n. The walkable area that surrounds a particular location, such as a retail center or transportation hub.
A High Activity Zone should serve a walkable ped shed with a 1/4 to 1/2 mile radius. If your downtown or other urban district is larger than that, you should have a series of High Activity Zones spaced 1/2 to 1 mile apart, each of which serve the surrounding area that is within a 5 to 10 minute walk.
Sub-urban parks will often include concrete or asphalt paving with comfortable widths of at least 5-feet. Playgrounds, pavilions, kiosks, signage and other features are common in such parks. Typically, sub-urban parks are meant for the ped-shed (adjacent area easily walkable) and therefore do not include off-street parking spaces.
The Beltway is irrelevant to the walkability question. If you add the mix of uses (i.e., housing) to the already-grid-ready layout of the Boulevard site, you've taken care of the entire ped shed in that direction from the Largo Town Center Metro. Everywhere else within the ped shed of that Metro station is or can easily be made as walkable as downtown Silver Spring.
Walkable catchments, sometimes referred to as "ped-sheds", are maps showing the actual area within a five minute walking distance from any centre, or 10 minutes from any major transport stop such as a railway station.