Double-checking to make sure
People normally don’t need signs to explain what another sign means, but here, things are different. In the USA, we have signs explaining what signs mean, just in case you don’t understand. For example, people can easily misinterpret this red, octagonal stop sign.
Stop means stop, not slow down enough to see whether there are cars coming from other directions. This is what’s known as a California stop and is grounds for police to give you a ticket. We only wish there was another sign explaining the second one, and possibly another one after that for good measure.