One of the things I respect about the Bay Area is the certainty with which a little rain will snarl a commute. To stay on top of traffic conditions, I recommend 511 Traffic, the Bay Area travel guide.
511 aggregates construction data and FasTrak data from CalTrans, accident information from the CHP, and real-time radar readings from SpeedInfo to give you the most accurate commute information available. Better still, travel times and traffic reports are available by phone and via the web. Give it a try before you head out for your next trip. The only thing more frustrating that traffic is traffic you could have avoided with an alternate route.