In SoCal, yes it is legal and I'm actually not sure why because it looks incredibly hair-ball unless cars are at a dead stop. I'll probably ride for a week before I even try it. It is going to feel nice cutting 30 minutes off my commute, though. SoCal's freeways rarely run the speed limit at all anymore, lol.
I feel its best use would be after a red light, you mosey on up to the front, take off, rinse repeat..