California law does not allow or prohibit motorcycles from passing other vehicles proceeding in the same direction within the same lane, a practice often called "lane splitting," "lane sharing" or "filtering."
https://www.dmv.ca.gov/portal/dmv/detail/about/lane_splitting
Vehicle Motorcycles: Lane Splitting (AB 51, Quirk) Current law does not change; lane splitting by a motorcyclist remains legal if done safely. This bill defines lane splitting as driving a motorcycle, which has two wheels in contact with the ground, between rows of stopped or moving vehicles in the same lane. The bill permits the CHP to develop lane splitting educational safety guidelines in consultation with other state traffic safety agencies and at least one organization focused on motorcycle safety.
https://www.chp.ca.gov/PressReleases/Pages/New-Traffic-Safety-Laws-Take-Effect-in-2017.aspx
Lane-splitting in California has been legal for years. The reasons why include the fact that many motorcycles are air-cooled and rely on air passing over the engine to keep the engine from overheating. It also allows motorcycles to position themselves so that they do not get “sandwiched” between two vehicles and crushed in a rear-end collision.
http://www.sfweekly.com/sponsored/lane-splitting-minimum-wage-among-californias-top-new-laws/
https://wheelsofgrace.com/california-legalizes-lane-splitting/
At about this time every year, the legislative session in California is winding down. Sept. 15 is the last day for the governor to sign any bill this year. As the legislative year comes to an end, I once again summarize several of the new laws that I believe are of most importance to our …