It certainly depends on the day of the week, weather and time of day -- along with 'not-so-predictable' accidents and road construction. Here is an excellent tool to study the traffic patterns over the time periods of your planned commute: http://www.sigalert.com/map.asp?region=Los+Angeles#lat=33.98 The traffic information comes from many different sources. In California, freeway speeds are primarily collected by Caltrans from sensors embedded in the roads. SigAlert downloads the raw speed data from them and displays it on their site in a (hopefully) easy to use format.
Los Angeles speed data comes courtesy of the Regional Integration of Intelligent Transportation Systems (RIITS) which is sponsored the Los Angeles County Metropolitan Transportation Authority (Metro).