Update: On Sept. 27, 2018, the California Highway Patrol released its “Lane Splitting Safety Tips,” two years after a new state law instructed the agency — along with other stakeholders — to develop guidelines.

If you’ve ever driven in California, you’ve probably had a motorcycle drive between you and another car.

This is called lane splitting (also known as lane sharing or filtering), and a 2014 study found that 80 percent of California motorcyclists lane split on the freeway, with more than half saying they do it “often” or “always.”

People have a lot of feelings about it, but they don’t always have all the information. So here are 10 things you need to know to understand lane splitting in California.

1. It’s legal

One of the biggest misconceptions is that when motorcyclists are lane splitting, they are breaking the law. They’re not.

“The practice of lane splitting or lane sharing has never been prohibited by California law,” said Sgt. Larry Starkey, who runs the California Highway Patrol’s California Motorcyclist Safety Program. “So therefore it’s always been an acceptable practice.”

According to the 2014 study, only 60.7 percent of vehicle drivers knew that lane splitting was legal.

Motorcyclists who are lane splitting still have to obey speed limits and other rules of the road, and can be ticketed if they don’t lane split responsibly.

2. A new law changed things … sort of

In 2016, Gov. Jerry Brown signed AB 51 into law, which defined lane splitting as “driving a motorcycle … that has two wheels in contact with the ground, between rows of stopped or moving vehicles in the same lane, including on both divided and undivided streets, roads, or highways.”

According to the CHP, this didn’t actually change state law. What it did was give CHP the power to develop guidelines around lane splitting.

This was important because the CHP put up lane splitting guidelines in 2013, but took them down a year later after a complaint was made to the state’s Office of Administrative Law. The guidelines could be seen as CHP establishing a new law.

AB 51 officially gave CHP the power to create those guidelines. Starkey said that after the law took effect on Jan. 1, 2017, the CHP worked with Department of Transportation, Department of Motor Vehicles, Office of Traffic Safety and motorcycle safety organizations to create new guidelines, which were released in September 2018.

The “Lane Splitting Safety Tips” begin with disclaimer that, “Lane splitting can be dangerous and extreme caution should be exercised. It should not be performed by inexperienced riders.” They include tips for motorcyclists (be aware of your surroundings, avoid lane splitting between large vehicles, danger increases at higher speeds and speed differentials) as well as messages for other vehicle drivers (lane splitting is legal, intentionally blocking a lane splitter is not).

3. It’s legal only in California

According to the American Motorcyclist Association’s website, every state except California bans the practice of lane splitting. Specifically, the states prohibit motorcycles from passing a vehicle in the same lane and riding between lanes of traffic or rows of vehicles.

4. Other states are trying to legalize it

At least nine other states have considered legislation that would allow lane splitting, and some are expected to take up the issue again in the next legislative session, according to Nick Haris, the Western states representative for the American Motorcyclist Association.

“I think as a nation, as we see traffic and attempts to address issues with gridlock, I think this is just going to be one of those things that’s identified as a good way to go,” Haris said of the increased interest in legalizing lane splitting.

5. It’s legal in lots of other countries

Lane splitting is legal and widespread in many European and Asian countries.

“It really is a way of life in Europe,” said Emma Booton, who grew up in England and has been riding motorcycles for 41 years.

“I’ve ridden in New Zealand where everybody just basically waved you around,” Haris said. “It was kind of surprising if you weren’t lane splitting.”

6. People have been lane splitting in California for a long time

News reports of Californians lane splitting go back to at least the 1960s. Starkey said the practice started as a way for motorcyclists to get through the state’s notoriously bad traffic.

“As the traffic got slower and slower throughout the decades, [motorcyclists] began the practice of lane splitting or lane sharing by moving in between the stopped rows of traffic on the highways,” Starkey said, “which prevented their vehicles from overheating, and also helped reduce congestion and ultimately let them get to their destination a little bit quicker as well.”

7. Very little data exists on how safe it is

One of the big problems with lane splitting is the lack of data on it.

A 2015 study from the Safe Transportation Research & Education Center at UC Berkeley found that 17 percent of about 6,000 motorcyclists who crashed between June 2012 through August 2013 were lane splitting at the time of their collision.

According to the study, “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.”

But the authors also say that the study cannot be used to compare the overall safety of lane splitting:

To estimate how the risk of being involved in a collision changes when motorcyclists chose to lane-split, we would require information on both the lane-splitting and non-lane-splitting riding that is done by some identifiable sample of motorcyclists. The collection of these data is fraught with problems, and the current study did not attempt to collect such data.

In other words, researchers would need to collect pre-crash data from motorcyclists who lane split and those who don’t, and see if one population is more likely to get into a crash than the other in order to determine the safety of the practice overall.

Data from the National Highway Traffic Safety Administration show that even though California is the only state in the country where lane splitting is legal, it’s in the middle of the pack when it comes to motorcyclist fatalities per 100,000 registered motorcycles compared to other states. But this doesn’t take into account other differences between state laws, including helmet laws and motorcycle riding culture.

At the end of the day, there are no conclusive data on how safe or not safe lane splitting is.

8. Here’s what motorcyclists say

Motorcycle riders say they lane split for two big reasons: speed and safety.

Lane splitting allows motorcycles to weave through traffic much more quickly than cars can, and bikers argue it also improves traffic for all commuters.

“You’re taking vehicles out of that line, and it’s less of a clog on the highways,” said Liza Miller, who runs a motorcycle repair and community gathering space out of her Santa Cruz home.

A 2011 study from Brussels — where lane splitting is legal — found that if you replaced 10 percent of cars on the road with motorcycles, it could cut the time everyone spends in traffic by 40 percent, in part because of lane splitting.

While safety is often used by drivers as a critique of lane splitting, motorcyclists say it actually makes them feel more safe while riding. Getting rear-ended while stationary or in motion is an especially big concern of motorcyclists.

“We don’t like to be sitting ducks,” Miller said. “When we’re going faster than the cars, we’re in control, and we’re monitoring and looking at what’s happening.”

Miller and Starkey also said motorcycles and motorcyclists can overheat if they are stationary for extended periods of time.

Motorcyclists also argue that drivers’ perceptions of lane splitting are skewed because they more clearly remember the times when a motorcyclists does it recklessly, at a high speed, as opposed to when a rider lane splits responsibly at low speeds.

“You might have 10 motorcycles go by that you don’t even give a second thought,” Haris said. “And then all of a sudden, either you’re not paying attention or someone goes by at a speed that you consider unsafe. And so that stands out in your mind. ‘Oh my gosh. That was dangerous.’ ”

9. Here’s what drivers say

The 2014 study found that 60 percent of drivers disapproved of lane splitting. More than half of those who disapproved said it was because they felt lane splitting was unsafe. Other reasons include fear that it will lead to a crash, it startles drivers, motorcyclists are going too fast, lane splitting is “unfair” or illegal.

“I think it’s rather unsafe,” said Edwin Martinez, who commutes to work across the Bay Bridge. “It’s unsafe for the person riding the motorcycle and also for the drivers of vehicles.”

Drivers have also reported motorcyclists intentionally knocking off side mirrors as they lane split.

“I have been clipped by a motorcycle,” said Gaurav Kapur, another daily Bay Bridge commuter, “and they didn’t stop for me. They didn’t stop and look what damage they did to the car.”

Kapur — who used to ride a motorcycle himself — said motorcycles can be hard to see when they lane split quickly through slow traffic.

“I love bikes,” he said, “but it’s just not safe. It’s highly unpredictable.”

10. Lane splitting misunderstandings can lead to road rage

If you search “lane splitting road rage” on YouTube, you’ll find lots of videos of lane splitting triggering angry reactions from drivers.

“For me, it resulted in somebody crossing a double yellow and running me up onto a sidewalk,” said Kat Taylor, who has been riding for about four years. “I ended up crashing trying to get away from them, and I broke 15 bones, and I got road rash over 40 percent of my body.”

Taylor said she no longer discusses lane splitting because of the disagreements it provokes, but other riders said they often have productive conversations about lane splitting.

“I think half the time they think it’s illegal,” Haris said. “So that oftentimes will change the discussion.”