...positioned to give you a 1-dimensional measurement: how far are you along this specific tunnel? Expecting triangulation is, well, overly optimistic. By an order of magnitude, at least (and would require a far denser mesh of beacons, whereas this only needs one beacon every $measure ; or two, for added redundancy).

In other words, it's still useful for navigation like "you are now 2000 meters/yards/cubits into the tunnel, therefore you should be taking either of the two middle lanes", but the trick is, this doesn't require the app to know your current lane.