You turn into an alley behind a row of shops. Unless a sign says otherwise, how fast may you drive?

Why

An alley is the handbook's narrowest class of road: any road no wider than 25 feet used to reach the rear or side entrances of buildings or properties. The limit inside one is 15 mph. Watch the two numbers that live next door. Twenty-five mph is the limit in a business or residential district, which is the street the alley opens onto, so it is the speed you are most likely to carry in with you by mistake. Ten mph is real as well, but it belongs to passing a bus, streetcar or trolley stopped at a safety zone or a traffic light. One permission worth knowing here: driving on or crossing a sidewalk is allowed when you are entering or leaving a driveway or an alley.