The word may also be used when referring to really dull signage for a government organization.
Perhaps prosaic or pedestrian fits the bill.
I'm going to go with insipid.
One possibility would be pabulum (also spelled pablum):
something (as writing or speech) that is insipid, simplistic, or bland
There are already a few fine candidates on this page (I like pedestrian and pablum), but I am not sure if any of them actually have the connotation of purposely.
One word that does have that connotation, at least to me, would be wishy-washy. However, I'm not sure it could be applied to "really dull signage"; I would rather call that one run-of-the-mill.
Anodyne, which strictly speaking means soothing, is often used to describe boring and/or non-commital official statements and publications.
For me at least, it carries overtones of wishing to avoid giving offense, with the (not always unintentional) side-effect of reducing both clarity and actual semantic content.
LATER - Several of the alternative proposed by others are words that can be used for OP's purpose, and to be honest OP's noun signage seems a bit too obscure to be worth looking for adjectives that do get used with that word.
But I'm still focussed on actual usage, and anodyne statement seems like a good enough variation to check. This NGram confirms it's not only used, but gaining currency by the decade.
It's called boilerplate and you usually find it in government and legal documents.
my main suggestion:
"monotonous" (which can, in fact, have a non-verbal meaning)
a few others:
"lacking personality" (which is actually two words)
"wordy" (not a word I would use, but fits the requirements)
"needlessly redundant" (again with two words)
"emotionless" (especially the government signage part)
Milquetoast! Such a colorful word for blandness.
As you say "really dull signage", I presume that you're looking for a negative connotation.
Stodgy, vacuous; turgid would be my favourite.
For the connotations of lack of style but still useful:
bureaucratese is more for longer stretches of words than a sign, but captures the sense of boring, like turhid and stodgy mentioned elsewhere.
Staid just means “Serious, organized, and professional; sober”, according to Wiktionary. As it makes something of a virtue out of blandness, it makes it sound like it might be intentional. Other words along these lines include sedate, temperate, moderate, and mild. But I get the feeling there is a much better one that I’m missing.
Another tack is to try words that describe things used intentionally to put people to sleep: lulling, soporific, anaesthetic.
I've also heard the word 'blasé' used to describe an object that was bland or boring and without any exciting or pleasurable qualities.
To make a precise and pointed reference to the fact that 'it had to be dull, so it is', we resort to 'Stupidized'. May not sound politically correct, but the term has been around in the technical field.