Include exit numbers in turn maneuvers

[1ec5]

In some cases, an at-grade intersection may have an exit number. OSRM treats turns at these intersections like ordinary turns, ignoring the exit number. OSRM should provide the exit number in the RouteStep object so that OSRM Text Instructions can relay that number to the user.

In the U.S., this occurs most often on expressways. For example, this at-grade intersection on the Saw Mill River Parkway in New York is tagged highway=traffic_signals;motorway_junction ref=12 exit_to=Hastings on Hudson:

Guide signage for the exit 11 and advance guide signage for exit 12.

OSRM treats this turn like any ordinary turn, omitting the exit number.

It’s a bit nonstandard, but traffic lights along stretches of U.S. Route 27 and U.S. Route 31E are also tagged with exit numbers according to smaller signage:

This Overpass query returns any node tagged explicitly as both a traffic light and an exit. There are only a few results.

This broader query returns any node tagged as a traffic light with an exit number. There are nearly 3,000 results, but many are false positives. For example:

• I just fixed some nodes that wound up with ref tags due to someone using Potlatch’s Repeat function on autopilot.
• A few nodes in the U.S. are actually part of an unrelated tagging scheme, indicating the route whose mileage is measured from that point (example). To better distinguish between these two tagging needs, I’d personally prefer that mileage reference points be indicated by a dedicated route relation role. But I’ll save that proposal for another day. 😉
• Results in Australia appear to be intersection inventory codes or somesuch (example).

Many of the remaining results show that explicit tagging as a traffic light and exit should be more common than it is currently, although probably nowhere near as high as 3,000.

A related feature is the intersection names used in Japan, Korea, and Nicaragua. Some have been tagged as ref (example), but most are tagged as name or reference_point instead.

/ref Project-OSRM/osrm-text-instructions#136 (comment)
/cc @MoKob @daniel-j-h

[daniel-j-h]

I think the root cause is OSRM only handles junction:ref way tags for exit numbers. And our rewriter only re-writes in unambiguous situations where the exit is a link class.

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