Speed limits will slow people down, just not nearly as well and as reliably as building roads properly. Despite having very little knowledge of Australia I have heard that police there can be pretty ruthless about speeding tickets, too.

Appropriately wide roads are better for everyone, even for drivers who don’t need to feel like they’re crawling when the speed limit is much lower than the road can physically support(even if it’s context cannot). Punishing some dangerous drivers after they’ve already done dangerous driving is so much less helpful than slowing people down to begin with.

And the best part is that slower roads are often made, especially when retrofitting existing ones, by putting in greenery and/or giving pedestrians and bicycles more elbow room and safe places to be.