[Ben73]

I don’t know what South Australian motorways look like, but Sydney has twice the population than the entire state of SA. Plus you have all the traffic just passing through Sydney. If you leave and 80m gap 16 cars will try to get into that gap. The only way to keep a permanent 80m gap would be to drive at about 30kmh slower than everyone else. That way as people pass you they will move into your lane and that gap will not take too long to increase to 80m.

Leave a 10m gap on a busy motorway and I will take less than a minute before 2 cars attempt to get in the gap.
Not saying it’s safe t drive this close. But it happens all day every day. It usually works out ok since motorways are designed to keep traffic moving at speed with almost zero hazards or delays.
That’s why there’s no intersections, no traffic lights, no pedestrians, no parking, pretty much nothing but driving at a reasonable speed.