When Lyft, the nation’s second-largest ride-sharing service, filed its IPO paperwork last week, it also warned of substantial risk, noting that it had lost more than $2 billion over the last three years alone. The critical step toward a profitable future, Lyft said, will be the introduction of fully autonomous vehicles that can eliminate the most costly part of its business: the drivers who operate its fleet of vehicles.

"We have incurred net losses each year since our inception and we may not be able to achieve or maintain profitability in the future,” the company said in its filing with the Securities and Exchange Commission.

Lyft could serve as a bellwether for the emerging mobility services sector, with Uber believed to be working up its own IPO plans. General Motors recently hinted that it would also like to go public with its self-driving vehicle subsidiary, Cruise Automation, as would Alphabet with its Waymo unit.

While still operating deeply in the red, ride-sharing services are expected to become a significant factor in personal transportation over the coming decades. The Boston Consulting Group has forecast that more than one-quarter of the miles Americans will travel by road in 2030 will rely on electrified, autonomous vehicles operated by ride-sharing services like Lyft.

That said, a number of recent studies have found widespread skepticism on the part of the American public — much of that centered around concerns about safety triggered by incidents such as the fatal crash last March involving an Uber prototype in Tempe, Arizona. Nonetheless, there are signs that potential users are warming to the technology.

“What I would expect to see is that once people see autonomous vehicles and ride one in a public arena we’ll see interest rise, and then their willingness to adopt the technology,” said Robert Lajdziak, a researcher with J.D. Power and Associates.

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One of the big questions is how quickly autonomous vehicles will come to market. Experts disagree, though there seems to be a retrenchment by some industry leaders, including Waymo Chief Executive Officer John Krafcik, who now indicate the effort may take longer than originally hoped.

Part of the debate centers around what the word “autonomous” actually means. In reality, there is a number of different levels. A number of manufacturers are already equipping their vehicles with advanced driver assistance systems, or ADAS, that can keep a car centered in its lane, automatically apply the brakes in an emergency, and even operate hands free for short periods on well-marked, limited-access highways.

The next step, “Level 3” autonomy in industry-speak, would allow a vehicle to operate under broader conditions hands free, though a driver would need to remain on the alert, ready to take control in an emergency. Tesla Chief Executive Officer Elon Musk last month said such a feature would be ready by the end of this year — though Tesla has repeatedly delayed the launch of that technology.

Waymo last December launched the world’s first autonomous ride-sharing service in the Phoenix area and plans to roll it out to at least 20 cities over the next several years, using modified Chrysler, Jaguar and Nissan vehicles. But they will, at least initially, require an “operator” to remain at the ready behind the wheel. Lyft, Uber and GM’s Cruise unit, among others, are testing similar technologies.

Eventually, the industry hopes to add even more advanced capabilities:

Level 4 autonomy would allow completely driverless operation, but such vehicles would be “geofenced,” able to operate only in well-mapped areas and only in good weather conditions.

Level 5 vehicles would be able to operate essentially anywhere and at any time, regardless of weather conditions.

Taking the driver out of the vehicle entirely would make it cheaper for people in urban and suburban communities to hail a ride than to own a personal vehicle, studies suggest. For companies like Lyft, the cost of operations would tumble sharply enough, the theory goes, that they could make substantial profits.

California, which has some of the strictest guidelines in the country, last month authorized testing of Level 4 vehicles on public roads. Companies must test autonomous vehicles to provide detailed reports on all “incidents,” including not only crashes but events where test vehicles had their automated controls disengage, requiring an operator to retake control. Other states, such as Arizona, have gained favor because they have less stringent testing requirements.

“It’s going to take a lot longer” than many have been forecasting before Level 4 and 5 vehicles are ready to go into commercial operation, said Sam Abuelsamid, a senior researcher with consulting firm Navigant. And it will be even longer before those vehicles are offered to retail customers because of the cost of the technology.

Nonetheless, Abuelsamid expects that fully driverless systems will eventually become an everyday norm and help transform the way we travel. The challenge for companies like Lyft, Uber, Waymo and others will be to build a sustainable business model until then.