With recent changes appearing on the Thai OSM, I have not been able to find much definitive advice on the above. A long time ago, I was advised on the Global OSM forum …
“If there is a physical barrier, draw as dual carriageway (divided road) … if its just a “painted island” meaning you can drive across it, then it gets drawn as a single carriageway.”
Seems in Thailand, its not really adhered to and I would like a consensus on this, and have it added to the Wiki.
My biggest argument is that if you if you draw roads with islands as dual carriageway, any right turns you need to make, (unless a link has been drawn), means the routing engines will send you onto the next available u-turn. This is technically incorrect, and makes a mountain of work drawing little access links to every POI on each side of the road to make the routing work correctly.
In OSM the directions are mapped as individual roads, U-turns connect them and are tagged with turn:lane=*
An example for other end of the spectrum for where the road is mapped bidirectional. The road has no physical separator in the middle, just a painted line. Legally it is not allowed to cross the line with your vehicle.
In this location we lack of imagery. Google shows again two lanes in each direction with a painted island in the middle. Adjacent to the painted island is a physical barrier. For u-turns it features dedicated lanes.
We have had this conversation before. I am in favor of mapping dual carriageways only when there is a physical barrier separating the lanes. Google displays these as dual separate lanes but I don’t agree with that.
In other parts of the world having a painted “no access” center lane might actually discourage drivers from cutting across it, but this is Thailand where such normal rules don’t apply. I always tag those with the appropriate number of lanes and otherwise treat it as a normal highway.
I don’t tag those lanes to the left side of major highways as cycleways or cyclelanes for the same reason. Someone is liable to be coming at you from the wrong direction, or there will be a small fruit seller kiosk built right in the lane, so one should not trust them. I’ve had pickup trucks use that lane to come back at me in order to avoid driving down to the next U-turn and then the next U-turn on my side. We wouldn’t want to inconvenience anyone, would we?
Here’s a photo of one from somewhere around Udon Thani. I consider this a four-lane highway and tagged it accordingly.
So having some examples to discuss about, I would say
A: borderline between single way or separated. Might be easier to handle if not separated. For high traffic roads I would consider separating it to route to proper U-turns.
B,C: These look like high traffic roads. People typically use U-turns here. So I would be angry if my routing tells me to cross that island at random places instead of guiding me to a proper u-turn. Mapping as individual ways sounds right here.
Exhibit D looks to me like a painted island type of situation west of the bridge. But at the bridge it disappears and does not start again before the road terminates. It’s hard to tell from the poor imagery what the situation on the ground really is.
The user who mapped this way is fairly new. His use of the dual lane structure east of the bridge could be a mistake, real and not apparent from the aerial imagery, wishful thinking, or his idea of what should be there. Without a ground check, we can’t know.
I appreciate Stephans efforts to reach a consensus, and of course, a lot is down to the clarity of the aerial, but in all cases, once we have surveyed it, then that takes precedence. However, I would add that if the aerial is not clear, tag as a bi-directional single way until a survey proves otherwise.
My comments on the pics are :
Exhibit A - Should be a single way, with no u-turn.
Exhibit B - Dual on the right side where we can see a physical barrier, then change to single heading to the left where the painted islands show.
Exhibit C - On the assumption its a barrier wall, then dual carriageways should be plotted.
Exhibit D - Absolutly single way … the U turn is a fabrication.
I think we can make things quite subjective using the interpretation of the aerials, unless you make the maxim simple - if the barrier is physical, make it dual. If not, then single way.
Now if Im heading East to towards Udon, and want to turn right onto the xx.2073, the routing will send me on the next junction where the minor road crosses, and force me to do a u-turn in the middle of the road. The counting down the GPS typically does to the junction will be wrong … it will send me straight past it, when in reality you move to the centre of the road and cross the island.
On the aerial the physical barrier making it dual carriageway starts another 1-2 km on. Now I think this was probably mapped before we had aerials, but illustrates my point. Without a survey, plot as single way then enhance as we find out the correct info.
Google Earth seems to support this convention too …
this is the same area.
I think I said enough on this now - you know my feelings ! Make it a simple rule, not open to discussion or differing interpretations ! Otherwise we will be continually changing each others edits.
Most of my mapping is in urban areas so the issue of painted medians per se isn’t something I come across often, but this, I think, is closely related.
Pardon the drawing; I couldn’t locate a real-life example just yet, but this is based on actual observations. (Note that this is one half of the road.)
Here, the physical barrier itself ends before the entrance to the soi, but the painted median extends past it. What I tend to do here is treat the painted area as an extension of the physical barrier, and map the street so entering the soi from the inner carriageway isn’t allowed. (While it’s actually physically possible to do so here, you’re obviously not supposed to, or they wouldn’t have painted it that way.)
This is quite similar to Exhibit B, so I would map the east section as a dual carriageway at least up to the first U-turn point. Further than that it turns into Exhibit C.
Now I’m not quite convinced it’s worth making the distinction between Exhibits A and C; it’s a pretty minor detail, quite too minor to base a recommendation on. I can hardly believe though that there isn’t an established tag for marking nodes where legal u-turns can be made on an undivided road. If such a tag existed I would favour mapping A and C as undivided. (Heck, we should probably invent one anyway, if we decide to go the undivided route, lest valuable information become lost.)
As an aside, I don’t think that “This is Thailand and traffic rules aren’t upheld” is a good argument to base our mapping on. If a street says no parking, it should be mapped as such. It’s the driver’s own decision to ignore the restriction and risk the possibility of legal action. The same should apply here.
Exhibit A is similar to road 217 from Ubon Ratchathani to Phibun Mangsahan. It was mapped as a dual carriage way, and I changed it to road with 4 lanes just recently.
With example B, I’d be interested if a little more to the left the road was physically separated again or not. If so, I’d likely draw that segment as dual carriage too, but won’t change it if mapped differently.
C: a road with 4 lanes.
D: roads with 2 lanes