Google Maps now has real-time traffic data. I checked Phoenix and I see lots of green. I assume that it means "go" but I have absolutely no context for what "go" would mean on a freeway. There's no legend anywhere on the page. Digging through the help, I find this article that indicates green means an average speed of 50+ MPH on that stretch. Yellow means 25–50 MPH and red means under 25 MPH (grey indicates unavailable data).
Yahoo's version is nice in that it shows average MPH at various points along the freeway, but you have to be at a sufficient zoom level for them to show up at all. Google's shows up from a citywide zoom level all the way down.
I guess to maintain consistency across those levels, you really have to boil down the presentation to something as simple as red, yellow, green, and grey. Still, it'd be nice to have actual speeds at some point…