Gene,
Here is my question. Do you want the viewing piblic to actually be able to read the signs or posters or advertising on your vehicles; or do you want scale relative to 1:1 real size?
Knowing you, the reason you are asking is so you can justify signs with your clever sayings on them that people do not need a microscope to read.
Here's my take. If the sign is on a vehicle, I wouldn't expect to read anything but the largest letters possible like the company name, but no funny slogan which is usually done in smaller type.
As for billboards, anything goes in my opinion, as long as you don't make a 4"x6" sign. That belongs in O scale. As long as you can read it with the naked eye, that would be acceptible to me and your viewing public. The font is of your chosing. Fonts have been around for a long time and only if you are concerned about time frame would some of the newer fonts be out of place.
There are a ton of signs available in preprinted form from many manufacturers, but it is obvious that you want to express you individuality here. My suggestion is to go for the smallest size font and type of font that you can read.
As for the construction of the actual sign, print the sign in color if you have one and experiment with it. No matter if you only have B&W, or color, once you print the sign on paper, glue it to a backing of cardstock. Then use strip wood to make a frame, painted of course in your favorite billboard color, can you say dark green, put a catwalk on it and a basketweave bottom, a couple of lights and.......golly! This is begining to sound complicated. Nevermind. Just buy a billboard kit or two from the people that have already done the work for you. You can always substitute the actual sign with your printed ones.
B-)