North American road signs are made of 16-gauge galvanized steel, which is coincidentally the same gauge of steel required to make SCA-legal armour and shields. This had led to the occasional unfortunate occurence of road signs being stolen by a prospective armourer.
It is important to note that stealing road signs is a serious crime in North America, and can cause injury or death to random strangers if they aren't warned about upcoming road hazards (the infamous "Octagon round" shield made from a stop sign is especially risky).
That being said, if you do have a contact in your local roads department, old road signs do get thrown out and they are a cheap source of steel. Unfortunately, even with the paint scoured off, they will always display a telltale hexagonal pattern from the reflective mylar facing which allows roadsigns to show up clearly at night. Road sign armour must therefore be painted.