mtanillo: in generally, trucks aren't 'banned' on certain roads. Unless you see a sign prohibiting any and all trucks, trucks can take it. What you can find is length, height, or weight restrictions.
For instance, the STAA routes (not necissarily the National Network, if you buy a RM motor carrier atlas, they're highlighted in orange). There are some exceptions. But, STAA means 13'6 tall, 8'6 wide, 53' trailer. When the STAA started in 1982, it basically 'froze' the individual rules of each state (they can still change, but barely) and created the national grid. A lot of states do not, by default, allow 53' trailers on their roads. Mostly, western states. Therefore, if you drive a 53' trailer through the state, it must be on the national grid. Since most interstate commerce trucks are 53' trailers, they avoid leaving the grid (except in cases of fuel, rest, and generally a 1-mi buffer).
If a state does allow 53' trailers and 80,000lb gross, they really don't need to mark which highways you can or can't take as they all are navigable. New Jersey has a width restriction, so they only place they allow 102" wide trailers is on select roads (and they sign them well). California has blanket restrictions to anything longer than 48' in the entire state that you must follow a 'truck route' if you're longer. Just because a road isn't a truck route doesn't mean thru trucks can't take it. Unless there is a 'no trucks' sign posted.
Maryland, New Jersey, Vermont, Connecticut, Kentucky, New Mexico, Arizona, etc have very few routes they allow max dimensions and weight, so they try to shoehorn all trucks onto the freeways and the most logical US or state routes. A lot of times, it's for the trucker's own good as the roads are ill-equipped to handle vehicles that long, wide, or tall. Mountains, tight turns, blind curves, no shoulders, dropped shoulders, no turn lanes, obstructions, low train bridges, tree branches, etc. It's easier to just not include it instead of listing dozens of 'restrictions' on each route.