It's a slick maneuver motorcyclists love, allowing them to cut between slower moving cars and shoot out ahead. Car drivers, often startled, hate it calling it brazen and dangerous.
Is it safe? Is it legal? Hoping to clarify the controversial practice, California officials have taken the unusual step of issuing a set of guidelines that affirm the move's legality but point out that there are only certain instances where it is considered safe.
California is the only state in the country that allows lane-splitting, also known as "white-lining," where motorcyclists pass vehicles in adjacent lanes by driving between them.
California Highway Patrol officials said they've posted the first-ever written guidelines on their website as part of a broader state highway safety initiative.
It comes as the number of motorcyclists in California is on the rise, as well as the number of motorcycle crashes.
"There is a need to acknowledge lane-splitting is being done in California, and a need to help people understand what is reasonable," said California Highway Patrol Sgt. Mark Pope. "Until now, no one in authority has said how to do it safely."
The guidelines say motorcyclists can ride between two cars if there is room, but must do it at no more than 10 miles per hour faster than the vehicles they are passing.
The guidelines also make it clear that motorcyclists should not attempt the maneuver at full freeway speeds, or in any traffic going 30 mph or faster. That essentially means the highest speed a motorcyclist should be going when lane-splitting is 39 mph.
CHP officials say lane-splitting is more risky at higher speeds because motorcyclists have less time to react when something unexpected happens.
"We are trying to get people to slow down," the CHP's Pope said. "A lot are traveling at 40 mph or faster. The faster you go, the harder it is to stop."
Pope said the guidelines do not reflect new policy. The CHP has long held that lane-splitting is legal in California because nothing in the state Vehicle Code specifically prohibits it.
Many states have explicit prohibitions against lane-splitting. Nevada state law, for instance, reads, "a person shall not drive a motorcycle between moving or stationary vehicles occupying adjacent traffic lanes."
Oregon, Washington and Arizona have similar language in their highway codes. Other states simply consider the maneuver unauthorized, according to the American Motorcyclist Association's national cycling rules database.
The practice is common in California. Some 87 percent of California motorcycle riders reported they lane-split, according to a recent state Office of Traffic Safety survey. Some motorcyclists call it lane sharing.
It remains a misunderstood and polarizing practice. The traffic safety survey found that only 53 percent of California drivers knew lane-splitting is legal, and 7 percent of drivers admit they have attempted to block a lane-splitting motorcyclist from passing them.
Several motorcycle activists lauded the guidelines for affirming California's unique lane-splitting privilege.
"They are very reasonable," said Greg Covel, executive director of ABATE of California, a motorcycle rights organization.
Now, "we know what the guidelines are, how close we are to staying within the law or pushing the law," Covel said. "If we get stopped for unsafe speed or lane changes, we know what we are getting into."
CHP officials say they sometimes ticket motorcyclists who are lane-splitting, but citations are based on an officer's determination that the rider is going too fast for conditions, or that the rider's lane changes are unsafe.
CHP officials acknowledge that motorcyclists get away with unsafe lane-splitting at times because it's hard to stop them.
"A lot of us are in patrol cars. It is hard for us to catch them," Sacramento-area Sgt. Mike Bradley told The Bee in an interview for a previous story on the issue.
CHP numbers show that more than 9,600 people in California were injured in motorcycle crashes in 2010, the most recent year measured, up 25 percent since 2000.
But state officials say they know of no comprehensive studies focused on lane-splitting dangers, and they do not have data on the number of lane-splitting-related crashes. Police say they do get reports of side rear-view mirrors being ripped off and occasional crashes, including fatalities.
Pope, of the CHP, and Chris Murphy, head of the state Office of Traffic Safety, said the state has engaged UC Berkeley researchers to study motorcycle crashes to reach better conclusions about motorcycling dangers in general, and lane-splitting in particular.
Depending on the Berkeley study results, the state could adjust its lane-splitting guidelines, Pope said.
Pete terHorst, spokesman for the American Motorcyclist Association, said the new California guidelines could be used by motorcycle advocates in other states to push legalizing lane-splitting elsewhere. But terHorst said advocates nationally typically focus on other motorcycling issues, including broader concerns about causes of crashes.
"We essentially endorse the California position, but we don't promote it in other states," terHorst said.
Some motorcycle advocates are leery of the CHP's guidelines. Gabe Ets-Hokin, editor of CityBike magazine in the Bay Area, said he appreciates the state's attempt to make a statement, but worries it could be a first step toward more restrictions on the maneuver.
"Lane-splitting is a unique lifestyle to California motorcyclists, a subculture," he said. "If we can do it safely, what is the problem?"
He and other advocates contend lane-splitting makes motorcycling safer by allowing riders to avoid dangerous situations in heavy traffic.
Motorcycle safety class instructors teach another technique, suggesting that cyclists ride on one side or the other of their lane, rather than in the middle. That way cyclists can avoid the oilier part of the lane, as well as see ahead better and give themselves more avenues to get out of trouble, Covel said.
The guidelines note:
Inexperienced motorcyclists should not lane-split.
On freeways (when traffic has slowed to below 30 mph), motorcyclists should lane-split only between the two fast lanes. The slow lanes are too dangerous because vehicles there switch lanes more often coming from onramps and getting to offramps.
Motorcyclists should not lane-split in toll booth queues.
Law enforcement officers can, at their discretion, determine that a motorcyclist is lane-splitting unsafely.
Other drivers should not try to impede motorcyclists from lane-splitting.