Here’s an exercise for the student: What, exactly, is the message that the PLOWS USE CAUTION sign is intended to convey? Three possibilities come immediately to mind:

1. People who drive plows do so cautiously. This is a simple matter-of-fact statement like SLIPPERY WHEN WET:


or the epimenidian NO TRAFFIC SIGNS:


Well, actually NO TRAFFIC SIGNS is ambiguous, as it could be read either as an assertion—There are no traffic signs up ahead, so you’re going to be on your own—or an imperative—Don’t post any traffic signs here [other than this one]. The same, for that matter, could be said of the iconographic version of the SLIPPERY WHEN WET sign:


which could just as well be read as a warning to look out for drunk drivers. So, perhaps a better example of a declarative would be WRONG WAY:


2. Watch out for plows! This message is a warning to the effect that you may encounter plows in the area and, if you do, you are advised to give them wide berth. Here, “USE” is to be taken as an imperative addressed to you who are presumably using a means of locomotion—car, bike, feet—less formidable than a plow (let alone a herd of plows). Compare the imperative DO NOT ENTER:


or the archetypical STOP:


though, as noted in the discussion of détournement in our Short Cuts: A Guide to Oaths, Ring Tones, Ransom Notes, Famous Last Words, & Other Forms of Minimalist Communication, “STOP” is ambiguous to the extent that it can be parsed as either a noun or a verb, which can be construed as intransitive or transitive, its object (yourself, your vehicle) being implied or made explicit by creative (if typically unsanctioned) embellishment, e.g.,


3. Hey, you guys driving the plows, be careful! Here again, “USE” is an imperative, but the addressees are plow drivers, not the motorist/cyclist/pedestrian.

So, how might we determine which of these three readings is the intended one, given the constraints on traffic signage promulgated by the Federal Highway Administration in its Manual on Uniform Traffic Control Devices (MUTCD) that include the admonition that “word messages should not contain punctuation, apostrophes, question marks, ampersands, or other characters that are not letters or numerals unless absolutely necessary to avoid confusion” and an almost impenetrable set of instructions governing the terseness and chunking of such messages (see “Section 2M.05 Message Length and Units of Information” at

 Supposing for the moment that you were unable or sufficiently unsporting to do the obvious, namely: google “plows use caution,” look at the first hit [], and call it a day, you might with confidence rule out reading #1 (the assertion of fact) on the grounds that not only is reading #1 silly, the shape of the sign and its background color are those conventionally associated with a warning—see, for example the current specifications for highway signage in Massachusetts ( ), which, like those of the other states, have evolved from the standards proposed by the members of the Mississippi Valley Association of State Highway Departments in 1923 who, according to historian Gordon Sessions, “agreed on a signing-and-marking plan which was destined to become the basis of the national standards agreed upon two years later” (see his Traffic Devices: Historical Aspects Thereof). The group proposed restricting signage to the following six shapes: round (to mark railroad crossings), octagonal (to signify “stop”), diamond (for regulatory information), square (for caution and “attention”), rectangle (for directional and regulatory information), and “route markers of some characteristic or conventional shape different from the above.” Lettering was to be black on a white background.

In 1925, the American Association of State Highway and Transportation Officials (AASHO) proposed “to introduce into the caution signs a series of distinctions which would indicate different degrees of danger,” in descending order of potential danger: circle for railroad crossings, octagon for “Stop,” diamond for “ordinary conditions of danger requiring precaution on the part of the driver at all times,” square “to be used where the dangerous condition is intermittent and involves little more than ordinary care or attention in driving.” All of these signs were to have black lettering on a yellow background. The STOP sign would wait to get its white lettering on a red background until 1955 by which time the science of industrial chemistry had evolved to make possible the development of a red pigment that would withstand inclement weather.

In addition to the STOP sign’s white-on-red makeover, the 1950s saw the introduction of the YIELD sign:


to which may be compared the STOP  sign formerly in use in England:


and the STOP sign still in use in Japan:


This new (to the US) shape and message betoken the evolution from the basically rural  society in which the first traffic signs appeared, thanks largely to the early recreational automobile clubs of the 1920s, to the increasingly urban and trafficky world in which we operate today. Small wonder that there is some confusion in the ever-proliferating traffic signage between the levels of danger originally encoded by the yellow-backed diamond, square, and rectangle:


And, yes,  there is an alternate reading of this message available at, perhaps a reflection of the snarkiness surely absent from the good old days of rural America that preceded the invention of the internal combustion engine:


But back to PLOWS USE CAUTION. Color and shape—not to mention the word CAUTION—clearly suggest that somebody’s caution is strongly advised. But whose? Here, context provides an important hint:


The setting is clearly urban, so we may guess that the kind of plows involved are the kind used to remove snow (as opposed to the kind used for tilling fields). As it happens, another piece of contextual information is offered by the railing of the highway overpass in the center of the view. This is in fact the MacGuffin in the story in which snow plow drivers are warned take it easy as they go about their business so as not to dump snow onto the unsuspecting vehicular traffic below.