I would like to discuss the current implementation of using the highest highway=*_link route possible for link roads.
Using this method there is inconsistent application. Particularly at turning segements which are also marked as link roads but commonly have the lower classification marked. It makes practical sense that it be the lower classification. But that's a big editing task (May be assisted by bulk editing which is easy enough) What's your opinion guys?
Using this method a car magically becomes a part of the highest route even though only one car may travel on that road per day, if it goes into a motorway, its a motorway link. The lower route makes the most practical sense and is what is used by Google, HERE and the state and national government mapping (Which records the official road classifications and heirarchy) of my area (Australia & Western Australia).
Additionally the current justification for higher classification - that link roads have the same attributes or restrictions as the higher classification road is untrue. For Australia wide (I have confirmed this information) it is a typical standard (I can't say for every single situation) that the speed limit remains that of the lower road classification until the slip road terminates. Though speed limit signs may be 150m or so prior to the actual "merge point" there exists no road here in government mapping, that is to say that 150m or so prior to the merge point the slip road is the main road. It is also at this point where you are on the main road that it becomes illegal to have horse drawn vehicles or mopeds or bicycles for example, depending on road. It is not illegal to have these on the link road (again can't confirm every single situation). Another note is that motorway links (controlled access highway slip ramps) can still have driveways or roads leaving and entering it provided there is unimpeded traffic flow whilst a motorway cannot.
As I'm exhausted at the moment XD I'll refrain from going into further detail.
Some clarification examples include:
12:00, 3 Feb 2015 (AWST)
Add comment here, include time and date.
so the admonition against tagging for the renderer does not apply here.
Everybody can stop reading after that half sentence. This is a rendering problem, so fix the rendering rules, not the tagging. I thought that this was clearly stated in the original mail discussion. --Cartinus 17:16, 2 July 2010 (UTC)
- What's the rendering problem? What's being said there is that whether it's tagged (for instance) primary_link or secondary_link makes no difference to anything except the renderer. --NE2 18:48, 2 July 2010 (UTC)
To render a link properly, it needs to start at one 3-arm node and end at another (3 or more). You get rendering artefacts if it ends at a 2-arm node (unless the two arms happen to be the same colour). The other thing you need to do is to put the link under the two 3-arm nodes at each end. So fine, you might think, just render links under everything (this is what Mapnik currently does). Unfortunately you need to render them on top of any intervening 3-arm node (eg a service road coming out in the middle of a link). The least amount of information you therefore need (to render it neatly) is the lower of the two classifications that the link joins. You render the link just under the lower of the two classifications, and any intervening even-lower classification road goes under the link. If you would prefer the link to be the colour of the higher of the two classifications that the link joins, you also need to know the higher-of-the-two classifications. The higher-of-the-two-classifications is (per the wiki) used as the basis of the highway tag (though this doesn't always happen, because it looks a mess unless it's reasonably symmetrical). Maybe the lower-of-the-two-classifications could be recorded with a links_lower tag. It would be simpler if we recorded the lower classification in the highway tag, and the higher one in a links_higher tag, but maybe life isn't going to be that simple. If you add both links_lower and links_higher tags, then a bot can sort it out one day.--RichardMann 22:30, 2 July 2010 (UTC)
How should links be named (to avoid them showing in red in potlatch)?
- Don't tag for the editor. If the link has a name, use that. If it doesn't, don't enter a name. (Roughly) all alerts of the data not complying with that/each editor's rules are notices only. Alv 22:23, 22 October 2010 (BST)
Observe this Y junction: 
2-way roads that split upon arrival of a crossroad should not be classified as links.
My reasoning is as follows
1. For a highway to be a link, it must branch off another highway that is of the same direction of travel. If a Tertiary Road ends in a Y Junction abutting a Primary Road, you do not have an option to take another route at the split. There must be options in order to classify something as link.
2. Routing is negatively affected by classifying them as links. A GPS giving turn by turn directions will state "Take the on ramp ahead and follow it onto the Primary Road" Since they are not thought as to be on ramps this can confuse travelers.
3. Each branch of the Y Junction is still considered the road which it split from. The name of the road should be added to the branches of the Y Junction. Therefore, it is not a _link because _link's do not have names
It seems there is a majority consensus on this based on what I see in OpenStreetMap. However, the picture on the _link page shows Y Junctions being classified as links (???)
If those Y Junctions are links then that would mean, for example in the picture on the official page, the Secondary Road (that turns into a Y Junction) would become a link at the previous node where two Residential Roads intersect the Secondary Road (which makes an elbow turn). BECAUSE at that point, there is no other option other than to merge onto the Trunk Road.
Let me hear your rallying cry and we can settle this!