Thanks for asking i've been wondering myself actually. I looked it up and organic maps doesn't actually do traffic, although magic earth does (another not foss but apparenty privacy respecting map app that uses osm) It says it's just crowdsourced from the general public who uses magic earth (in an anonimous way, I guess there are enough magic earth uses for it to work since Some people say it works really well. (although others say it doesn't you should probably try it for yourself.)

On another note that I also found from my research just then, traffic knowing apps don't actually improve travel times but they do make previously congested places more congested. sources:

source 1

source 2

source 3

Edit: Magic earth is not foss so there is no way to varify that the app is actually respecting your privacy like it says it is, i'm sorry for any confusion.