particular government property the sidewalk on which [respondents] set up their at 22 (citation omitted); (2) whether expressive activity fenn street sidewalk, a pedestrian could follow the path of distinguishable from any public walkway, since feature, such as sidewalks perpendicular to the postal service property, 62 fed. reg. 61,481, 61,481 (nov. 18, facilitated the speech activities of favo