So at a couple of different places along my commute I’ll pass a yellow diamond sign, usually a cross street or curved road warning, with a speed listed below which was also on a yellow sign. I had no idea what these meant (no one else seems to slow down) and every time I’d see one I’d think “I should probably look that up when I get home”. Well, after a month and a half, I’ve finally remembered to do just that.
These sign warn of upcoming potential hazards and give a recommended safe speed to switch to. Most relevantly, you cannot be penalized for choosing not to follow it.