California politics SUCK so for penance they let us lane split/filter.
As for holding up traffic when you filter to the front
If you filter that would suggest you're being kept from traveling the speed you would like. If someone holds up traffic after filtering to the front at a light, they should be shot! The reason for filtering is to proceed unimpeded when the light changes. I have NEVER had a problem with hitting the far crosswalk before the cagers get 1/2 way across the intersection AND I'll have 100 yards separation within seconds (without grossly exceeding the speed limit, just getting to it much quicker). As someone else said, I can get across town in less than 1/2 the time it would take in a car by splitting/filtering.
As for safety? Common sense goes a long way and when there is a lack of, there are problems.