From The Collaborative International Dictionary of English v.0.48 [gcide]:
main road \main road\ n.
A major road for any form of motor transport.
Syn: highway.
[WordNet 1.5]
From WordNet (r) 3.0 (2006) [wn]:
main road
n 1: a major road for any form of motor transport [syn:
highway, main road]