CHICAGO, Nov. 25 – If this were any other city, perhaps it would not matter what kind of roadway was underfoot in the back alleys around town. But with nearly 2,000 miles of small service streets bisecting blocks from the North Side to the South Side, Chicago is the alley capital of America. In its alleys, city officials say, it has the paved equivalent of five midsize airports.

Part of the landscape since the city began, the alleys, mostly home to garbage bins and garages, make for cleaner and less congested main streets. But Chicago’s distinction is not without disadvantages: Imagine having a duplicate set of streets, in miniature, to maintain that are prone to flooding and to dumping runoff into a strained sewer system.

What is an old, alley-laden city to do?

Chicago has decided to retrofit its alleys with environmentally sustainable road-building materials under its Green Alley initiative, something experts say is among the most ambitious public street makeover plans in the country. In a larger sense, the city is rethinking the way it paves things.

In a green alley, water is allowed to penetrate the soil through the pavement itself, which consists of the relatively new but little-used technology of permeable concrete or porous asphalt. Then the water, filtered through stone beds under the permeable surface layer, recharges the underground water table instead of ending up as polluted runoff in rivers and streams.

Some of that water may even end up back in Lake Michigan, from which Chicago takes a billion gallons a year.

“The question is, if you’ve got to resurface an alley anyway, can you make it do more for you?” said Janet Attarian, the project’s director.

The new pavements are also designed to reflect heat from the sun instead of absorbing it, helping the city stay cool on hot days. They also stay warmer on cold days. The green alleys are given new kinds of lighting that conserve energy and reduce glare, city officials said, and are made with recycled materials.

The city will have completed 46 green alleys by the end of the year, and it has deemed the models so attractive that now every alley it refurbishes will be a green alley.

“It is now business as usual,” Ms. Attarian said.

But all these improvements come with a cost, and some people around Chicago have begun to wonder if a city that hardly recycles its trash and has a hard time keeping its trains and buses running should be spending money on fancy alleys.

Judy King, putting all her household refuse into one bag on Tuesday and tossing it into a bin in a green alley, said: “How do you decide where your priorities are? It’s a hard one. I’m bothered that there isn’t more recycling.”

The city has lately begun having serious talks about a comprehensive recycling program to replace the uneven guidelines now in place. But beyond recycling, it has a vast array of “green initiatives” that put it at the forefront of environmentally conscious cities.

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