Can I add my voice to encourage people to use OSM for lots of what's being talked about in this discussion (and the discussion on CityCyclingGlasgow). I'll mention the limitations of OSM in a moment, but first I think it's worth throwing some clarifications and positive notes about it in...
(disclaimer - I may be talking to people who know this already - sorry if I am - but lots of people don't so it seems worth a few words)
It's not helpful to think of OSM as having 'overlays'. What's great about OSM is that it's a database of information not a single map. There are hundreds of maps and apps using this database, and it's not too complex to create maps that do what you want for a given purpose. There are some maps which do use an overlay (like the Waymarked Trails site). Buy maps like the commonly used OpenCycleMap rendering are a complete re-design of the map using the same data. You might not notice this looking very different, but try something like the OpenCycleMaps Outdoors rendering style for a different look.
Why this matters is because it makes the whole thing MUCH more powerful for collecting and using the kind of data everyone is talking about. If we wanted a very different map, which only showed cycle routes and area names,like the tube-style diagram of Edinburgh's off road network for example, we can do this from the data.
There are holes in the data - stuff like path surface tends to be mapped much later than the actual route of a path, but I'd strongly encourage people to add this data to OSM rather than to try to create overlays of information elsewhere. And if there isn't an OSM rendering available which shows what you/we want, work on creating one... lots of people will thank you for it.
By the way, if there are objective things which aren't yet being mapped at all, the beauty of OSM is that you can still use it to map these. The 'tagging schemes' for the data aren't limited to what's already being tagged. If there's a need to record something new then it's possible to work with others to find a sensible way to begin mapping this.
To illustrate OSM as a database, take a look at something like Overpass Turbo which with a little bit of learning about OSM will allow you to extract data to use yourself.
And then I'd also say that if anyone does think about working with data as an overlay to a map, use OSM for the background too. It might take a little learning to do this, but you'll quickly end up with something more flexible than anything based on a copyright map.
Finally a couple of words about what I do think are the actual (and important) limitations of OSM when we're talking about cycling infrastructure. Where it does start to fall down badly is if you want to get into street-level details. Take a look at this rendering of the Haymarket junction for example. OSM works by mapping the routes of roads and other features, rather than by mapping the boundaries of these roads. That works really well for lots of what we need a map for, but it can't easily cope with the kinds of detail which make it so apparent that Haymarket is a disaster for people on bikes. You'll see that while the direction of the roads and tram lines at Haymarket (on this rendering) are correct, the rendering of the road has a single steady width, when in reality at a location like this the actual road area and lane layout is very relevant to us. As far as I know the mapping of road areas is still discouraged - although you'll find that areas are mapped for some other features like rivers, or pedestrian areas.
Sorry - that turned into a rather long post. Hope it's useful to those not familiar with OSM rather than just filling otherwise useful space...
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