When I moved to the United States six years ago, center turn lanes were a new thing to me. Two Way Left Turn Lanes (TWLTL) , as they are officially called, make traffic safer on busy roads where left turns are made frequently, by offering a dedicated lane for left turns:

Source: MUTCD

When I first started to map these lanes, there was no documentation on the OpenStreetMap wiki on them, so I just started to map ways that have them with center_turn_lane=yes, thinking that I’d revisit them later if the community came up with more sensible tagging scheme.

In 2012, infamous US mapper NE2 created a page dedicated to TWCTL, adopting the British spelling centre_turn_lane=yes. There are around 6000 uses of this tag, most of them in the United States. The page was created without much if any discussion in the US community: there is only one reference to it on the talk-us mailing list[1].

Since 2012, more elaborate tools and tagging schemes have emerged to map lane configurations. I particularly like the Turn Lanes Editor JOSM plugin which, together with the Lane and Road Information paint style, offers an intuitive and visual way to add lane details to ways.

As you can see, when a two-lane road with a TWLTL is tagged using the plugin, the map paint style offers the correct interpretation. The tags on this way would be as follows:

lanes=3; lanes:forward=1; lanes:backward=1; lanes:both_ways=1; turn:lanes:both_ways=left

Even though this looks much more complicated than just centre_turn_lane=yes, the plugin makes it straightforward to map (it even remembers recently used configurations so you can easily apply to other ways) and it uses the generic lanes tagging schema that is already in wide use, and allows for tagging even the most complex lane configurations.

TWLTL have been a topic of discussion in the Telenav mapping team. We have been adding a lot of lane configuration details in Detroit and Phoenix. For now, I gave the team the following guidance:

• For ways that don’t have TWLTL tagging yet, use the plugin and the generic lanes / turn:lanes schema.
• Where (local) mappers have already added centre_turn_lane tagging, leave it alone.

I would rather have one consistent tagging style for the United States (and other countries where TWLTLs are common. I would prefer to use the lanes / turn:lanes tagging for that. What do you think?

[1] The OSM mailing lists are notoriously hard to search. I used wget -r -np -l 1 -q -A gz https://lists.openstreetmap.org/pipermail/talk-us/ && find . -name \*.gz -exec gunzip {} + && find . -name \*.txt -exec cat {} + | grep centre_turn_lane