To get into the traffic signage industry, there's a few things to understand. I was with OKI as a regional sales manager for Colorpainter before Mimaki took over the sales/service. The key word here is whether you'll be printing "Regulated" DOT Signage. There are different regulations for Federal Highways, as well as State. Most local governments follow state regulations. If the sign required to conform to DOT regulations, then it can only be produced by methods prescribed by the regulation. Screenprinting, Cut Vinyl Overlay on Vinyl, and digital printing. For Digital Printing, the regulations also state specifically what sheeting, inks and overlaminate must be used. Certain states only allow certain sheeting. Some haven't approved Digital Printing at all and some have approved digital, but not the specific "recipe" that has to be used. OKI developed the M64s-Nikkalite version to be paired with a specific Nikkalite sheeting. TX DOT approved the printer, inks, sheeting and overlaminate that have to be used. The attached letter is specifically for the OKI "Recipe" but I'm sure each manufacturer has their own certification letter. The nikkalite
There are many traffic signs applications that aren't regulated. As you mentioned, Novelty signs, and pretty much any reflective sign that isn't on a "regulated" roadway. The issue is, if you want to get into the traffic sign business, and produce regulated signs, then you'll have to go through the process of getting approved by the regulating authority, and the sheeting supplier will need to certify you for the warranty. For instance, just because you use inks/material/laminate that is certified by the 3M, doesn't mean that 3M will honor the MCS Traffic warranty. They come in periodically to "certify" you. I haven't been through this process myself, but I spoke in depth with a 3M traffic sheeting rep that mentioned it, and they want to see your working conditions, how you store your media, etc. And many times 3M provides sheeting directly to the traffic printing companies instead of through vendors. I know Grimco carries some of the different DOT grade sheeting, but I think they can get a better price by going direct.
Also, I can echo what some have said about the 365 being used for traffic signs... We placed at least 5 machines in existing traffic print shops, and each one of them had the 365 that 3M had "subsidized" because of the volume of sheeting that they were buying. None of them were using the latex anymore because of issues with speed, color and ink durability, plus the other issues with Flexi that have been mentioned.
TLDR: I would stick to what you're doing now, unless you want to jump into traffic signs with both feet. Keep printing the novelty signs and you won't have to deal with the headaches.
one guy on here printed a bunch of signs on 3290, didn't overlam it since they were temp....and within a week he got a call because they turned into blanks. 
