Lining the arterial road system of the country, road signs are a crucial component of keeping traffic moving and automotive occupants safe. Many of the signs come in different shapes, from triangles to octagons, to denote different meanings. Here’s why.

When the state of Mississippi opted to institute a uniform standard for road signs in 1923, they took issue with the white square Stop sign that was currently in use. Officials with the highway department instead suggested that signs use shapes to denote the level of danger ahead. A Yield sign, for example, could have three sides. A Stop sign could have eight. If a sign was informational in nature, it could be a simple square.

The system, which is based on the belief that our brains will recognize more complex shapes as something worth paying attention to, has endured. Simple shapes, like a rectangle on a street sign, don’t carry urgency. A Stop sign does. In fact, the Department of Transportation’s Manual on Uniform Traffic Control adheres to a philosophy on shapes. According to the DOT, a triangle can represent instability and conflict—perfect for Yield signs. Circular signs are more often found in signs representing safety and security, like community-based signs.

The DOT has standardized signs. A yellow diamond shape almost always indicates a warning like merging traffic ahead or an added lane. Service signs like gas or lodging are typically square. Regulatory signs are usually a complex shape colored in red.

Combined with colors, shapes allow road signs to be quickly understood. Whether our brains recognize more complex shapes as a rule or because we’ve simply learned via exposure to road signs, the end result is the same. A circular Stop sign just wouldn’t have quite the same effect.

[h/t Reader’s Digest]