The majority of alleyways have low traffic and infrequent repaving cycles, resulting in back roads with potholes and puddling that are uninviting, unattractive, and almost always inaccessible.
Whether in a downtown or residential context, alleys often serve freight loading and deliveries, trash collection, or other large vehicle access that would otherwise use main street curb space. During the short periods where large vehicles are present, the alley becomes inaccessible to pedestrians using mobility devices who have difficulty crossing the curb.
Alleys may contain overhead power or underground utilities, which require bioretention facilities to be set back to maintain access for utility companies.
The design of green alleys should strive to balance their necessary utilitarian features with their placemaking potential. Green alleys use sustainable materials, pervious pavements, and effective drainage to create an inviting public space for people to walk, play, and interact.
Construct green alleys with permeable pavements such as pervious concrete or permeable interlocking concrete pavers with high reflectivity to reduce heat island effects and provide stormwater treatment. Avoid locating permeable pavements in areas where waste or recycling containers may be picked up to avoid having debris deposited onto the pervious surface. In areas with closed depressions that result in puddling, assuming the underlying soils are feasible for infiltration, permeable pavements could be used without having to install storm collection structures and an extension of the storm drain main.
Bioretention facilities can bring greenscape to the alley and create gathering spaces adjacent to the planters. This may require regrading the alley to direct flow that is usually down the center of the alley to a channel along the side and into the bioretention planter.
Bioretention facilities may need to be lined to prevent groundwater migration into adjacent structures and underground utility trenches. Review the condition of adjacent structures during the survey and planning process, such as presence of basements and utility corridors.
Intersections between alleys and sidewalks have intentionally short sightlines, and should be designed to minimize the speed of motor vehicles (if permitted). Raise the intersection or the entire alley to the sidewalk grade and optionally use textured pavement to further reduce speed. Motor vehicle traffic in the alley approaching the sidewalk is typically controlled with a STOP sign (MUTCD R1–1).
Where permeable pavement or bioretention is not suitable, direct the alley drainage to a bioretention facility located in the planting strip on the downstream street.
|Potential GSI Features
|Property Line / Adjacent Properties
To foster a safe, inviting street environment, green alleys should have human-scale lighting and clear sightlines to the street. Use light fixtures that focus their illumination toward the ground and minimize light pollution.
Alley greening and bioretention maintenance may be initiated and carried out by residents or neighborhood associations, or coordinated with city maintenance schedules.
Alleys provide direct property access and eliminate the need for driveways along main roads where people are walking and biking. Consider the use of alleys in all new developments and renovations to existing properties.
Green alleys have different maintenance responsibilities than conventional alleys. Use of textured pavements and other materials may be challenging to existing street sweepers and snowplows. Similar to shared streets, alleys may benefit from the application of snowplow compatible materials and provisions for maintenance equipment access. Partnering with local businesses that are responsible for clearing any existing sidewalk of snow is often the best way to ensure prompt first-round snow clearance.
Freight vehicles may use green alleys for loading and unloading, which reduces double-parking on neighborhood streets. Low mountable or flush curbs provide pedestrians access around large vehicles.
Vehicle traffic can be restricted during non-delivery hours for outdoor seating or other uses. Where vehicle access is permitted, alleys should be designed for very low speed movement with adequate space for delivery trucks and other freight vehicles. In narrow green alleys, permanent street furniture is minimal, allowing easy access by people and vehicles to any door, storefront, or loading dock. On wider green alleys, bollards and other street furniture and designated loading zones may be desirable; furnishings should not block loading paths. Since freight is often conveyed on hand trucks or cargo bicycles, careful attention should be paid to the location of curbs, if applicable, and access paths from loading zones to building entrances.