Was hoping the article might shed some light on the ambiguity of some of the prohibition signs in the UK and parts of Europe, e.g. the "no motor vehicles" sign[0]. Given that the ones with the red circle but without the red slash typically mean up to what is shown is allowed and anything above is not (e.g. maximum speed, maximum width, maximum height), the one showing a car and a motorbike could reasonably be interpreted as anything up to the size of a motorbike and car are allowed but anything bigger (such as a lorry and a bus) are not. Adding a simple slash, as the rest of the world seems to do, and as per other prohibition signs in the UK and Europe (e.g. no right turn) would remove all ambiguity. Wonder if there's some strange history behind that apparent anomaly.
> that the ones with the red circle but without the red slash typically mean up to what is shown is allowed and anything above is not (e.g. maximum speed, maximum width, maximum height)
These number are limits on a continuous domain, where inclusion or exclusion of the limit number itself is basically meaningless. The list of vehicles is a set (without necessarily a clear order) which has fundamentally different properties so it's not surprising the meaning is slightly different. You couldn't have a sign that forbids pedestrians and cars but allows motor vehicles otherwise.
The rules are simply that when a red on white sign displays anything other than a number (with or without a slash) it means this thing is forbidden. If it's a number it means a high limit.
Back in the days when the League of Nations was deciding all this, there was a road sign in the German Empire consisting of a red circle around a white background with a black arrow pointing straight ahead (and no slash). This did not mean it's forbidden to go straight ahead, it meant it's _mandatory_ to go straight ahead - the thing that later on was changed to the white-on-blue arrow. (examples: https://de.wikipedia.org/wiki/Bildtafel_der_Verkehrszeichen_... but the sign must be older than that).
If you want to be picky, the "you must" signs generally had a narrower red ring than the "you must not" ones, but even that rule is not always adhered to on the linked page.
I think the slash was added in some cases because there's an old German sign with a black arrow on a white background with a red ring, that means _you must go this way_ instead of _you must not go this way_ as we'd read it today. So the slash on all such signs (no left turn etc.) is to make it doubly clear this is a prohibition not an instruction.
Fun fact: the German Empire had an additional convention that if the symbol (e.g. a motor car) was shown in outline instead of filled in, then the prohibition only applied on Sundays.
EDIT: looking at https://commons.m.wikimedia.org/wiki/Category:Diagrams_of_hi..., the 1927 German sign for "parking _allowed_" was a black P on a white background with a red circle; the prohibition sign contained the text "Parken verboten" (parking forbidden). So I can see why the German delegate might have objected to making the new "no parking" sign their old "yes you can park here" sign!
I always thought the slashed ones were ambiguous, as adding a slash on top of a red circle which already signifies prohibition could be interpreted as a double negation.
%2C_um_1927_-_Deutsches_Museum_Verkehrszentrum.jpg){kind=link}
[0] https://en.wikipedia.org/wiki/Prohibitory_traffic_sign#No_mo...