But since we don’t tag for the router (an often repeated mantra), the question is: when do you tag
designatedas opposed toyesand why?
Maybe in retrospect it wasn’t wise to choose “yes” as bicycle value for the situation that they are allowed on footways, because the are not on par with pedestrians there, they may only go at walking speed and give way to pedestrians, which means not really comparable to either the street nor a cycleway or an unspecified generic path.
Cheers Martin
I’m in the UK, so the answer is “never”.
See my earlier reply for what is I think a workable definition or let’s discuss why that is not a good definition.
For how things are different per country OSM_tags_for_routing/Access_restrictions is likely a good source. For the UK, the table shows no use of bicycle=designated following the answer of @Richard.
The Default access restrictions for all countries unless otherwise specified on that page has bicycle=designated and checking all countries I see most countries follow that except for a few.
As an aside, it should be noted that the table at https://wiki.openstreetmap.org/wiki/OSM_tags_for_routing/Access_restrictions#United_Kingdom implies that access restrictions across the United Kingdom are uniform. They are not. As an example, the rules in Scotland are substantially different to the rules in England (and Wales).
The rest of that paragraph is largely rubbish too - the statement "The legal status of cycles on public rights of way in England and Wales is disputed. On pavements, it is illegal to cycle unless designated … but on public footpaths the law is less clear " is one that is only advanced by the extremist end of the pro-cycling lobby; even Cycling UK (who can be relied upon for many daft suggestions) aren’t sure and only “suggest that it should be allowed”.
I can give a specific answer to that for “foot” access - here (although that’s been heavily remapped since I used “foot=designated” here). There is a legal “access on foot” right of way that joins the road from the south and the north, but it’s actually a dangerous road to cross. There were blue signs telling pedestrians from the south to turn right, walk along the cycleway to the roundabut cross at the crossing there, and then walk back the other side. I tagged those sections as “foot=designated” as opposed to “foot=yes”. Since then another mapper has added “foot=designated; bicycle=designated” to the entire length of https://www.openstreetmap.org/way/894921545 which in a sense isn’t wrong (the cycleway was created for use by pedestrians and cyclists), but it does lose the sense of “pedestrians are specifically requested to do X” that was there before.
Other than that, @Richard is correct (“never”) for England and Wales, although Scotland has different rules and I’d defer to a local on how best to tag things there - I’m unsure how the Core Paths network handles “intended for mode X” in addition to the “Core Path” designation and the basic legal right that I linked to in the other post.
Speaking a bit for the US here (though even that is a patchwork of different rules), there are very few cases where a cycleway is compulsory for bicycles and forbids other transportation methods. Usually the only implied forbidding is motor_vehicle. Everything else is generally allowed, though there may be different right of way expectations.
So when I’m mapping here, I follow the recommendation that @emvee noted, and interpret bicycle=designated to mean that the way is designed for bicycles, and bicycles are expected to be the primary user. This is stronger than “yes” because in a common law community you can ride bicycles nearly anywhere, but some routes are preferred because they were built for bicycles.
So to map this to the German case, the official blue traffic sign is the highest form of designated and it is backed by law. This is very uncommon in the US - if we followed this convention there would be almost no use of designated here for bicycles (or horse or foot, for that matter).
For the “case 2”, clearly identifiable as a cycleway, but not compulsory, then I’d ignore the law for the moment and focus on the intent. This would map in the US to something like this 36 Bikeway where I live - it is 27km long, wide, signed for bicycles, named for bicycles, etc. You could walk on it or ride a skateboard or crawl on your hands and knees, but the intent is clearly bicycles. In your case, because of the above convention about designated implying no for everything else, you’d want to then add the various yes's for foot, horse, etc.
My 2cts:
b. No; it’s not a cycleway. Goto 2.
2 Is this a type of way where bicycles normally may ride AND where bicycles do not have an alternative they are supposed to use?
This is country-specific. Use the access-wiki with access rules by country and by road type.
Yes: you’re fine.
No: if it’s an exception. tag bicycle=yes.
Not clear: tag bicycle=yes to be sure.
If bicycles have an alternative they are supposed to use (e.g. a lane or a side path) tag it on the highway (e.g. with bicycle=lane or bicycle=use_sidepath)
In all non-standard situations, tag explicit access. E.g. if a moped is allowed on the separately mapped cycleway as well as on the road, tag moped=yes on the road AND on the cycleway.
In short,
bicycle=yes means access for bicycles.
highway=cycleway means it’s a cycleway verifiably (signs, markings, construction) made for or assigned to bicycles; bicycle=designated is implied, but other access rules (exclusions/inclusions) may vary.
bicycle=designated on a different road type or path means it’s not a cycleway per se but it’s formally dedicated firstly to bicycles, while not implicitly excluding other traffic.
I’m in the US (CO), and never use ‘designated’, always ‘yes’ if they are allowed. I don’t see the value in ‘designated’. There are very very few paths where bikes hold a priority over walkers.
I’ve used bicycle=designated to mean designed and managed for bicycles as documented on the wiki. However, this piece of information is not about access so it would probably be better if wasn’t part of the access tagging scheme. I consider bicycle=designated and bicycle=yes to have exactly the same meaning with regards to access.
So “but on public footpaths the law is less clear” should be removed?
(I added " Note: access restrictions across the United Kingdom are NOT uniform, see this note" already)
I think that was British Cycling being daft rather than Cycling UK (and yes, the organisation names are confusing!).
In Australia we use bicycle=designated where it’s signposted or otherwise marked as for use by bicycles, this might be a bicycle logo painted on the surface, or a bicycle sign. bicycle=yes is for where bicycles are allowed to use the way, but it’s not signposted or marked as for bicycles (ie. implicitly).
Shared paths are common here which are equally for pedestrians and bicycles and we tag them highway=cycleway + bicycle=designated + foot=designated + segregated=no.
Not that our use of bicycle=designated doesn’t mean cyclists have to use that way rather it’s signposted that they are allowed to use it.
I would say bicycle=designated is redundant for this case. A cycleway is a path designated for cycling, as you described.
This is an important point. In countries that adhere to the Vienna Convention, mandatory signs are an important category of signs that correspond to certain traffic rules. 
is part of this convention.
By contrast, in the United States, mandatory signs officially don’t exist at all, and neither does the legal basis for them. The closest thing is the occasional “must” on a regulatory sign, such as 
obligating cyclists to make a two-stage turn. But regulatory signs about access either say that you may or you may not, so there isn’t an obvious opportunity to use access values such as designated and use_sidepath.
Some jurisdictions have antiquated laws requiring cyclists to use a sidewalk if provided, but people don’t tend to map such laws because they’re uniform across the whole region and not well known. Or the laws are full of obscure conditions about age, age of the accompanying adult, etc. that no one knows how to tag.
I tend to use bicycle=designated instead of bicycle=yes because iD’s Cycle & Foot Path preset uses bicycle=designated foot=designated. If that’s imprecise, then at least there’s little practical harm compared to omitting foot=* from a highway=cycleway, which does break routing.
That said, there are situations where I find it important to distinguish between bicycle=designated and a mere bicycle=yes, for example:
I would interpret the icons on this sign as foot=designated horse=designated bicycle=designated wheelchair=designated moped=no hunting=no. Although it’s perfectly legal to bike on the parallel residential road (bicycle=yes), you would “go out of your way” in doing so. (The ⅓ route shield calls for a route=bicycle relation but doesn’t affect the access tags in any way.)
This trail is officially a multi-use path for pedestrians and cyclists. An occasional park rules sign reminds pedestrians to keep off the paved trail and stay on the sandy shoulder when possible. This rule isn’t strong enough to relegate pedestrians to foot=no shoulder:foot=designated, but it does establish cyclists as having the right of way. (Unusually, cyclists even have the right of way over motor vehicles at crossings.)
This expressway=yes has a bike lane. Can you see it? It’s indistinguishable from a shoulder, except for an occasional “Begin Turn Lane – Yield to Bikes” sign at an off-ramp. The local highway department has a policy of keeping the bike lane as low-key as possible. They don’t want casual cyclists to find out about it, because the heavy traffic at 50 mph would be hazardous. Nevertheless, this is a popular commuting route for more experienced cyclists.
In the West, some stretches of Interstate freeway allow cyclists because there’s no other connection, short of dismounting and climbing a mountain. But even though these freeways have all the relevant signs allowing cycling, they would be only bicycle=yes because they only allow cyclists by necessity.
Aside from these cases, if you prefer to tag a bike trail as highway=path instead of highway=cycleway, then bicycle=designated is the most obvious way to refine it as a bike trail as opposed to a trail that happens to allow cyclists. Iterative refinement is also possible with path=*, but that key is about physical construction rather than usage.
In Japan, there is no common tagging scheme for bicycle=* yet. I’m just proposing a new idea to distinguish between bicycle=designated and bicycle=yes on the talk-ja mailing list.
There are two types of signs in Japan that indicate bicycles are allowed on footways. These roads have legally different traffic rules.
(1) A “For bicycles and pedestrians only” sign (a sign with a picture of a bicycle and pedestrians)
(2) A “For pedestrians only” sign with a “Except bicycles” auxiliary sign (see photo)
In the proposal, they are distinguished by bicycle=*:
(1) → highway=footway/pedestrian + bicycle=designated
(2) → highway=footway/pedestrian + bicycle=yes
I’d map that as a cycleway:right=lane; cycleway:right:oneway=yes. Notwithstanding, that doing so, might make casual cyclists end up there. To consider hazards is not fun: Imagine a family with kids on holidays getting routed there. This especially, when realizing that openstreetmap data gets used as the one and only source of information more often than welcome.
Back on Topic: In a Vienna Convention country, this might very well carry a mandatory sign, the meaning of would be, people are to cycle on the designated lane, and not on the carriage way there. There really is not more to this. Anybody disagrees?
Yes, there is often subjectivity to where cyclists should go – on this expressway, I often pass by cyclists enjoying themselves on recumbent bikes. However, my point is that some bike lanes are pointedly discreet.
Within this region, a normal bike lane would be festooned with bike-related signs, but here it’s just the bare minimum that the local authorities can get away with. They wanted to prohibit cycling and walking, but cycling advocates pointed to state law that only allows them to prohibit cyclists and pedestrians from freeways, not expressways. As a compromise, they took down the prohibition signs but don’t facilitate non-motorized traffic.
Though this is an extreme case, I think it’s on topic in the sense that the presence of cyclist-oriented accommodations is a major factor in bicycle=designated tagging in the U.S. The original post asked for perspectives from outside Germany, but it didn’t say all the responses have to come from the same planet. 
I wound map this as shoulder=yes together with shoulder:width.
As the Wiki says:
In many legislations, it is allowed to ride a bicycle on the shoulder if it is present, which renders even slim shoulders a useful safety feature for cyclists - same for hikers.
Yes, this does address the question of how to describe the physical characteristics of this lane. In terms of access restriction tagging, something along the lines of bicycle=yes or shoulder:bicycle=yes would be more appropriate than bicycle=designated or shoulder:bicycle=designated for the reasons above. Or maybe discouraged, but those typically have “not advised” signs around here. In other words, the absence of a sign can sometimes be as meaningful as the presence of one.
Yes, designated does not fit here.
Following “map what is on the ground” I would indeed go for bicycle=yes or shoulder:bicycle=yes, discouraged only if there are sign or marking indicating that.
