Assemblyman Tom Lackey (R-Palmdale), a retired state highway patrol sergeant who co-wrote the bill, called the new law a "groundbreaking step."
"This is a huge win for roadway safety,” Lackey said in a statement. "We are now giving riders and motorists clear guidance on when it is safe."
Lane splitting, in which a motorcyclist passes other vehicles by riding between them along the lane line, has long been a controversial issue.
Technically, it has not been legal or illegal, falling in a gray area where it was treated as acceptable by law enforcement agencies. But when the CHP published guidelines on the practice in 2015, a citizen complained that the agency should not be allowed to create public policy. In came AB 51.
Quirk's original bill proposed that lane splitting could occur legally only when a motorcycle was moving no more than 15 mph faster than the traffic around it, and it prohibited the practice at speeds above 50 mph.
Several motorcyclists' groups objected to that, saying the limit was too low. Other groups and individuals, who believe that lane splitting is dangerous regardless of speed, objected to the proposal entirely.
The revised bill, which sailed through the legislative process, provides a basic definition of "lane splitting" and leaves the rest to the CHP. Quirk has said it has many benefits, including reducing traffic congestion and promoting safety.
"I am thrilled to see that California is once again at the forefront of common-sense road safety legislation,” Quirk said. "Signing of this bill will bring legitimacy to this practice and help to keep our roads safer and our drivers – both motorcyclists and motorists – better educated.”