What is defined as a road not used primarily for through traffic but provides access to rear entrances?

The correct choice is defined as an alley, which serves as a narrow passageway that typically runs between or behind buildings. Alleys are primarily designed for local access rather than being main thoroughfares for heavy traffic. They often provide service access to the rear of properties, allowing for deliveries, parking, or other activities that do not involve the main street.

In an urban environment, alleys help maintain the aesthetic and function of main streets by keeping traffic and access to rear entrances less disruptive. While other options like a side street may also offer access to residences, side streets usually connect to main roads and serve through traffic to a greater extent than alleys do. Main streets are intended for significant volumes of traffic and primary access, thus lacking the specific functional definition focused on rear access. Backroads typically refer to less traveled rural routes that may not fit the urban context described by the question.