There’s only two entrances; the R St one off Bladensburg is much more relaxing. Bladensburg south of there, towards H St., is bad but has sidewalks. Another option is to use West Virginia (from the SW) or 18th St NE (from the N) to get to Montana Ave, which crosses Bladensburg at a light and becomes S St, just one block from the arboretum’s R St entrance. Montana itself is an ugly near-highway, but you’d only be on the street for 1-3 blocks.
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