Humorous messages on electronic signs discouraged by new guidelines from US Federal Highway Administration

It's an understandable mistake considering states are legally required to adopt the MUTCD itself or their own modified versions of it as design guidelines, but engineers technically don't have to abide by guidelines in such a document when they have a good reason. Subjectivity is built-in, which is why the document and others like it distinguish between Standards (rules) and Guidance (implementation suggestions and examples). There are lots of situations where a particular interpretation of a federal or state standard fails in an edge case. So no one will be sent to prison for non-compliance with a guideline, it just irritates the Federal Highway Administration.

The FHWA has this weird two-sided relationship with the MUTCD. When they want to get their way, they talk about how states legally need to have regulations for highways and imply that the MUTCD specifically is that legal regulation. When they dislike a project that isn't in compliance (mostly pedestrian safety infrastructure), they will use this reasoning to withhold funding. But when criticized for its insanely outdated safety regulations (even after its most recent revision), they admit that the MUTCD is not a law, just a guidebook, and engineers can do whatever is appropriate!

BTW, the vague text from the MUTCD's 11th Edition in question (Part 1):

This Manual presumes the user of the MUTCD has sufficient working knowledge, professional training and experience, and education in the principles of traffic engineering. Other resources can be consulted to understand the basis for decisions that are made in which engineering study or judgment will be applied.

The MUTCD is not a roadway design manual, and engineers seeking guidance on design should refer to appropriate roadway design guides recognized by the Federal Highway Administration as needed for the design application.

They are very carefully couching their language to avoid devaluing the power of the document while also admitting that, indeed, "other resources" can be consulted and that "judgment" is a legitimate action in some circumstances. The second quote refers to roads and not devices, but the FHWA's roadway design guides are also more squishy than they claim to be.

Additionally, from the executive summary on the Federal Register:

There are many factors that go into making a road safe, including the surrounding land use, the geometric design of the roadway, and the uniform and consistent application of traffic control devices. The MUTCD is a set of technical criteria for the latter, and does not preclude action that State, local, or tribal decision makers might take on the first two.

The MUTCD is developed and organized for the purpose of establishing national standards for traffic control devices on any roadway, bikeway, or shared-use path that is open to public travel. It is not intended to inform State or local policy on the design and character of communities or the geometric design of roadways, to prioritize a travel mode, or to influence land use or access by any mode of travel. Relevant local authorities and roadway owners determine land use, such as transit-oriented development, and roadway design to safely and conveniently prioritize walking, bicycling, public transit, motor-vehicle travel, or a combination of modes.

In theory, a sign on the highway is strictly a traffic control device, but in practice, you can argue that literally anything is related to "surrounding land use" and "the design and character of communities," including road signs that contain phrasing which is arguably humorous in nature. For this reason, there are tons of roadways around the country that aren't in keeping with what the MUTCD implies is necessary, like defining speed limits by the 85th-percentile (which is unsafe and therefore summarily ignored by younger engineers). Non-compliant traffic control devices also exist; they are uncommon, mostly because it's expensive to maintain non-standard equipment, but the idea that all municipalities are in strict compliance with every regulation's supplemental guidance ever issued (or that regulators care) is a joke. Still, you can see how this wording tends to keep engineers compliant.

The language this article refers to is full of "shoulds" and not "musts" or "shalls." It's in Section 2L. The sections that describe humor are specifically labeled "Guidance" and do not carry the same weight as other parts of the chapter (labeled as "Standards"). For example, the MUTCD suggests that messages not be displayed which "could adversely affect respect for the sign" (whatever that means), but this is just guidance. Here is the exact language (2L.07.04):

A CMS should not be used to display a traffic safety campaign message if doing so could adversely affect respect for the sign. Messages with obscure or secondary meanings, such as those with popular culture references, unconventional sign legend syntax, or that are intended to be humorous, should not be used as they might be misunderstood or understood only by a limited segment of road users and require greater time to process and understand. Similarly, slogan-type messages and the display of statistical information should not be used.

The actual Standard does not mention humor. It says: "Traffic control messages shall have priority over traffic safety campaign messages." i.e. a traffic safety campaign message is a secondary use of a CMS; so any use of the CMS for a safety campaign can't adversely affect traffic control. Notice that this is not specific. It is meant to be interpreted by engineers using "judgment" and "other resources." The text I quoted is marked as "guidance" and is full of "shoulds." They do this in part because this guidance is effectively unenforcable. The definition of humor extraordinarily vague and inherently undefinable.

In any case, states can choose to adopt broadly similar but slightly different versions of the MUTCD which satisfies any legal requirement. These rules can vary slightly. A case in which variation would be permissible is the removal of guidance disapproving of the inclusion of humor in a road sign that exists as part of a traffic safety campaign. From the CFR 655.603(b)(1) (Code of Federal Regulations) which defines what "substantial conformance" means:

The guidance statements contained in the National MUTCD shall also be in the State Manual or supplement unless the reason for not including it is satisfactorily explained based on engineering judgment, specific conflicting State law, or a documented engineering study.

In other words, anything that is marked as "guidance" can be completely ignored by states if they choose.