The word may also be used when referring to really dull signage for a government organization.
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.
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)
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.