Should the link between roads be tagged based on the road type your head to or the road type you’re coming from?
If the data is going to be used for a navigation system, then I think you would use the road type you’re going to:“Take the ramp to I-270.” If you want to know what agency/entity maintains the road including the link (ramps) then it would make sense to use the road type you’re coming from and then use another tag to indicate what road you’re going to?
In most cases, we match whichever is the highest up the hierarchy, regardless of whether it comes before or after the link road. As far as I’m aware, this is what navigation services expect, especially with highway=motorway_link.
If there is an intersection with a tertiary and a secondary road then all the links need to be secondary because the secondary road is higher up the hierarchy. Is there hierarch of the roads on OSM wikipedia?
Yes, they should be secondary. The hierarchy is as follows:
motorwaytrunkprimarysecondarytertiaryunclassified/residentialThese are listed in order on the wiki here.
Part of the reason for going for highest, is that we don’t have an unclassified_link or residential_link. The way that highway tagging ended up means you either use the highest, or you deviate from the standard sometimes.
At least in the U.S., the norm is for roads to be classified based on importance to the overall transportation network rather than a particular agency. So in most parts of the country, it isn’t too hard to find secondary state highways and primary city streets. There are also some locally maintained freeways built to Interstate standards and even designated as part of Interstate routes.
Navigation systems assume highway=motorway_link is a freeway ramp. If there’s a sign indicating that it leads to I-270, tag the ramp with destination:ref=I 270 (and also make sure the highway=motorways are all part of a network=US:I ref=270 route relation).
I always had difficulty to properly map highway links. Motorway and trunk highways don’t offer much difficulty, although there are exceptions. So, links which connect roads at different levels are somewhat easy to map. The problem is with roads at the same level.
This problem can be seen in the main wiki and also in some discussions about road links:
So, the general idea about mapping links with the higher hierarchy it’s not as static as it seems.