A side street is defined on dictionary.com as a street leading away from a main street; an unimportant street or one carrying but little traffic. An alley is a a passage, as through a continuous row of houses, permitting access from the street to backyards, garages, etc.
Both of these items can be compared to a stack. When I think of an alley, I think of a dark scary place with criminals (outcasts of society, kind of) lurking in every shadow. This may not be an accurate conception, but an alley is a place for hiding things that no one wants to see or have seen. Alleys are home to secondary buildings used for storage, and used to be outhouses. As we walked along the alleys, we passed many trashcans and every spare inch of ground filled with gravel and used for parking. Along the alley ways we came upon sewer vents that are not found on side or main streets, which makes another point for the stack comparison. When you are walking along an alley, most of the time you feel somewhat confined because of the fences residents put up, creating a front between their land and the access way. The fences represent their turf as well.
Side streets can be thought of in the same negative light if in a certain context, but my first thoughts when I hear the words 'side street' are of suburbs and children playing in the streets of their neighborhoods. Side streets are connectors; they connect one street to another, and one family to another. Without side streets where would parents teach their children to learn how to ride a bike? and where would those kids then practice after taught? If you think about it, side streets can be an escape from the busy world. When someone is on a big road that only serves the purpose of transporting traffic from one side of town to the other all they are focused on is getting where they are going. When you turn off of that very street onto one of its side streets you are immediately absorbed into a slower pace and calmer atmosphere. The scenery changes from shrubbery and directional signs to houses and garages. Side streets can be considered beats, but I do not think of them as hectic interchanges.