Routing — `exit_to=` to `destination=` rewriting

Posted by daniel-j-h on 26 February 2017 in English (English)

In which I explain the differences between the exit_to= tag on the highway=motorway_junction node and the more recent destination= tag on the way branching off of the motorway. In addition I will show the need for handling both tags and leave you with a tool for automatic rewriting.

To provide great guidance for our Open Source Routing Machine users we want to include ramp information for highway navigation.

There are two established tagging schemes for highway ramp signage:

  • Using the exit_to= tag on the highway=motorway_junction node where the ramp branches off of the motorway. This tagging scheme was established mainly because the default Mapnik style displayed the tag quite nicely.
  • The more recent destination= tag on the way branching off of the motorway.

Why do we have two completely different tagging schemes? Have a look at the following example where a ramp branches off the continuing motorway:

a . . . b . . d .
          ` . c .

In this case the exit_to tag is placed on the node b together with the highway=motorway_junction tag. So far so good. Now consider the common situation where multiple numbered exits diverge at a single point.

            . c
a . . . b `
          ` . d

Now there are destination signs for the ways bc and in addition for bd. There are workarounds like exit_to:left and exit_to:right tags but as you can imagine these do not suffice for more complex situations.

For these situations the destination tag is the better alternative.

Here in an example for where we need such a workaround.

Mapillary View

Photo by Mapillary user andrewsnow under CC BY-SA 4.0.

The Open Source Routing Machine supports destination= tags for quite a while now. Unfortunately handling exit_to= tags on nodes is not easy to implement for us, since we would have to create the road network representation first in order to derive the ways the destination signage is meant for.

There is a similar problem with stop sign nodes: without direction tag we have to find the nearest intersection to apply the stop sign penalty to the right turns. But in order to find the next intersection we already need the road network. It is simply not compatible with how the routing engine is structures to allow for high-performance parsing at scale.

Unfortunately the exit_to node tag is still quite popular, especially in the US. See the Bay Area as an example:


In contrast, the destination way tags:


We clearly have to support both tagging schemes at this point.

This is the reason I spent some time to build a tool for rewriting exit_to node tags to destination way tags where unambiguously possible:

It is quite conservative, taking care of oneway link roads, destination tags that are already present, making sure the highway=motorway_junction tag is present and more. It prefers correctness over quantity.

For the Bay Area it is able to add roughly additional 800 destination tags in a matter of seconds. A viable pre-processing step for making highway navigation with the Open Source Routing Machine more user-friendly, especially in the US!

Check out the tags' Wiki pages for more detailed descriptions:

Check out our recent Open Source Routing Machine v5.6 release in case you want to give it a try yourself.

Comment from Claudius Henrichs on 27 February 2017 at 09:07

Thanks for the great summary and sharing the tool. Do you know of any other QA tool to verify destination tagging apart from Check Autorität?

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Comment from Claudius Henrichs on 27 February 2017 at 17:33

Btw. Up until checking actual usage I was under the impression that exit_to was more popular than destination. Good to see that destination already has a healthy 5x advantage and growing:

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Comment from daniel-j-h on 1 March 2017 at 15:46

I think Osmose has some destination checks. Not sure what kind of specific QA you want to do on destination tags, though. Care to elaborate?

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Comment from EdSS on 3 March 2017 at 20:27

Since 'destination' is explicitly about the signage I make a plug for leaving abbreviations as they are, neither abbreviating nor expanding abbreviations.

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