Google Maps includes a ‘live traffic’ feature which apparently exploits crowdsourced data from GPS-enabled phones to estimate traffic speeds on main roads. There’s a screengrab (click to embiggen) of central London below, with the light colours indicating higher speeds. The coverage is a little patchy as it presumably depends on there being enough GPS-enabled phones moving around for it to work.
This isn’t really ‘data’ in the sense that we usually cover on this blog as it is proprietary and, thus far, not reusable. But you can see how the underlying database could be very useful for transport planning or road safety purposes.