In CA, interstates are 12' wide, and maximum vehicle with is 9', most cars and trucks are much less than 8' wide. That leaves from zero to 4'+ between vehicles. California Department of Motor Vehicles: “California law does not allow or prohibit motorcycles from passing other vehicles proceeding in the same direction within the same lane.”
is a link to NHTSA crash stats for 2015 nationally. They don't break out splitting, but do helmets, bike size, rider age etc and state-by-state stats.
There was an extensive study of splitting in CA done by a Berkeley PHD here (and referenced in Motorcyclist mag):
- partially quoted here:
"Of the 5,969 collision-involved motorcyclists we studied, 997 were lane-splitting at the time of their collision (17%). Motorcyclists who were lane-splitting were notably different from those that were not lane-splitting. Compared with other motorcyclists, lane-splitting motorcyclists were more often riding on weekdays and during commute hours, were using better helmets, and were traveling at lower speeds. Lane-splitting riders were also less likely to have been using alcohol and less likely to have been carrying a passenger.
Lane-splitting motorcyclists were also injured much less frequently during their collisions. Lane-splitting riders were less likely to suffer head injury (9% vs 17%), torso injury (19% vs 29%), extremity injury (60% vs 66%), and fatal injury (1.2% vs 3.0%). Lane-splitting motorcyclists were equally likely to suffer neck injury, compared with non-lane-splitting motorcyclists.
Lane-splitting appears to be a relatively safe motorcycle riding strategy if done in traffic moving at 50 MPH or less and if motorcyclists do not exceed the speed of other vehicles by more than 15 MPH. A significant number of motorcyclists lane-split in fast-moving traffic or at excessive speed differentials. These riders could lower their risk of injury by restricting the environments in which they lane-split and by reducing their speed differential when they do choose to lane-split."