Google tells me that, in the UK, this might be a peculiarity that has come about thanks to health and safety regulations for the construction industry which are particular about "Signing, Lighting and Guarding (SLG) guidelines":
What are the main provisions of the amended Regulation 97?
(a) There must be on site, at all times when the works are in progress and workers are on site, at least one person who has been issued with a valid construction skills registration card relating to either “health and safety at roadworks” or “signing, lighting and guarding”. Furthermore the works must be supervised by a competent person who has been issued with a valid construction skills registration card relating to signing, lighting and guarding on roads.
In other words, as the OP suspected, this is very likely jargon particular to the construction industry. One explanation for the use of signings could be due to their temporary nature (during construction). I couldn't make out if these regulations also involved a person manually directing traffic (often seen in some countries), a requirement which would make the use of signing more obvious.
I also have no idea if the occurrence in the US is a one-off or not. But I did come across one reference which might suggest that it is possible.