Undivided dual carriageways

I’ve come across this situation many times and don’t know how to best map it. I finally decided to ask the local community about it. There are many multi-lane highways in Thailand that have cross-hatched center strips but no physical dividers separating the lanes.

Sometimes, depending on the mapper, these are drawn as two separate lanes with link roads added for u-turns just as you would draw a “real” dual carriageway, other times as one highway. See the image in the link below:


My inclination is to map these as a single highway, especially here in Thailand, because cars make u-turns or cross a through-intersection wherever they want without incurring any penalty. Those hatched lines don’t mean much. The situation is quite different in the U.S. and Europe where one had better pay attention and not cross those “invisible barriers”.

The question has come up before:


That looks like a road with 4 lanes, not like a dual carriage highway with 2 lanes each.
During armchair mapping, it can be difficult to distinguish between these two types, depending on the quality of the imagery.
I’d suggest to use the lanes tag here.

I’m inclined to agree, as I said, but I won’t change any of the many similar highways drawn as divided (dual)-carriageways. I just wanted some opinions about future mapping.

Thanks for your reply.

I believe the existence of a physical barrier has long been the OSM standard for determining whether or not to map a dual carriageway. So a painted median, however wide, won’t count. (This is one thing OSM does differently from some other mapping services, included Google Maps.)

I’ve actually quite often thought about this. There are situations, e.g. those involving large intersections, where mapping painted islands as if they were physical would make life a lot easier. I still avoid doing so if possible though.

Regarding the current situation, I’ve seen quite a few highways that aren’t correctly tagged according to the criteria. But I’m not sure whether it’d be worth the effort to “correct” them. For all we know, a physical barrier might be installed in the near future, making the whole thing a waste of time.


Thanks for that clarification. I believe that some folks, when checking their work against what Google shows, choose the dual-carriageway mapping because that’s what Google does. I don’t think that’s completely unreasonable (or illegal) but it makes mapping any intersections with residential ways needlessly complicated. The actual mapping of the primary highway shown in my example photo has primary_links connecting the two carriageways at such intersections. In real life, and especially in Thailand, cross traffic can simply ignore the hatched line.

Another concern I have is that by using a false dual-carriageway, one gets an incorrect representation of the reality on the ground. There are not two highways separated by a wall or median present but only a very wide multi-lane highway.

I am in agreement with Dave & Paul … its Thailand, and people can and do turn on these “painted islands”. I tend not to correct other mappers work if it exists already, but if Im doing new stuff, then I tag the painted islands roads as single.
Agreed there is a fine line between where we draw a link to show a u turn OR go from dual to single and back to dual again. I guess that largely depends on the number of physical intersecting ways… ie, 1 or 2 ways that connect would get a link, whereas 3 or more would influence me to draw this as a length of single section carriageway.