[OSM-talk] Using destination_sign relations for complex exit ramps?

[Phil! Gold]

I've been trying to figure out how to tag some moderately complex exit
ramp systems when a single motorway_junction doesn't seem adequate.  An
email to the newbies list led to an IRC discussion that led to a solution
that I like, so I wanted to present it here to see what other people think
and possibly what other people have done in similar situations.

As background, most highway exits are simple and a motorway_junction with
a ref= for the exit number captures all the important information about
the exit.  That approach works best, though, if there's only one
destination from the exit.  I know several places where the reality is
slightly more complicated than that.  One example is the exit from
westbound I-70 in Maryland (in the US) onto I-695[0]; there is one ramp
from I-70 with exit number 91 that splits a short while later into exits
91A (I-695 southbound) and 91B (I-695 northbound).  I want to be able to
capture all of that information in OpenStreetMap so that a theoretical
routing agent can say, e.g., "Take exit 91 to the right, then take exit
91A to the left."

  [0]: I-70 westbound exit 91 http://osm.org/go/ZZd8vPYT

The IRC discussion pointed out that this falls more or less into the same
domain as the destination_sign relation.  The only thing it's missing is
the exit number.  What I think would work would be to use the
destination_sign relation with the destination= tag containing the target
road (I-695, in my example), possibly extended with other destinations
from the signs in order of increasing distance (so exit 91A might have
"destination=I-695; Glen Burnie; Washington"); and the ref= tag containing
the exit number (so exit 91A would have "ref=91A").

I'd like to know what other people think about this idea.  What other
approaches have people used for my situation?  One person on IRC suggested
that ref= on the destination_sign relation sould be the ref of the
destination road, but I think it makes more sense to have it as the exit
number (perhaps there could be a destination_ref= tag).  Are there any
opinions one way or the other on that?

-- 
...computer contrarian of the first order... / http://aperiodic.net/phil/
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