Every California driver's seen it: While you're sitting in your car on the freeway, creeping through traffic, one motorcyclist after another zips by, riding between lanes. It may seem unfair (why does he get to go ahead!?) and unsafe (won't he be hit!?), and for those reasons, illegal.

It's not. At least not in California. Lane splitting, also called lane sharing and traffic filtering, is 100 percent legal in the Golden State, simply because it's not illegal. It's not mentioned anywhere in the rules of the road.

Now, California politicians are looking to make it legit, with a bill that would make lane splitting explicitly legal—and, naturally, regulate it a bit. The bill sailed through the state Assembly without debate, and is headed to the Senate.

And if you care about congestion, carbon emissions, or motorcyclist safety, that's a good thing.

From Not Illegal to Legal

Every other state in the union explicitly bans lane splitting, according to the American Motorcyclist Association, although some have reconsidered in recent years. It's legal in Europe and Asia.

The California Vehicle Code does not mention lane splitting at all, which is precisely why it's not illegal. The closest thing motorcyclists have to lane splitting rules are guidelines the California Highway Patrol issued in 2013. They're not legally binding, but advise motorcyclists who traffic filter to do so when traffic is going 30 mph or slower, and not go more than 10 mph faster than the speed of traffic. They also recommend riding between the left lanes (where drivers are "more accustomed" to it) and not lane splitting near trucks, at night, on unfamiliar roads, or in bad weather.

The bill moving through the legislature runs along the same lines, but makes the issue much clearer. It "unequivocally authorizes motorcycles to drive between stopped or slow moving vehicles," as long as traffic is moving at 50 mph or less and the motorcyclist doesn't outrun traffic by more than 15 mph.

Advantages

The big upside of riding between cars is obvious for motorcyclists: While the chumps on four wheels are crawling over the Bay Bridge at 3 mph, cyclists are zipping through. But it's good for drivers too, because motorcyclists who lane split reduce congestion and carbon emissions. That's according to a 2012 study by Belgian research firm Transport & Mobility Leuven.

Studying one congested stretch of road, the researchers found replacing 10 percent of cars with motorcycles would cut time stuck in traffic by 63 percent—for everyone. Carbon emissions drop by 6 percent, due largely to smoother traffic flow. The study didn't specifically consider lane splitting, but noted that "when traffic comes to a complete standstill, it can be assumed that all motorcycles drive between two lanes."

Seemingly counter-intuitive, traffic filtering is actually a viable safety technique. Motorcycle Safety Consultant Steve Guderian

On top of all that, there are safety advantages to lane splitting. Hard to believe—it's so easy for a driver to hit someone riding by!—but riding between cars rather than in front of and behind them seems to reduce the risk of a deadly collision, specifically from being rear ended.

"Seemingly counter-intuitive, traffic filtering is actually a viable safety technique," motorcycle safety consultant Steve Guderian wrote in an August 2011 study. It "removes the motorcycle and rider from the danger spot behind a stopped car, and places the motorcycle into the more secure safety envelope that is created between two larger vehicles." Guderian found that California had significantly fewer motorcyclist fatalities from rear end collisions than other states.

California Assembly Member Dan Quirk, one sponsor of the bill, points out these benefits. He also argues that once lane splitting is officially legal, it'll be easier to slap on some rules to regulate what's safe and what isn't. “Hav[ing] clearly established guidelines in state law will make it easier for the California Highway Patrol and the Department of Motor Vehicles, for example, to educate drivers on this practice," he says.