Alain, lane splitting is also called lane sharing. In California and in many European countries it is permitted to ride between two cars/trucks that are side-by-side in adjacent lanes. As you can imagine, in LA's daily traffic jams while everybody else is bumper-to-bumper either stopped or traveling at very slow speeds the motorcyclists are riding between the lanes of cars/trucks at a reasonable speed and cutting their commute times by 1/3 to 2/3's.

Fred