course, autonomy is the ultimate safety system. So, it makes sense that large-scale implementation of Level 4 autonomy would intersect with that safety benchmark that year.

Tech: Volvo has openly listed the tech that will allow its future cars to drive autonomously. They include (but are not limited to) trifocal cameras, ultrasonic sensors, surround view cameras, surround radar, long-range radar and combined radar and camera units.