Crowdsource the data from people running an Openstreetmap mobile app with location data collection, which doesn't exist and wouldn't be accepted by the sorts of privacy-conscious people who prefer Openstreetmap, or
Buy traffic data from a company like INRIX that actually goes to the expense of building and maintaining sensors along roadways, which it can't afford because it doesn't get that sweet exploitative advertising or surveillance capitalism revenue.

I suppose it's possible that apps like OsmAnd could provide a feature for users to log into and receive traffic data from their own INRIX subscription that they bought themselves directly, but that would be such a niche feature it probably wouldn't be worth the development effort.