A Rider in traffic uses the width of the bike to his or her advantage by "splitting the lane". Some feel it is extremely dangerous. Here is a breakdown:
1. It's illegal, mostly
In all states except California it is expressly stated in the law that passing between traffic on a road or highway is illegal. In California it is not expressly stated, but "reckless driving" is, and that's the ticket a rider will get if they actually are doing it recklessly.
2. It's dangerous.
Yes, it is, but not that dangerous. It is certainly a risk for the rider and that means it is technically dangerous. Done at 30 mph over the speed of traffic it is very dangerous. At slightly higher than the speed of traffic it has actually been found to be rather safe. In fact, states that do not allow lane splitting have more motorcycle fatalities from cars colliding into the rear of the motorcycle than California, where lane splitting is allowed. It just doesn't help that it scares the heck out of the Driver's as the Rider passes.
3. There is "lane-splitting"....
Also called "White-Lining". This is when you are moving in traffic, at a reduced pace or normal speed, and you may be alongside another car and suddenly a motorcycle zooms right between both of you. Many people are understanding of the driver that gets impatient sitting in traffic and just wants to move ahead, but it upsets many people as well. It is the proximity of a passing bike that bothers most people, as well as the noise. Others Drivers simply hate the fact that someone is getting ahead of them. Many Riders will tell you that some people deliberately try to cause the rider to have an accident. It happens - it's road rage. Crazy, isn't it? A person stuck in traffic might actually not care if they cause physical injury or death simply because they are stuck and the other guy isn't. Anyway, if you see a motorcycle doing this in your mirror you get two points. Keep an eye out because only the driver of the car can get points this way.
From the perspective of a Rider, he or she can see every inch of the bike from left to right. They see exactly the edge of the vehicle. This is not always possible in a car. A Driver's left side door is easy enough for them to sense, but the front right? - not so much. The Driver's right front fender is almost always overcompensated for because it can't always be seen from the driver's seat. This doesn't happen to the Rider. Like a cat using it's whiskers, they can tell exactly where they can fit, and they go for it. The risk is that sometimes that space closes as they ride into it, especially if they are going fast, and it all goes painfully wrong. Other times it comes out just fine, with only the Driver's ill will left in the wake.
4. ...and there is "filtering."
Filtering is different from lane-splitting in a very slight but important way. Filtering is done at a stop light and the Rider advances to the front of the line while the other vehicles are stopped.
The term "filtering" comes from the visual from above the street. The cars and trucks are stopped and the motorcycles just push through the spaces between. Many people find this far less rude. The cars are stopped and the bikes fit so why not?
For a Rider there is an extra benefit. Many Rider's get hit from behind at stoplights by Driver's that are just not looking. A bike that is filtering is now no longer the last bike in line for that inattentive driver to hit. By filtering, the Rider is now protected by the other cars around them from a strike from behind. They use the cars around them as a shield.
In Europe this a not illegal. In fact it is encouraged because of the effect it has on easing traffic congestion in busy cities.