Staggered intersection are very short zig-zags of only a few meters.
They are common in rural and exurban areas, especially in the US.
(In addition, these cases could as well be tagging issues)
We do not want to announce these short left-rights or right-lefts:
* -> b a -> *
| or | becomes a -> b
a -> * * -> b
Here is one example:
- https://www.openstreetmap.org/edit#map=20/39.26017/-84.25182
And here are two edge-cases that we don't handle at the moment:
- http://www.openstreetmap.org/edit#map=20/38.87900/-76.98519
- http://www.openstreetmap.org/edit#map=19/45.51056/-122.63462
and probably should not handle since the distance in between is
quite long (roughly 7-15 meters). For these we want to announce
two turns to not confuse the user.
Thanks to @1ec5 for raising this issue and @karenzshea for
providing additional US examples and cultural insights.
After half a day of looking at the tagging and the data came to the
following conclusion:
We can't keep the user to the innermost / outermost lanes depending on
the exit the route takes: we found situations where both heuristics were
wrong.
Even on popular roundabouts the tagging is often wrong or in the best
case not present at all.
There are at least two different ways to interpret roundabout
indications: 1/ where e.g. a right arrow on the lane indicates turn
restrictions for the roundabout and the need to take this lane to exit
the roundabout to the right (possibly skipping multiple exits) and 2/
where a right arrow just means this is a lane in a immediate right turn.
Example: Australia marks lanes with arrows that seem to indicate
"angles you can exit the roundabout from", for example, these two ways:
- http://www.openstreetmap.org/way/320941710
- http://www.openstreetmap.org/way/42918021
Whereas Germany marks lanes with "directions you can travel in these
lanes immediately after entering the roundabout":
- http://www.openstreetmap.org/way/52578338
These two different interpretations of how to draw the arrows on the
roads mean we have conflicting solutions to "which lanes can you use to
take exit B from entry A" based on locality.
Continuing to tag ways based on lane markings is no problem, but
unfortunately, we can't reliably resolve good advice for navigation
system users (like "use the inside lane to take the second exit at the
roundabout"), there are too many situations that would generate bad
instructions (instructions that tell users to go into a lane they
shouldn't use).
Before we asserted on unique lane indications per lane. Turns out the
OSM data contains lane strings such as:
left;left|right
Which represents two lanes as in:
<< >
|| |
The two left indications _on a single lane_ look like data issue.
And we can't represent this with our enum-approach at the moment.
We don't want to crash there, so silently swallow this and
generate a single left|right for it.
This changeset implements Lane Anticipation on roundabouts, delimited
by enter / leave step pairs. It does not handle lane anticipation
within a roundabout.
Lane anticipation happens on the granularity of a valid roundbaout:
We discard partial roundabout (enter without exit or exit without
enter) or data issues (no roundabout, exit before enter).
Related:
- https://github.com/Project-OSRM/osrm-backend/issues/2626 for lanes
within a roundabout
- https://github.com/Project-OSRM/osrm-backend/issues/2625 for handling
going straight in lane anticipation
