http://www.sigalert.com. It only does Southern California, San Francisco, Sacramento, and Phoenix, though. incidents and accidents marked, with timestamped CHP details via pop-up tool tips, exact speeds at each sensor.
The City of LA exposes traffic data from its Automatic Traffic Surveillance and Contol (ATSAC) traffic-signal system at http://trafficinfo.lacity.org, which shows surface-street speeds on selected arterials throughout the city.