Traffic in the Bay Area is predictably unpredictable. Fortunately, 511.org provides real-time traffic updates that are accessible over the web or from your phone by dialing 5-1-1. There is also a test site where you can customize your driving time updates for up to four trips.
511.org covers the entire Bay Area including San Mateo, Santa Clara, Alameda, Contra Costa, San Francisco, and Marin Counties. Silicon Valley coverage includes all the major thoroughfares like US-101, I-280, I-880, Highway 92, Highway 85, and Highway 84, plus both the San Mateo Bridge and the Dumbarton Bridge.