Alley may be paved, or unpaved, and a blind alley is a cul-de-sac. Modern urban developments may also provide a service road to allow for waste collection, or rear access for fire engines and parking. Because of geography, steps (stairs) are the predominant form of alley in hilly cities and towns. This includes Quebec City in Canada and in the United States Pittsburgh (see Steps of Pittsburgh), Cincinnati (see Steps of Cincinnati), Minneapolis, Seattle, and San Francisco as well as Hong Kong, Genoa and Rome. Some alleys are roofed because they are within buildings, such as the traboules of Lyon, or
Alley many no wider than a truck, and typically flanked with brick houses. A typical house on these alleys or lanes is called a Philadelphia "Trinity", named because it has three rooms, one to each floor, alluding to the Christian Trinity. These alleys include Willings Alley, between S. 3rd and S. 4th Streets and Walnut and Spruce Streets. Other streets in Philadelphia which fit the general description of an alley, but are not named "alley", include , , Phillips Street, South American Street, Sansom Walk, St. James Place, and numerous others. Steps, Pittsburgh's equivalent for an alley, have defined it for