What do the signs look like?
State highways and all lesser numbered roads are considered on a state-by-state basis, according to regional usage. That's why we specifically do not have a "standard" for this -- each state has its own system, and therefore its own nomenclature. (While there may also be regional nomenclatures for US Routes and Interstate highways, these are nationwide systems, and therefore have nationwide standards.) Maybe it's easier to look at per-system rather than per-state. Either way, as long as each state's standard matches local usage and looks something like "State Hwy xx", "SR-xx", "M-xx", or "LA-xx", it should be fine. (We shouldn't spell out what can be abbreviated -- TTS will read "SR" as "State Route" -- but we also cannot abbreviate what must be spelled out -- TTS does not expand "SH" to "State Highway".)
TTS expands "CR" to "County Road", so we should maybe consider using that for the sake of brevity. Perhaps someone looking at "CR" out of context wouldn't know what it means, but in context it should be relatively easy to understand, I would think -- especially once the user hears "County Road" spoken aloud.
Before making a decision, I would want to know (1) what the signs say and (2) what street/postal addresses on these roads look like before ditching the letter. "CR-B-15" looks weird, "CR-B15" should be okay.