The Cambridge Guide to English Usage is arranged a bit differently to other guides, so it took me a while to work out that the book does not appear to give explicit advice on the subject of short signs. However, the book does have some text that implicitly gives away the Cambridge University Press style:
"in the conventional sign NO ADMITTANCE."
"An official NO ENTRY sign makes access by"
"as when the sign says PROCEED WITH CAUTION rather than DRIVE CAREFULLY."
Notice the NO ENTRY version is italicized but the others are not. None of the examples I could find in the text were quoted, but all were fully uppercase.