Warning Signs.
—The name indicates their purpose, that they are intended to warn against danger or to indicate that extra precautionary measures should be exercised. They give notice of sharp turns, bad bridges and culverts, railroad crossings, cross-roads, and so on. Frequently the word “danger” is printed followed by a descriptive word indicating the kind of danger; for example, “Danger—sharp curve,” “Danger—bad bridge,” “Danger—railroad crossing.” Or there are used such words as “Slow” or more simply “Slo,” with a graphic or descriptive word following, as, “Slow—school,” “Slo—drawbridge.”
The International Road Congress suggested a standard graphical sign, 24 by 16 inches, with white symbols on a black background. Some American roads use a modification of these, [p. 440].