The large valuations of Uber Technologies Inc. and Lyft Inc. may be justified, if Goldman Sachs is right about the ride-hailing industry ballooning to $285 billion by 2030.

The estimate from Goldman’s analysts uses the taxi industry as a base model, and sees ride-hailing overtaking taxis in most global cities. As ride-hailing companies turn to autonomous fleets, the analysts see the need for drivers disappearing, but not necessarily lowering costs for consumers.

Goldman estimated the base case for the market by focusing on San Francisco, the hometown of Uber and Lyft, where they say ride-hailing is more than four times the size of the taxi market. As ride-hailing companies like Uber Technologies Inc. and Lyft Inc. continue to grow, they expect other top tier cities, including New York, London and Tokyo, to reach San Francisco’s level of ride-hailing usage by 2030.

That should boost the overall ride-hailing market to $285 billion in the next 13 years, while the taxi market shrinks to a little as a quarter of the size.

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Right now, they estimate the global taxi market is worth $108 billion, which is triple the size of the $36-billion ride-hailing market. At the same time, they calculate an average of 15 million ride-hailing trips a day globally, which they expect to increase to 97 million by 2030.

That would translate to $65 billion in revenue for the ride-hailing companies by 2030, based on the assumption that the companies would take a 23% commission from gross market sales.

Uber is the most highly-valued private company, with a current valuation of $68 billion, according to The Wall Street Journal, which would make it more valuable than some Dow Jones Industrial Average DJIA, -0.87% components, such as Caterpillar Inc. CAT, -0.96% and Travelers Companies Inc. TRV, -0.87% . Lyft commands a valuation of $7.5 billion. Neither are profitable, partly because the companies spend money to attract drivers and subsidize rides.

Retaining drivers and growing the driver network has been paramount for the companies’ growth. But the relationship has been fraught, as drivers have argued that they’re not independent contractors and have asked for tipping options on Uber. Recently, Uber drivers were angered after the company revealed that it owed its drivers millions of dollars due to an accounting error.

As autonomous vehicles enter the equation -- with Lyft pairing up Waymo and General Motors GM, -1.31% and Uber developing cars with Daimler and researchers at Carnegie Mellon among others -- the analysts see the cars displacing approximately 6.2 million drivers.

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That would cut out the cost of driver subsidies for the companies, but the analysts don’t anticipate those savings translating to cheaper rides for consumers. They argue that the companies or manufacturers will likely take on additional expenses coming from the cost of operating fleets, and have the potential of facing additional taxes as authorities look to recoup funds lost from the taxi industry.

That doesn’t necessarily mean the costs would fall on Uber or Lyft, but on whomever manages the fleet. The analysts point to Daimler, in the Uber and Daimler partnership, for example, saying that ride-hailing companies will wish to remain “asset-light.” However, providing the autonomous fleet may be the biggest revenue generator in the new market, bringing in an estimated $220 billion by 2030, they say.

Other experts have argued that autonomous cars will eventually bring down the cost of each ridesignificantly, with the expectation that Uber and Lyft would lower fares as the cars can be on the road 24 hours a day, unlike cars with drivers.