The one about cows is no joke. Hitting a cow in a sedan at highway speeds is more or less the same as hitting a brick wall.
Edit: actually, given the choice, take the brick wall, because at least a simple masonry wall will crush and crumble and absorb some of the impact. The cow will just gain a sudden increase in its net velocity at your expense.
The one with "No speed limits" doesn't mean that you can go 200 in a 90, it just means that previous speed signs no longer apply. So if you had a speed sign of 70 in a road that typically allows to go 90, you can do 90 again.
The kangaroo one just makes sense, it's only exotic because you're the only place with kangaroos, I bet they don't think twice about the same signs for their local wildlife.