No, it is not optional. It is optional for the motorcyclist who chooses to lane split or not, but it is not optional for the car or other motorcycle who has their lane effectively narrowed by the motorcycle doing the lane splitting. At least I am not aware of any signal that a motorist can give to tell a motorcycle that they don’t want their lane shared. So, it is a one-sided optional and that was my point.
I am fine with filtering at a stoplight or a traffic jam where the cars are stopped or effectively stopped (<10 MPH or so), but lane splitting/sharing at normal road speeds is not a good idea, in my opinion.
If you consider a lane filtering/sharing/splitting motorcycle a threat to your own safety, I can understand your opposition, Voyager.
"Studies" may exist, quantifying and qualifying peril (as in, higher accident rates, etc.) caused by the practice, but I'm unaware of any.
In years and thousands of miles ridden in the California environment, I never felt threatened by the presence of a lane-splitting motorcycle, whether I was on a bike or in an automobile. Not to say I was not merely unaware of the danger posed by the practice.
Some time spent in your own observation, and perhaps participation, in the California system might corroborate your opinion, or contribute to a revision of your views.
Regardless, fear not. My prediction, lane filtering/sharing/splitting will never become legally approved in Pennsylvania.
Yet, why concern ourselves with this issue? In ten years time, we are told there will be no internal combustion engines, thus no motorcycles or automobiles as we know them, nor any cows and consequent beef, milk, butter, ice cream, cheese, yogurt, leather, etc.!