Dual carriageway junctions

Hi All,

I’m currently working on checking and updating bus routes throughout Bristol, as well as checking pedestrian crossings. These projects have overlapped often at large junctions which often need tag and geometry improvements.

Could anyone please help me decide whether this road junction would benefit from a little simplification? I’m coming at this;

  1. from a useability point of view,-
  2. to simplify tagging updates (particularly for route relations & road naming) and,-
  3. to improve the accuracy of representation (my interpretation of the wiki).


My questions are:-

  1. Along the A432 (Stapleton Rd), should the two short ‘straight-ahead’ ways be combined into one way with appropriate access tags? It is one carriageway on the ground, with a mini separating traffic island (no pedestrian crossing island).
  2. Should the right-turn filter lanes on the A4320 (Easton Way) meet at a common node on the A432 way (with turn restrictions), instead of tracing the swept paths? Similar-ish to this A37/A4174 junction
  3. Should the pedestrian areas be re-tagged to traffic islands? This looks to be an old mapping practice from before area:highway=traffic_island existed. Is that a fair guess?

I’ve decided to post in UK, as I’ve seen it’s very common here to map two ways around a mini traffic island (e.g unmarked pedestrian crossing islands). I currently sit on the side of the discussion which views this as incorrect accoring to the wiki and road-classification logic (i.e, two ways only for dual carriageways and extended traffic islands), but don’t want to make lots of these changes if I’ve misunderstood things as a newbie!

For clarity; existing:

Proposed:

Massive thanks for any help you can offer!

2 Likes

Hi
I’m looking at something similar in Wigan where there is a series of junctions and new segregated cycle lanes.
With respect to the pedestrian areas I would base my decision as to whether pedestrians use them - in your case they clearly do. There are areas which are not used by pedestrians or cyclists and these I have seen with area:highway=traffic_island and sometimes with barrier=kerb, although reading the wiki the barrier=kerb is wrong.
highway=traffic_island is not rendered on the standard carto but it would be useful if it did - but I would still map them cos I am sure they are useful when rendered.
With respect to your ideas about the roads on the junction I have different ideas depending on thie situation - but in this case I would leave them as they are and add a small traffic island on the northbound Stapleton Road where the bus lane is segregated by the island with the bus lane traffic lights. Keeping that separate bus_lane is I think important to assist routers as your proposed may allow cars to proceed north boud and possibly turn right - which in my view of the aerial is not possible.
Good luck with these complex junctions

Thanks for your thoughts, Tony.

The pedestrian use of the traffic islands is covered by the footways crossing them. To map both the footway, and the area as pedestrian seems to me to create two features for one physical instance. The proposal I’m making is covered in the wiki, and I don’t see traffic islands as being similar to pedestrian areas

As for the road ways, there would be no risk of affecting routers, as they would be guided by access tags, and turn restriction relations (as shown in the proposed JOSM screenshot).

So This is how I would map it.


(ignore turn restrictions, I didn’t bother fixing them)

The only thing I’ve really changed is, I’ve split the duel direction way in the middle of the junction. although merging is technically correct it also make the junction a lot more complex and harder to work with so I would split it. The ways should remain separate as they are since they are separated by a physical barrier, the islands. You could have the filter lanes meet in the middle and limit them with restrictions however i believe keeping them separate is cleaner and simpler. The only other change I would recommend is moving the split points on the north and south road closer to the intersection as they a currently placed where the lanes begin. not where islands split the road.

Your islands should be highway=traffic_island as not all of the island area is intended for pedestrian use. You could add a pedestrian area within the island however in this case i wouldnt bother since the pedestrian area doesn’t really stretch any further than where the footpath way is already.

Mapping pedestrian areas on top of footpath ways is the correct way to do it. this is done for backwards compatibility so that nav engines that don’t support pedestrian areas can still function. It the same reason why we keep building=* areas even if we’re adding building:parts=yes so that renderers that don’t support building parts can still render the basic building.

with regards to crossing islands, both mapping a island as a node or as a area are equally acceptable however a mapping as a node is more of a less detailed short cut to allow lots of features to be mapped quickly. Please if a island has already been mapped as a area with a highway split around it please do not simplify this as you would be destroying detail a previous mapper spent time creating. There is a recent debate about this here Residential Road with Separated Lanes

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Hi Kitsee
I do like your style on this - the roads are easy to read the flow. For completeness I would add the pedestrian island areas on the southern part of Stapleton Road

Thanks for your input, @kitsee. I remember reading your discussion the other week and concluded I was “pro- node” camp for mini crossings. My thinking being that a map is inherently an abstraction of on-the-ground reality; that’s what makes it useable (instead of a ba-jillion byte digital-twin model of the Earth!). As it happens, it’s a slighty different scenario here anyway, as it’s not a pedestrian refuge.

As for your point explanation of keeping the lanes split:

, would you mind elaborating a little on those challenges / risks, please? I feel there’s some bit of knowledge here which would be really useful for me to be aware of for future highways edits!

Definitely on the same page as you here ^

Questions like that give me flash backs of school exams for me :joy:

Its not a easy question to give a straight answer for. The most obvious answer is that it just visually looks better. Although we should generally avoid “mapping for the renderer” it can help guide you down the correct path since looking better visually prob means that its a closer representation of reality. The original junction is hard to understand at first glance as a user and as a mapper. Merging in the middle of the junction is not a normal thing to do so it can require extra thought to understand it.

From a user’s perspective it also not a very clear representation of reality since imagine if you were using this map as a satnav. as a bunch of ways just merge together in a single node and it can sometimes be hard to tell which ways connect to which. The ways have also been shifted so that they all meet at a single node too and as a result doesn’t follow the path that the vehicle follows.

From a mappers’ perspective, again if lay out more normally then it easer for a mapper to understand it quickly from a glance. It also spreads the turn restrictions out so that they are not all just dumped on a single node but spread out over a number of nodes.

Here are a few examples of well mapped intersections atleast in my opinion to give you some kind of reference.

Tbh i’m not sure if this is all that useful or if i’m just rambling now but i hope this is of some use.
On the side note I just noticed while writing this, the turn lanes should be highway=primary_link rather than highway=primary.

I thought something similar as I was typing that!

I’m generally onboard with your reasoning - I can see how the turning ‘links’ make it clear at a glance that they are turning lanes. Just to highlight with your second example:

; It looks like it should be possible to turn right from Moira Terrace on to Fitzalan Place, however the ways don’t allow it. Feels like this error wouldn’t have been made if the junction was created by mapping the roads, rather than vehicle routes. Mapillary view of turn lanes approaching the junction from Moira Terrace.

Separately, I actually meant to ask about having Stapleton Rd ways separate for the bus gates. I’ve proposed merging them on the basis that the forward/backward directions are not separated by a significant traffic island / reservation. Can you explain why you would retain the split?

The ways do actually allow it in a odd way but I agree it looks like a bit of an oversight on this one that I missed as well. Prob could do with an extra turning way to clean that one up.

Although its not significant, they are still separated so I would but don’t take my words as gospel here. This is very much entering the personal opinion territory. I’m a micro mapper, I like making the map as detailed as possible so this is what I would do. You don’t have a bad argument here for it being a single way and if I saw it mapped like that and I wasn’t micro mapping this area I prob would leave it like that too.

however I would say that now that it has already been mapped as two ways and the islands have been mapped as well. simplifying it would sacrifice previously mapped detail which is a big no no in my book. OSM should only ever become more detailed over time and never go backwards, becoming less detailed.

Also just like to mention that although area:highway=traffic_island doesnt get render. for mapping it quite nice for tying other features together like footpath=traffic_island, dropped curbs, lamps eta. For example, that junction we were just discussing

Ah, yes. I guess this case rather supports my case for not adding these so-called ‘turning ways’ as they don’t physically exist. Following your logic through, surely there should also be ‘turning ways’ for the remaining two directions:


This is surely mapping for the renderer/router so I’ll avoid it unless there’s a separate *_link way approaching this junction (as in my original case).

As for the busway;

, I do agree with this, except surely where said details are ‘wrong’ according to the community consensus? In this case, as I’ve not had any support to change it, I’ll leave it be. It will still grate on me every time I cycle through that junction though!

Cheers for your help, I’ll crack on with the pedestrian edits, and leave the roads for the time being.

This is borderline mapping for the renderer. I wouldnt add the extra turning ways since the existing one are close enough to the actual path the car would take, the one i did add is quite different to the existing path. like i said logically the turning lane is not needed, the only reason i add it is to be more accurate to the vehicles path to make the data more usable for uses such as sat navs. they are mostly optional tho.