Increase MAXIMAL_ALLOWED_SEPARATION_WIDTH to 12 meters

the check should cover merging of roads at intersections similar
to https://www.openstreetmap.org/node/53020993#map=18/37.86590/-122.25083

CHANGELOG.md

@@ -1,5 +1,7 @@
 # UNRELEASED
   - Changes from 5.15.0:
+    - Guidance:
+      - CHANGED #4835: MAXIMAL_ALLOWED_SEPARATION_WIDTH increased to 12 meters
     - Profile:
       - FIXED: `highway=service` will now be used for restricted access, `access=private` is still disabled for snapping.
       - ADDED #4775: Exposes more information to the turn function, now being able to set turn weights with highway and access information of the turn as well as other roads at the intersection [#4775](https://github.com/Project-OSRM/osrm-backend/issues/4775)
@@ -702,4 +704,4 @@
         - `properties.traffic_signal_penalty`
         - `properties.use_turn_restrictions`
         - `properties.u_turn_penalty`
-        - `properties.allow_u_turn_at_via`
+        - `properties.allow_u_turn_at_via`

src/extractor/guidance/mergable_road_detector.cpp

@@ -469,7 +469,7 @@ bool MergableRoadDetector::HaveSameDirection(const NodeID intersection_node,
         1, node_based_graph.GetEdgeData(rhs.eid).flags.road_classification.GetNumberOfLanes());
 
     const auto combined_road_width = 0.5 * (lane_count_lhs + lane_count_rhs) * ASSUMED_LANE_WIDTH;
-    const auto constexpr MAXIMAL_ALLOWED_SEPARATION_WIDTH = 8;
+    const auto constexpr MAXIMAL_ALLOWED_SEPARATION_WIDTH = 12;
 
     return distance_between_roads <= combined_road_width + MAXIMAL_ALLOWED_SEPARATION_WIDTH;
 }