SAN FRANCISCO — A fare war between Uber and Lyft has led to billions of dollars in losses for both ride-hailing companies as they fight for passengers and drivers.

But in one way it has been good for investors who snatched up the newly public companies’ stock: The losses have scared off the competition, giving the leaders a duopoly in almost every American city.

The two San Francisco companies have already lost a combined $13 billion. And with no clear road to profits ahead, no one else has much of an incentive to mount a challenge using the same model that relies on human drivers.

That allows Uber and Lyft to continue offering low fares to grow, which will be good news for riders. But it figures to test the patience of investors.

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