支路 contrasts with 主路 (zhǔlù, 'main road') in traffic contexts and with 干路 (gànlù, 'trunk circuit') in electronics. The image is the same: a 'branch' coming off a trunk, whether of asphalt or wire.
grammar
Used with the measure word 条 (tiáo) for the road sense — 一条支路 'one side road' — because 条 covers long, narrow things (roads, rivers, snakes).
