Recent question of some person, who uses maps for Garmin navigation devices, derived from OSM data, brought up a good example of tagging for navigator. He was dissatisfied by his Garmin navigator (actually, BMW branded Nav V) making straight routes instead of turn-by-turn ones in case, when destination is somewhere in the end of private road, tagged with access=private. He acknowledged, that Garmin Basecamp software works okay with the same map. Then he asked about proper way to tag roads, say, in suburban residential areas, where access is limited by locked lift gate, and where only residents have a key.

Several OSM contributors told him, that it is enough to put a barrier=lift_gate node on that way and to tag this node with access=private. Wiki suggests this as some sort of “intermediate” or “minimal” tagging method too (and obviously, it doesn’t call it a “bad practice”). And it will help to avoid that issue of Garmin routing. But is it right and consistent?

First, access= key is intended to tell us, that access to tagged object is limited (or, otherwise, granted). So, tagging a lift gate with access=private literally means “this is a private lift gate”, which only partially makes sense, since you still can approach it, but you can’t operate (open) it.

Second, access regime of road is not necessarily created by physical barrier such as lift gate. It could have a sign. Or just a note. Or it could be limited by law. It means, we can’t eliminate adding limited access tag to highway-tagged way itself completely, in any situation. Therefore, it makes that scheme with access=* on a barrier node obviously inconsistent.

Third, having some limited amount of data in bounding box around the point with access=*, it is impossible (without any additional tag, which should store the direction of propagation for this access rule along the way it belongs to) to find out, in which direction does this rule propagate. Another argument is that access rules for roads should be inherited from an enclosing territory, where this rule applies. But it’s completely wrong, because there are access rules applied directly to roads, not to a territory, where this road is situated.

Indeed, it could be hard to tag every road behind the barrier with access=*. But that’s another question - question of data completeness. Situation of incomplete tagging is normal for OSM, and it doesn’t create any issue. But removal of access=* tags from roads does, because it increases an incompleteness.

So, that’s why we should, ideally, tag roads with limited access tags and avoid relying on barrier node. Especially, if this tagging method is inspired by an attempt to compensate bugs of certain navigation system by changing data for it.