Usability 
The Weather Channel's mobile site (xhtml.weather.com) has traffic maps for most US metropolitan areas. A traffic link appears below the current weather conditions if there's traffic information available for the area. The maps are resized to between 100 and 300px wide to fit the screen width of the phone you are using.
I think combining traffic and weather makes sense for mobile. The weather has a big impact on traffic and checking both before you go somewhere is a good idea.
The Weather channel, which aquired Beat The Traffic in 2011, no longer shows traffic information on weather.com