I lane split every day on my commute on Hwy 94 San Diego. All statistics, unfortunately I don't have them say splitting lanes is safer than sitting in stop and go traffic. The chances are way higher that you will be rear ended than cut off. If done properly and at the right times and speeds it is a safe way of riding.
I will lane split at any speed as long as I feel it is safe, or putting me in a safer spot. If I am following a clump of traffic at 60-65 and see open hwy ahead, I will split to get out of the clump and on to the open road, where there is less chance of any one hitting me from any direction.
I do agree that it takes a certain kind of mind set and rider confidence. I have friends that have been riding for 15-20 years that do not feel comfortable splitting in traffic unless its stopped. And sometimes even then they don't feel comfortable.
One rule of thumb I use is not to split if the car has a space to get in to on the other lane. I only split when cars are side to side and wait for that moment. If there is a spot on the lane your chances that a car will take it are high, so let them and split right after they do.
In my 20 years of riding Kalifornia, I have had almost none lane splitting incidents, compared to 1 rear end, 1 dog, and multiple cars pulling in front of me or turning in front of me.