Interestingly enough, southern California has experienced wildfires which demand excavation. These have occurred with no significant disruption or turmoil. Typically the refuge centers are not fully utilized. This may have something to do with a fairly robust infrastructure and generally competent emergency services.
It had less to do with the refugee shelters and more to do with being able to sleep on friends couches. Despite what was being broadcast on the television, half a million people did not end up at the Q or Del Mar. Everybody I know that evacuated ended up at family or friends houses. Also, traffic was a problem in the hours following the evacuation orders. Not a bad the the recent power outage, but compatible to peak rush hour.
On a side note, there was the normal panic buying of gasoline and bottled water. I think that no matter what the emergency, do not expect to be able to obtain those once the public becomes aware of the problem.
IMHO, unless evacuation is required, it's best to stay put and wait it out. At the very least, you'll miss the worst of the traffic while using your stored supplies.