If you examine the wording on cautionary road signs, you’ll realize they aren’t wholly consistent.  Yet as native speakers of English (or with working fluency of English) we usually have no trouble interpreting the intended meaning.  The basic semantic task is to decide whether the words on the sign are descriptive or advisory — or both.

The single word slow is advisory — it’s shorthand for a command to go slowly, whereas the words bump and dip are descriptions of conditions of the road ahead, not verbal commands to perform weird dance steps.  The phrase fog area is descriptive, alerting the driver of a condition, and fog is a noun used as a modifier, it’s not a verb directing the application of, e.g., insecticide to a locale (another possible interpretation).  However, slow children is both advisory and descriptive; it’s not a unit phrase parallel to fog area, but rather a command to proceed at slow speed because there are children in the vicinity. Slow deaf child is another such example, although here, as interpreters of the sign we split the modifiers apart; the first one is a command to the driver whereas the second modifier adds more detail about what type of child is in the vicinity. A different interpretation would assign both modifiers to child and refer to something else, either a physically slow-moving child or perhaps one with limited mental capacities.

Yet another contrastive pair is deer crossing and blind driveway; the first phrase describes a location where a certain species of animal traverses the roadway frequently,  but the second phrase does not refer to a place where unsighted persons drive vehicles — it refers to a low visibility condition that applies to a stretch of private pavement where a car could be entering the main roadway.

Road signs, including cautionary and regulatory signs, need to be brief and unambiguous.  Context obviously plays a big role for narrowing down the possible interpretations of the words chosen on these signs.  Shape and color of signs can establish some context for interpreting linguistic phrases.  No left turn appears on a white rectangular sign whose shape and color convey the meta-meaning of regulating driver behavior, so no left turn is a command. No train horn appears on a yellow diamond which means a warning, so the expression is interpreted as a descriptive alert, not a command.  Nevertheless, even when it’s understood the context is regulatory,  the quirks of language can leak through. So, it’s hopefully assumed that drivers will interpret the sign use two lanes as a shorthand command to get out of a third lane that is going away shortly (due to road work or some other condition), or fan out into two lanes from a narrower single lane, but will not interpret the sign as a command or (invitation!) to straddle the dividing stripe of two lanes with their vehicle. 🙂  (The alternate phrase form two lanes makes the intent clearer.)

A cautionary sign that seems peculiar to New England (or even to just Massachusetts?) is thickly settled. It’s a command actually, to not drive faster than 30 mph because houses and buildings are closer than 200 feet apart. People new to the area are sometimes puzzled by this sign, as well as the regulatory no salt zone.

The inherent ambiguity of language is further reduced by using broader, more universal symbols, including lines, arrows and numbers and other non-linguistic graphics. Is there universal psychological validity to some of these symbols, or is their acceptance just a consequence of historically widespread dissemination of certain symbol conventions?