Sign: bicyclists don't ride on the sidewalk
I'm not sure how effective they will be, but there is definitely a need for signage about cyclists and sidewalk riding.
In the area where I saw the signs, it isn't illegal to ride on sidewalks.
The problem is that many cyclists riding on sidewalks ride faster than pedestrians walk, and when in "mixed traffic" cyclists should ride "as if they are guests of the pedestrians" meaning at or slower than walking pace.
Cyclists riding on sidewalks significantly faster than walking pace are dangerous.