This is the part of the PRP clarification I like to highlight:
Basically, if it seems like it’s on PRP, it’s ineligible. So, for instance, even if there is a mural on a back fence or garage in the alley but it’s part of PRP, it’s ineligible. Alleys are also typically meant for vehicle traffic in most residential areas, and that could possibly be seen as an unsafe pedestrian area.