Yeah, I was confused at first too. It is mentioned lower down in the article.
> In your example Fremont should presumably be highlighted yellow. I think the renderer puts far too little visual emphasis on the bike network roads (and omits the markings from the key!)
Yeah, although the Seattle yellow highlights are missing a lot (e.g. the Westlake cycletrack, which is one of the few "green" corridors in the city and used by tons of cycle commuters, isn't highlighted). The Elliot Bay trail, another "green" corridor along the bay, is highlighted around the interbay area but not the portion that heads into downtown. The Interurban North multi-use path is also missing from the highlight. But that's just bad data; it can be fixed.
Edit: Oh wow. It turns out some highlighting isn't shown at some zoom levels. (In particular, the zoomed out view I would use to try and plan a route is missing some highlighted segments that show up at higher zoom.) I saw a random block highlighted yellow; zoomed in and it is part of a much longer segment. That is a big UI problem.
(Fremont Ave is highlighted at that higher zoom.)
Part of what loeg is saying is that the dedicated bike infrastructure they have is not especially bike friendly. To the extent people are following OpenStreetMap's "on the ground" rule, it will be hard to extract information about that from OSM.
Here the main highways are US Bike Routes, which means they get highlighted on the Mapzen style, with the exception of someone doing a through trip (where they are the logical route), they are the absolute worst places to cycle in town.
Since it is open street maps, it relies on manual curation. My guess is that some curators use the official city bike maps, and others editorialize.
This seems to do a great job where I am, though it crashes repeatedly on both iOS devices I tried. It definitely needs better testing with mobile safari (and an app with some sort of tile caching / offline mode, unless I'm missing something)