The official word from the CHP
is that "Lane splitting in a safe and prudent manner is not illegal in the state of California."
They also give some general guidelines:
1) Travel at a speed that is no more than 10 MPH faster than other traffic – danger increases at higher speed differentials.
2) It is not advisable to lane split when traffic flow is at 30 mph or faster – danger increases as overall speed increases.
3) Typically, it is safer to split between the #1 and #2 lanes than between other lanes.
4) Consider the total environment in which you are splitting, including the width of the lanes, size of surrounding vehicles, as well as roadway, weather, and lighting conditions.
5) Be alert and anticipate possible movements by other road users.
And they state quite clearly that "Lane splitting should not be performed by inexperienced riders. These guidelines assume a high level of riding competency and experience."
Honestly, I look at it similarly to overall speed (specific legalities aside). The speed limit is clearly posted, yet drivers routinely travel 5-10 mph over that without incident. And some drivers go even faster. But regardless of whether you set the cruise control strictly on the posted limit, or push a little more, it is still your
responsibility to make sure that you're traveling in a safe and prudent manner. There are times when it is reasonable to push a few mph over, and times when the "safe" speed is actually below what's posted (weather, visibility, construction, traffic congestion, etc.). The driver must make the call, and drive appropriately for the current conditions. And if a LEO thinks that you're not driving driving in a reasonable and safe manner, then they can pull you over for a little chat . . .