Normally, a channelized turn lane is separated from the main road by a grassy space or a raised median. In OSM, this is usually mapped as a short highway=*_link that branches off from the main road. In Thailand, however, many intersections have a channelized turn lane separated by only a painted island.
In general, a painted island should not affect a highway being mapped as a dual carriageway, according to discussion at https://forum.openstreetmap.org/viewtopic.php?id=57544 and wiki. For these channelized turn lane, a small one may not be a problem, but refusing to draw them separately for the larger ones may cause navigational issues. For example, if the point of turn is distant from the intersection center, the navigation software may direct the user to make a turn at the wrong location. Moreover, we’ll have a difficulty with detail mapping if we wish to tag the lanes=* to the road around an intersection.
For example, see these location. (a street view can show it clearly)
https://goo.gl/maps/hfC1nWj35d5kTkTA6
https://goo.gl/maps/yp2jgUbCYztWrPef6
https://goo.gl/maps/9STuh6CJg5BLSsie7
Is it better to tag these kind of roads as a separate way?