Southbound means travelling towards the south. The southbound carriageway, a southbound train, southbound traffic.
It is the word used on signs and in traffic reports, where the direction is more useful than the destination. In migration, southbound is the autumn direction in the northern hemisphere. Note the -bound ending, meaning headed for, which comes from an old Norse word for ready and not from binding.
1adjectivesouthboundTravelling towards the south.
“The southbound lanes of the motorway were closed after a lorry overturned.”
southbound is a direction of travel.
Middle English
south
English
south
English, today
southbound
From south + -bound. south comes from Middle English south. From Middle English south, from Old English sūþ, from Proto-West Germanic sunþr (“southern”), from Proto-Germanic sunþrą (“south”), from Proto-Indo-European sóh₂wl̥ (“sun”). 2
Etymology from Wiktionary, CC BY-SA