The Central American squirrel monkey (Saimiri oerstedii) is designated a vulnerable New World monkey. It doesn't seem to be so uncommon in the places it lives. It's just that there aren't many places where it does, actually, live.
Steve, Michael and I ran into a small machination of the monkeys on the Osa Peninsula in Costa Rica a couple of years ago. They were jumping through the trees right along the single dirt road that winds through the peninsula.
The monkey-crossing sign was actually photographed at the Wilson botanical area closer to the Panamanian border. You have to love a place that troubles itself about such things.