The side of the road with the single lane is always marked with this road sign.
I have been in a car many a times with different people and upon seeing this sign have heard the comment, "Well, duh, isn't that obvious." And I always find myself explaining the following:
The sign means that if you are in the single lane and the opposing lane has the extra passing lane, you may only pass if there are no cars in either of the opposing lanes. In other words, if there is a car in the far left opposing lane but the opposing lane closest to you is open, you may not use the open lane to pass because there is still opposing traffic present.In sum, the context of where the sign appears is what makes the message on the sign not quite as obvious as one would think.