I've done it... and lane splitting can be done safely... I stay at about 20 mph faster than moving traffic and once the traffic reaches 50 or more I get back in the lane. Those bikers who move between traffic at 50 mph or more than the surrounding vehicles are looking to die.
The "Slower Traffic Keep Right" rule only applies to "limited access highways"... that means freeways here in California. A large 6 lane road does not have that requirement and you will not find that sign on those roads.