Well actually it's aluminium.
Having reduced my motorway drawings down to simple symbols, I've had them cut out of 4mm aluminium. One of the symbols is 6ft wide, so it'll be interesting to hang.
In doing all this I have discovered that aluminium has a slight grain, something I only found out after attempting to remove some backing glue. Aluminium is very soft and perhaps in future I might try Stainless Steel.
But nevertheless, it's looking good.