I came across a few tagging quirks to do with bus access and specifically 
"service=bus"
First of all there's some roads with service=Bus (uppercase B). So that's 
definitely a mistake.Only exists in a cluster in Manchester ...plus a small 
cluster in Sao 
Paulo!:https://overpass-turbo.eu/?w=%22service%22%3D%22Bus%22+global&R

The idea of the service=bus tag is to allow a bus routing system to know that a 
service road is for buses, with the implication that all other service roads 
should be not for buses. That was a surprise to me. When I configured a bus 
routing system (doing this as part of my day job at TransportAPI), it didn't 
occur to me to disallow buses driving down service roads, but maybe I should 
rethink that.
In any case it seems to me that the access tagging should be regarded as 
orthogonal to this. What I mean is, while we might tag a road as service=bus, 
we should also make sure that all these special roads for buses ...allow buses! 
If they're access=no, then we need to also make sure they are bus=yes (or 
psv=yes). Sounds reasonable?
So I came up with this Overpass query to try to find those problems 
http://overpass-turbo.eu/s/pDR  Not too many.
Should we go ahead and fix those things? Quite a small job. I can do it myself 
actually.

There are many other problems (bigger problems!) we could fix with bus data in 
the UK. For example an extension of this idea would be to check for access 
tagging on all ways which are part of bus route relations. And of course 
there's coverage of the bus route relations themselves. That's something we 
were discussing here as a possible UK quarterly mapping project: 
https://www.loomio.org/d/uBc7x1Ok/quarterly-project-july-sept2017
Harry







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