Transit, a 'trunk road' might be understood differently by different English speakers, so it would be helpful to provide some background or context to the question. In general, the 'trunk' in 'trunk road' is similar to the 'trunk' in 'trunk call' - for communication beyond the very local.
I agree, John. With some exceptions, I think we'd only recognize the term when applied to telephony, as in, "the trunk between the Lehigh and Trafalgar exchanges." Even then, that concept has faded into history.
I didn't realize the term was in use any longer when applied to roads. There was the Grand Trunk Road traveled by Alexander the Great in India.
US Highway 66 would seem to qualify as a "trunk" road. It became known as Route 66, and was the subject of an eponymous American TV series in the 1960s.