Road sign colors

As a result of recent exercises, I've spent the last couple of weeks sharpening my awareness of colors.  As I was about in the city, I ran through the mantra of color combinations (redgreen blueorange violetyellow) to keep myself focused on color (where typically I am more aware of form).

One of the images I captured for my exercise made me consider - why are highway signs blue? I googled a bit and found regional and country-specific standards.  It seems in Europe and the US, highway directional signs are green, but in Europe non-highway (or on-motorway) signs may be green though in several countries they are blue. Besides the color scheme, there are also standards for shapes.  Triangles and diamonds, for example, typically carry warnings.

How were these decisions made?  Was there any research done to suggest certain colors or shapes are related to particular modes of discourse or states of mind?  The lack of uniformity across countries suggests the system grew organically and only later as drivers began motoring in other countries was some kind of uniformity required.

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