What's a word that can be used when something, such as a document, is purposely bland?

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

Solution 1:

Perhaps prosaic or pedestrian fits the bill.

Solution 2:

I'm going to go with insipid.

Solution 3:

One possibility would be pabulum (also spelled pablum):

something (as writing or speech) that is insipid, simplistic, or bland

Solution 4:

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.

Solution 5:

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.