When entering a high capacity roundabout about there’s often, at least in Sweden, a separate way acting as a shortcut allowing motorists to skip the roundabout if they are making a right turn.
Example: http://www.openstreetmap.org/?lat=59.29688&lon=15.22749&zoom=17&layers=M (Going from north to west)
I suppose that should be mapped as the linked example, as the right turn “shortcut” is a link between a trunk road and a motorway_link. If both ways were trunk roads there should be a trunk_link, right?
The problem is that the shortcut never gets favored by the routing engines (or Garmin maps created with standard TYP-files and Mkgmap). Is there a better way to map these shortcuts to ensure proper routing?
Note: Going straight south and into the roundabout and then take the first exit right is of course also allowed in the example linked above.
Note: Tested with http://www.openrouteservice.org/ with an route from “Pos@: 15.229254 59.299783” to “Pos@: 15.220886 59.295488”
mkgmap processes highway=motorway_link (the “shortcut”) with road_class=3 road_speed=2 whereas the trunk road that forms the roundabout is processed as road_class=4 road_speed=5.
This is probably why mkgmap favours the roundabout. You could change the mkgmap lines rule file so that motorway_link is given higher speed and class if you liked, but you’d need to be comfortable that this was appropriate for the wide range of motorway_link roads that your router will encounter.
I had once a similar problem in Hungary, where two primary roads meet. There I tagged the “shortcut” simply as “primary” and since then, my Garmin routes properly.
An other solution could be to map the shortcut as short as possible (means starting it nearer to the roundabout in the linked example).
Does any device display the value of the maxspeed-tags? That could also be a possible solution.
csdf, GPS-Mic: Thanks for your suggestions. I successfully changed the mkgmap rule with higher speed and got the correct routing. For a more permanent solution I’ve changed the tagging on the source data in OSM to “trunk” as I didn’t want to shorten the way as it would affect the turning instructions.
GPS-Mic: I don’t really understand your question about maxspeed display and how that would affect the routing? As for display the current maxspeed in Garmin units I think only the NT format supports that feature and mkgmap cannot generate NT maps.
Sorry, I wrote it very incomprehensible.
I just thought, if no device shows the maxspeed that is tagged, it doesn’t matter what speed you write in the “maxspeed”-tag. Like this, you could increase the maxspeed there to influence routing.
No problem, I understand your point clearly now and yes, a temporary solution could be to change the default
junction=roundabout & highway=trunk [0x0c road_class=3 road_speed=2 resolution 18]
to
junction=roundabout & highway=trunk [0x0c road_class=3 road_speed=4 resolution 18]
However, editing the source data (OSM) would not be a good solution as other devices (non-Garmin, i.e. smartphone applications and alike) displays the value for the maxspeed tag in the same fashion as Garmin devices does for NT maps.
If I recall correctly there was some discussions on the mkgmap-dev list a while ago and while maxspeed might not be a large factor while routing (I belive the Garmin devices mainly uses the road_class) it still factors in the max_speed.
To sum it up, tagging these “shortcuts” as roadType and not roadType_link (i.e. trunk instead of trunk_link) seems to be the best solution.