I learned today that in the US, according to federal regulation, the only traffic sign that is allowed to be round is the “Railroad Crossing Ahead” sign.

The “Do Not Enter” or “X is prohibited” signs are not round! They are white rectangular signs with round images.

Edit (5/29) - Why is this the only round sign? According to Richard C. Moeur’s Manual of Traffic Signs:

The greater number of sides on a sign, the more critical a message it conveys. This is why a circular shape was adopted for the Railroad Advance warning sign (i.e. infinite number of sides), as a collision between a train and a typical vehicle will nearly always end badly.