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.