It's a Surveyor's stake not a utility marker. "R/W" stands for Right of Way. Right of Way is a legal term for property owners. Meaning, although you don't own the property...the sidewalk area...you and your neighbors have a legal right to use it to get to/from your property...aka easement.
It may seem silly to identify a sidewalk. All these property lines and coordinates are drawn on a map and filed with the city or county. All these lines and coordinates, however, just are not identified in the real world, so a surveyor needs to temporarily drawn them for construction.
Utility markers are painted onto the ground. Different colors mean different utilities. Orange...I think...may be for communications. Your HOA may be laying another sidewalk or something. Any picnic areas back there?