The innovation of the green cabs, which have meters, was meant to help solve the problem of an unequal distribution of taxis in the city, with so many concentrated in Lower Manhattan and Midtown and so few available elsewhere. Its ancillary effect is a marginal disruption of the feudal system that brings enormous wealth to owners of yellow-taxi fleets, in possession of medallions that now cost more than $1 million apiece, as drivers themselves struggle to make any sort of reasonable living. The price of a permit for a green cab is $1,500 for three years, making it possible for drivers themselves to own them and ultimately profit from any appreciation.

In July, Ebony Davila, who had been a livery driver in the Bronx for several years, received her permit for a green cab. She began driving it on Sept. 5 and within 10 days she had covered the cost of her permit and some additional fees, she told me. Increased passenger volume has meant that she has been making more money than she did in the past (she can still take flat-fee dispatch calls through a service), but her experience indicates how difficult the life of a driver in New York remains; the mother of a 9-year-old son, she clears about $840 a week after fees and leasing costs, putting in 10-hour days Mondays through Thursdays and working between 5 p.m. and 2 a.m. on the weekends.

One morning last week, I accompanied Ms. Davila as she drove on East Fordham Road toward the Grand Concourse in the Bronx and through Washington Heights and other quadrants of Upper Manhattan. (On the West Side, green-cab drivers are limited to picking up passengers north of 110th Street, and on the East Side, to the area north of 96th Street.) Midmorning, activity was minimal. In the two hours I spent with Ms. Davila, she picked up one passenger, in Morningside Heights, who was going to East 72nd Street.

Green cabs, in some sense, offer another lens into the vast differences between Manhattan and so much of the rest of the city. Doing business in the Bronx is particularly challenging, Ms. Davila told me, in part because so few customers have credit cards and because so many are accustomed to negotiating flat fees for livery service. This has left Ms. Davila to focus her efforts in Harlem, the area around Columbia University and Inwood, where she has done well.