Uber just discovered a new activity to integrate into its app: walking.

Yes, using your feet is an essential ingredient in Uber Express Pool, a new option users in select cities will begin seeing in the Uber app. Similar to standard Pool, Express Pool combines up to three individual rides into one, but instead of forcing the car to zig-zag across blocks to make three separate pickups, the app asks riders to walk 1-2 blocks so the car can stay on a central, straight path.

If this sounds eerily like the bus, you're onto something, right down to the cheaper fees. Uber says the rates for Express Pool will be up to 50 percent less than those for standard Pool, and 70 percent cheaper than its main service, UberX. So if an X ride costs, say, $15, Pool would be about $9, and Express Pool would be $4.50.

Those aren't quite bus rates, but they're pretty close. But Uber insists it's not trying to encroach on the domain of public transportation.

"There are no fixed stops, no fixed routes, no fixed schedules -- everything in this is dynamic," says Ethan Stock, director of shared rides at Uber. "You're getting a custom route and custom trip for you. There are corridors and routes where it makes sense to put a 60 passenger vehicle on that and drive it back and forth in a straight line. That's very much not what we're trying to do here."

Uber says Express Pool solves the main pain point of standard Pool -- the wild inconsistency of travel time, which happens when you get matched with someone who takes the driver hugely out of the way. Express Pool all but eliminates the problem, first by by keeping the driver on a generally straight path, and second by increasing the time it takes to make a match: when you call for an Express Pool, the app will take 1-2 minutes to find your best matches in the area.

"What we're seeing is the total trip time is less for Express Pool than it is for Pool because the quality of matches in that initial matching window means you get much straighter, high-quality routes," Stock says.

By contrast, standard Pool would try to find your match in more or less real time, limiting the number of passengers it could group with you. Giving the app more time means Uber's AI-driven matching tech can put more riders into the mix, upping everyone's chances of a smooth ride. Express Pool cars can take up to three riders, and they can all have different destinations, although, again, you may be asked to walk 1-2 blocks so the car can stay on a straight path.

Of course, all this speaks to the kind of customer for Express Pool: Riders who aren't in a hurry and are extremely budget-conscious. That might include all Pool users, were it not for one thing: the walking. Uber will be keeping both options in cities where Express Pool is launching since the company acknowledges there will always be customers who are unwilling or unable to use their feet.

From the driver's perspective, Express Pool works almost exactly the same as Pool: They get paid based on the distance of the ride, not the number of riders. Drivers can opt into whatever Uber services they choose (provided their vehicles and equipment meet the requirements), so they don't haveto offer Uber Express Pool, but they'll obviously have more opportunities for rides if they do.

Uber piloted Express Pool in Boston and San Francisco, and the company says feedback from riders was positive, so it's now launching officially in Denver, Miami, Los Angeles, Philadelphia, San Diego, and Washington D.C., as well as the two original cities. More cities, including New York, will come later.

"We built a completely new back end for how we match drivers and riders," says Stock. "New York was a prototype of the walking aspect of this early last year, and it was built on the prior generation of technology, so it's going to take us some extra time to make sure that we land this well in New York."

Uber's new service definitely rubs against public transportation more than anything it's done before, though as low as the rates are, it's still not as cheap as a subway ride. It's also far from the first car-sharing service to integrate walking for a better rate -- that's pretty much Via's whole business model.

Still, Uber's dominant market share in the U.S. gives it an advantage in a product like Express Pool, which only works as well as there are riders close to you also in need of a ride. By encouraging them to use their legs, Uber is offering something most of us would stand up for anyway: cheaper rides.