There's a lot of design elements that we take for granted, and that's intentional! But a really simple mental exercise is working backwards, break down the elements, and ask yourself how you would approach it.
As a first example, take traffic signs. They're simple, they're ubiquitous, while almost universally understood. But how did they achieve all of that at once? Let's break it down:
Simple shapes! The dimensions should be appropriate for each situation. The thinner, taller signs are meant for drivers who are closer to the sign, and the environment may have limited space. Avoid repetition so people could recognize a silhouette easily--a Stop sign should NOT be confused with a Speed Limit sign, which should NOT be confused with a Yield sign.
Colors! While deceptively simple, there are a ton of factors to consider. Try to limit the palette to 3 colors or less, 2 main colors are optimal. Colors should HEAVILY contrast each other: A No Right Turn is distinct from a Right Turn Only sign because of the bold red circle-slash going over the arrow. Furthermore, the colors should be easily distinct in most kinds of lighting, hence the stark white background with bright red or dark black text.
Typography!* Ain't no sign should have cursive or comic sans, it's gotta be DISTINCT and BIG--note that all text is in UPPERCASE!! The font should be standardized & recognizable from a distance: The Speed Limit sign has a giant "50" that's twice of the "speed limit" text, and is always on the bottom half. Text should come secondary to shape & color, but it's also important.
Icons! This is technically a subsection to shapes and follow most of the same rules, but they have unique challenges. A Pedestrian icon is a distinct shape of a person, it can't be confused with a Truck icon! The hard part is using icons which are universally recognized to explain complicated ideas, which is more cultural context than literal. For instance, the Cancel icon on the No Right Turn is well understood now, but its design is a bit abstract. The same goes for the Arrow icon. They don't have one-to-one real-world counterparts, so its meaning had to be proliferated culturally.
Altogether, these design elements combine to make a traffic sign that a person only glances at for a couple seconds at most. It has to explain how the road works, what's coming up, what the rules are, and how other cars or pedestrians should behave. As much of an imperfect system it is, it's fascinating to take a couple minutes to think about the design challenges & solutions instead of letting it be invisible to you.
