I'd say you got three options:
1. Keep doing what you're doing. (Doesn't meet specs and you can't print it.)
2. Print it on clear, laminate it and apply that to the HI. (Doesn't meet specs but you should have no problem printing it.)
3. Outsource to a company that does this type of work (Will meet specs. and you can start printing something else.)
Good luck!
Good points....
Here in the UK there is only one way to manufacture highway signage... the way the Ministry of Transport regulations tell you.
Font, character size, kerning, colours, production methods, aluminium grades, reverse power coating colour, post specification and BS EN numbers. You name it and it's laid out in the regulations in anally retentive detail.
If you want to print highway
signs it makes sense to consult the regulations first... printing might not even be an approved manufacturing method in which case your products won't be accepted even if they look correct.
This is all a lot easier over here because there is absolutely no regional deviation allowed... the regulations apply everywhere. This also applies to emergency service vehicles which are all fitted with reflective high vis graphics regulated to Home Office approved designs, materials and production methods.
Before you waste a lot of time and money have a quick look at the regs.