Happens all the time in San Diego, so much so that I've given up and just roll with it. Without fail there will be a motorhome coming home from the desert and 20 vehicles will be stuck behind them for the entire two hour drive since they never pull over. I've gone the gamut from wishing for a highway patrol to wishing for a tow missile launcher, but in the end all you can do is go with it.
Oh, and I could be wrong, but I swear there are more slower traffic keep right signs than there are speed limit signs on the road.