Being Australian, I accidentally rode my bicycle down the entry lane to a German autobahn after seeing a sign with a bike and a circle around it (not through it).
White (yellow in some countries) signs with a red circle are always prohibition signs (or speed/weight limits), no matter if there is a bar or not - actually only the "no stopping" and "no parking" signs have bars (two for no stopping). If something is allowed, the sign is white on blue.
That's an interesting example, as white arrow/arrows on blue in circle also exists, which limits (or otherwise indicates) the directions you can continue.
The one-way traffic sign does fit in, but most commonly you see it in situations where it's actually just informational, like the entrance of a road. In this case it doesn't limit you in any way, it's not like you have to enter that road. You will see a no entry sign on the other side of the intersection anyway, which does limit you. But it does inform you that you shouldn't expect oncoming traffic.
The horizontal one-way road sign is less common, and is often used in T-junctions. There it arguably does double duty.
It does mean that after entering that road you aren't allowed to turn around and go back, so there is a bit of a mandatory aspect there. Nevertheless I think it still fits best with the informational signs, also to separate it from the white arrow on a blue circle.
Well, not really. This sign is an indication that you will not encounter oncoming traffic.
The other end of the road will see a "No Entry" sign (red disc with a white horizontal bar) and any side street will get a "mandatory direction" sign (a blue disc with a with circle and a white arrow) that shows which direction is mandatory or a "no turning" sign showing which direction is prohibited. The square blue sign with an arrow is an indication that someone else has mandatory or prohibition signs.
For example here: https://maps.app.goo.gl/PwAYqUffz6uPLVZW6 you can see a "no right turn", a "one-way traffic" sign, and a "no entry" sign (all saying "except bikes"). The "one-way traffic" is definitely informative, it would be a disc otherwise.
it seems like it's less universal than I thought. Using just a rectangle with an arrow when coming from a side street rather than a circle makes no sense to me unless there are also no entry/no right turn/mandatory left turn signs as well.
