Someone was still interested in trunk road.

I looked it up and here's what the shorter Oxford Dictionary has to say;

1. "Main body of tree opp. to roots and branches; human or animal's body without head & limbs & tail; main part of any structure"

2. (Also ~line) main line of railway, canal, telephone main line [...]

9. [...] ~road; main road."

The origins are Middle English from Old French "tronc" and Latin "truncus"

I reckon the use for main roads etc. has come out of the first meaning, especially the main body of a tree. A road system has parallels with a tree with main roads having minor roads branching off them.