Watching these workmen this morning, as I sit here waiting for the bus, it strikes me that users don't need to know why a design works to know how or when to use it.
So these guys are working on the road verge, essentially in the bike lane, while traffic whizzes past at morning-rush-hour speed. To make the situation even more dangerous, they are often crouched down low and are located on the crest of a hill.
As is likely dictated in their OH&S policy, they have the witches hats out - and I'm sure they feel slightly more comfortable as a result. But, I'm thinking "They only have a few out. Wouldn't you want to close that bike lane out sooner, like over the other side of the crest?"
Then it strikes me that the physical shape (not to mention colour) of the witches hat allows it to be seen from behind small crests. Being a little over a foot high means that it sticks up above the hill and is more visable from a distance on gently undulating road surfaces.
It could have been designed more cheaply like sports field markers, which are little half spheres, but it seems that the conical shape is, so far, winning the "evolutionary" race in this part of some commercial ecosystem. Incidentally, sports field markets are used on flat surfaces - possibly supporting the hypothesis above.
It is clear that a taller safety marker is safer (in a variety of situations) than a shorter one.
So thinking about all of this, I wonder what the answer would be if you asked these users (the workmen) why these safety markers are shaped so. I didn't have the guts to ask before my bus came but I wouldn't mind betting that one answer would be "So that they are easier to pick up and put down".
This brings me to the fundamental point...
Users don't need to understand every detail of a design rationale to experience its utility or usefulness.
If the first step on the path to designerly enlightenment is realising that you (the designer) are not your user, then maybe the next step is realising that your users are not designers. Not sure what the final step is but these things usually come in threes.