How far along is self-driving technology? When can consumers -- and investors -- expect fully self-driving vehicles to come to market?

Those questions are on the minds of a lot of investors these days. But they're not simple to answer. It turns out that the answers to those questions depend on what you mean by "self-driving," "consumers," and "come to market."

What we mean by "self-driving" -- the levels

In articles about self-driving technology, you often will see references to "Level 4" or "Level 5" systems. Those levels refer to vehicle-automation standards set by SAE International -- the professional association of automotive engineers -- and are widely accepted around the world.

You can learn more about the levels of self-driving technology here. But for our purposes, here's what you need to know:

Level 2 is an advanced driver-assist system, one that can control both steering and braking under certain circumstances, but isn't considered self-driving.

Level 3 is a system that can actually drive the car, but only under limited circumstances. Level 3 systems require an alert human driver who can take over on short notice.

Level 4 is "high automation." In practice, that has come to mean a system that can do most of what a human driver can do, but only in a limited (well-mapped) geographic area.

Level 5 is full-blown autonomous driving: The car can go anywhere and do anything an experienced human driver could.

There's one more critical fact to know: The levels are set by the manufacturer of the systems -- there's no third-party authority evaluating them -- at least not yet.

Obviously, a Level 5 system is much more advanced than a Level 3 system. So when will these different levels come to market, and how?

Levels 2 and 3 are here now

General Motors' (NYSE:GM) new Super Cruise highway system and Tesla's (NASDAQ:TSLA) current Autopilot are borderline Level 3 systems. Both drive the vehicles under limited circumstances, but require a human driver to be ready to take control.

But both GM and Tesla have shied away from applying the Level 3 label to their systems, because Level 3 is controversial: It's the first level that implies the car can actually drive itself. And the systems raise a big question: How quickly can a distracted human turn to take control of the vehicle in what might be a sudden emergency?

The questions have led some companies, including Ford Motor Company (NYSE:F), to say that they will skip Level 3 and work toward Level 4 instead. But others are pressing on. Audi will soon introduce a highway-driving system on its 2019 A8 sedan that it's calling a Level 3 system. Is it more advanced than Autopilot or Super Cruise? It's hard to say yes: Frankly, on paper, it sounds similar to Super Cruise. But Audi gets to call it Level 3 if it meets the standard.

Level 4: Coming soon, but to which market?

Several companies (and groups of companies) have Level 4 systems under development, and at least a few are expected to become available in some way by 2020. Notably, a partnership that includes Intel and giant auto-industry supplier Delphi Automotive have promised to have a Level 4 system available to automakers by the end of 2019. Cars using that system could start appearing on the roads a year or two later.

Some could appear sooner. GM's first Level 4 car is nearly ready for production, but GM isn't planning to sell it to consumers. Instead, it'll go into service with ride-hailing companies like Lyft. You may be able to ride in a Level 4 car a few years before you can buy one.

Other first-to-market possibilities include Tesla, which could offer a Level 4 system before the end of the decade, and Daimler AG (OTC:DDAI.F), maker of Mercedes-Benz vehicles and heavy trucks. But it's likely that most buyers won't be able to buy a Level 4 vehicle until 2021 or 2022 when the systems begin appearing as options on cars, trucks, and SUVs aimed at consumers.

Level 5: A big challenge

Bryan Salesky is the CEO of Argo AI, a self-driving software company partly owned by Ford. Salesky is a veteran of the Google Self-Driving Car Project (now known as Alphabet (NASDAQ:GOOG) (NASDAQ:GOOGL) subsidiary Waymo), and a legitimate expert on the state of self-driving technology.

In a recent Medium post, he had some sobering thoughts for those hoping to see fully autonomous vehicles become widely available soon:

We're still very much in the early days of making self-driving cars a reality. Those who think fully self-driving vehicles will be ubiquitous on city streets months from now or even in a few years are not well connected to the state of the art or committed to the safe deployment of the technology. For those of us who have been working on the technology for a long time, we're going to tell you the issue is still really hard, as the systems are as complex as ever.

There are several different groups working on full-blown Level 5 technology. But whenever I talk to experts, their answers are similar to Salesky's: A Level 4 car that can work in a mapped area in good weather is feasible in the near term, but unless someone makes a big breakthrough, full Level 5 driverless cars are much further way.

Summing up: How far along are self-driving cars, really?

Here's what we've learned:

Late 2018: You may be able to ride in a Level 4 vehicle via a service like Lyft in a major city.

2021: Vehicles with Level 4 technology might be starting to arrive at dealers for sale to consumers. But at first, it will likely be an expensive option limited to luxury cars.

By 2024 or so: Level 4 will probably be widely available, at least as an option, on many mainstream vehicles. By then, Level 4's geographical limitations may not be a concern, as the 3D maps will have expanded to cover most of the developed world. But the systems still may not work well in severe weather.

??? years from now: The first true Level 5 vehicles will start to appear.

Long story short: Self-driving cars -- in the Level 4 sense -- are indeed coming soon. But they'll be limited for a while yet, and cars that can do anything a human driver can are probably much further away.