[jeroenhd]

There are government databases and various public-private partnerships, but the most common method of traffic information is by collecting users' information while driving. There are companies that resell access to that data too: https://www.mapbox.com/traffic-data

OpenStreetMap doesn't even seem interested in things like "this street is closed next week Tuesday", but maybe there's another open project that collects this information for maps apps. I don't know any, though.

[habi]

> OpenStreetMap doesn't even seem interested in things like "this street is closed next week Tuesday"

OpenStreetMap data consumers can have much longer update cycles than "next week", so it makes sense to have such information outside of the map data if you are implementing a router.