Use low plantings in bioretention facilities and landscaped curb extensions near intersections to maintain sight clearance; plants should grow no higher than 24 inches above the sidewalk grade.
The width of a curb extension varies with street type. Curb extensions are typically recessed 1–2 feet from the outside edge of the right-most travel lane, though width may be tailored to accommodate emergency or large vehicle access, or other existing conditions.
The curb return from bump-out edge to original curbline should be designed to enable street sweeping along the curb edge, typically angled between 30 and 60 degrees relative to the curbline. Steeper return angles will usually require hand-sweeping.
Design inlets and outlets to resist incursions by vehicles and bicycles, as motor vehicle wheels may be prone to enter, especially during parking maneuvers. Metal lids are an effective design strategy to block vehicle entry.
Inlets should include a presettlement treatment, especially along the curbline where runoff flows straight from the gutter.
No lateral offset distance is required along the edge between the curb and bioretention area, as parking is not permitted adjacent to the curb extension. An 18 to 24-inch level area may be included along the curb to ease maintenance access from the street.
Utility corridors often determine where cells can be installed. Utilities may be sleeved, bioretention cells can be set back, or curtain liners may be integrated into design to prevent loading of infiltrated runoff onto subsurface infrastructure.
The intersection corner area may be used for plantings or even bioretention area, though designers must carefully balance project goals and uses. Corner plantings increase the lateral offset of crosswalks, directing pedestrian paths around planted area and potentially decreasing visibility for turning vehicles.
Locating curb extensions in areas where on-street parking is already prohibited, such as near fire hydrants or driveway setbacks, can enable GSI implementation without affecting parking or curbside access. Ensure emergency responders have necessary access to utilities at all times.
Walled bioretention cells may incorporate seating and furniture around edges.