As part of my ongoing scenery development with signs, I have done some research into the US Primary Interstate Highway system.
This system has signage that looks like this:
The blue field has a number inside it to indicate the route number.
The Primary Interstate Highways are the major interstate highways in the United States, and are assigned a one or two digit route number.
Even route numbers are assigned to east/west routes, and Odd route numbers are assigned to north/south routes.
Lower numbers will be seen in the south (even numbers) and west (odd numbers), and higher numbers will be seen in the north (even numbers) and east (odd numbers).
There are 66 Primary interstate routes, starting with '4' and ending with '99'. Obviously not all numbers between 4 and 99 are used.
The good news about this numbering is that anyone wanting to make their own Primary interstate route sign can use numbers that don't actually exist, thus keeping their towns and cities in the realm of the game setting preventing anyone from knowing extactly where your city is located.
For a complete list of signs and more information please click here.