Here in the U.S., we have been using the convention of tagging exit information on the
highway=junction node using the poorly documented
exit_to tag. The rest of the world has been using
destination=, and now that I start to think about this problem more, I can see why.
Let's take this example of the junction of I-70 and the Baltimore beltway I-695.
(Have you used Mapillary? It's pretty awesome and unlike Google Streetview, OpenStreetMap explicitly does have permission to map from the images. It's pretty easy and fun to contribute your own, too.)
How to tag this so that navigation and other software could make sense of it?
Currently, the only relevant tags on the junction node are
ref:right=91A indicating the exit / junction numbers. There is nothing on the `_link' ways to indicate destination.
One way I have seen people map this information in the US is roughly
exit_to:left=I 695 N;New York;Towson and
exit_to:right=I 695 S;Baltimore;Glen Burnie. This is not great for a number of reasons:
- There is no way to distinguish the
I 695 Setc) from the names reliably.
- The US seems to be the only jurisdiction where
exit_toon the junction node is used, the rest of the world uses
If we were to adopt
destination= in the US like everywhere else, most of the signpost information would be on the
_link ways after the junction node. Here is the situation in OSM:
According to the
destination documentation, the first way would get
destination=New York;Towson and the second way would get
So far so good. But there's a few pieces of information that are not captured yet:
1) The exit (junction) numbers 91A and 91B. Oh wait - these were already on the junction node as
ref:right=91A. This works, but intuitively it would make more sense to have this information on the
_link ways as well. I am not sure how and the wiki does not give any guidance. (Actually it suggests using
ref on the junction node for exit numbers.)
2) The road numbers the exits connect to, in this case I-695 N/S, leading to I-95 N/S.
destination:ref seems to be used for that quite a bit. That would work for I-695 but I don't know how to capture the connection to I-95.
Following the documentation as best I can, this is what I come up with for the highlighted _link ways:
I think this system makes more sense than the
exit_to convention that is poorly documented, doesn't scale well to cover complex cases, and is used nowhere else in the world. The only good reason I can think of why people use it is because it's used much more (about 18,000 times) than
destination= in the U.S. Elsewhere, from what I can see in TagInfo,
destination= is more popular.