Facts about traffic sign shapes include the fact that the circle is used exclusively for railroad advance warning signs and the octagon is solely used for stop signs. Another fact about traffic sign shapes is that the crossbuck is used only for railroad crossings.
Traffic signs with more sides usually correlate to more critical messages. This is why the traffic sign for railroad advance warnings is a circle; the circle has an infinite number of sides which symbolizes the extreme and numerous dangers associated with the collision of a train and a vehicle.
There are exceptions to this general principle. For instance, the trapezoidal sign that is used for recreational areas has more sides than the equilateral triangle yield sign.
The diamond shape is used for warning signs, and the pentagon is used for county-marker signs and school advance-warning signs. Vertical rectangles are used for regulatory signs while horizontal rectangles signify guide signs, temporary traffic signs and warning signs.
Pennants are used only for no-passing-zone signs. The shape of a pennant sign is also meant to represent an arrow instructing drivers to get back onto their side of the road. Any other shapes seen are usually used to indicate route markers.