When a rider takes an UberPool, Uber matches them with other riders on similar routes. An UberPool can have two users per ride, but each can bring one person along with them for a total of four passengers.

In a blog post, Uber said that UberPool helps “materially reduce the cost for riders” and “cut mileages and emissions.” Passengers receive a discounted fixed fare for the inconvenience of sharing a taxi with a stranger who might take your ride slightly out of your way. According to Uber, UberPools should be 25% less expensive than Uber’s cheapest private taxi service, UberX.

Uber has a checkered recent history in the U.K., including a long-running court fight over the legality of its app. In October, it won its court case, meaning that it did not have to change its service to comply with existing rules that govern London’s official black cabs. However, Uber still faces a ruling from London’s transport regulator which could decide to enforce additional rules in the British capital.

UberPool has been expanding rapidly in recent weeks, adding Chicago, Miami, and Bangalore, India to the cities where it is available during the past month. London also joins other major metropolitan areas such as San Francisco, New York, Paris, Los Angeles, and Austin, Texas.

Uber’s primary ride-hailing competitor, Lyft, has a similar car-sharing feature it calls Lyft Line.

Where UberPool is available, it appears to be a popular option. Uber CEO Travis Kalanick said in June that half of all Uber rides in San Francisco are taken on UberPool. An Uber blog post said that the company has had “millions” of UberPool trips and that “thousands” of users use the service to commute five times a week. Having a healthy number of users is critical for UberPool, because the more passengers there are the more likely UberPool is to find two routes going roughly the same direction to combine.

Kalanick has called UberPool a “major innovation” and has directly tied the product to Uber’s mission statement to make “transportation as reliable as running water.” It’s Uber’s least expensive service, and the company hopes that reduced prices means that riders will opt for it more often than they would more expensive Uber services or traditional taxis. One of Uber’s goals is that its services, including UberPool, could one day be a substitute for car ownership. “As more people in more cities use UberPool, it will help contribute to the future that Uber has already begun to create: fewer people owning cars, and fewer cars on the road,” Kalanick told Uber employees in June.

Uberpool “ultimately takes cars off the road by making the need to own your own car a thing of the past,” the company said in a blog post announcing the London service.

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