Who can answer US traffic law questions authoritatively?
Does someone in the government answer questions about laws or do we only find out in court?
For example, can we expect answers from someone in the police department?
The only really authoritative source of answers is a court interpreting the laws on an as applied basis (and there are many U.S. traffic laws, one in every state and sometimes additional local ones, not a single U.S. traffic law).
An answer from a government official or police department is not authoritative, although it may be informative of how the official in question would enforce the law.
The executive branch may certainly publish the laws and educational materials about how they enforce them. The judicial branch has its own interpretation and may also publish guidance in addition to specific case reports. Your own attorney may earn a living by answering your questions about the likelihood of success on the merit of any particular case, in light of the foregoing.
Even if you were to convince a police officer or motor-vehicle administrator to give you a "firm answer", it would not necessarily be binding upon them or anyone else, if it turns out they were incorrect. Your lawyer, on the other hand, has a duty to be zealous and diligent on your behalf, and has malpractice insurance that may cover the costs resulting from mistaken advice.