In this discourse, I'm treating 'filtering' as riding a motorbike between queues of stationary cars; 'lane splitting' is a similar thing, but the cars are actually moving.
Whether the practice is legal or not depends on where in the world you are. It's legal in California, but nowhere else in the USA as far as I know. Filtering and lane splitting are both legal in the UK and mainland Europe. In the UAE, where the legality of most things on the road seems to be decided ad hoc, it's apparently not allowed in Abu Dhabi, but may or may not be permissible in Dubai. Not that this stops most motorcyclists from doing it anyway.
The UK's Highway Code (Rule 88) includes the phrase: 'Additionally, when filtering in slow-moving traffic, take care and keep your speed low.' OK, I know that this rule is specific to the UK.
The definition of 'low' speed in this context is a grey area. Personally, I slow to a crawl so I can stop if the taxi driver in front of me suddenly opens his door to spit in the road. Random door opening seems less likely when traffic is moving, but sudden lane changes become more likely. Hence 'take care'. Various highways experts and driver pressure groups have opinions published on the internet and elsewhere, but the consensus seems to be not to lane split at more than 30mph above the prevailing traffic speed. This seems on the high side to me.
From a motorcyclist's point of view, it's only reasonable to move to the front of a traffic queue and be away with the green light, rather than to sit in temperatures in the forties Celsius with the engine blowing more hot air on his legs. Cars, at least, usually have air conditioning. This option should always be available on normal roads where a car is typically 1.8m wide in a 3.6m traffic lane. At roadworks, for example, traffic lane widths may be reduced, and my monstrous 101cm wide bike won't fit through the gap. I have to queue and watch the pizzas speed past me.
The advantages of allowing filtering and lane splitting don't stop with preventing bikers from getting hot legs. From their perspective, not getting stuck in traffic queues speeds their journeys and they're presumably not late for their appointments. This alone should make motorcycling an attractive proposition for commuting. A modal switch from cars to motorbikes offers advantages for everyone else on the road:
- When traffic is moving freely, a motorbike takes up pretty much the same amount of road as a car. That is one whole lane width, plus a gap in front and a gap behind. As congestion develops, the bike moves into its own narrow lane between queues of crawling cars and the space the biker was occupying becomes available for a car. The bike essentially disappears from the traffic volume.
- There's a safety issue. A bike is invisible in the middle of a traffic lane behind a bus or truck, and it's to everyone's advantage if the biker moves to one side so that the machine is visible in the mirrors of the vehicle in front. Moving to the side is also likely to reduce the chance of the bike being rear-ended.
- A motorbike, even one with a big engine, usually uses less fuel than a car. It uses a lot less than a car of comparable performance. Although motorcycle exhaust emissions are dirtier than those of modern, cat-converted car exhausts, it is fundamental that if you burn less, you pollute less. If you're not sitting in a traffic jam for half an hour, you're not polluting for 30 minutes at zero miles per gallon.
- Having arrived at his destination, perhaps his office, our hypothetical biker can park his machine in a much smaller space than a car. You can get three or four large motorbikes into one car space, and if the bike's parked in an alley or on a hardstanding where cars can't go, it's taking up no car spaces at all.
You can achieve these last two by car sharing. Four commuters in one car take up only one parking space, and quadruple the effective gas mileage. But you lose convenience and flexibility in travel arrangements.
Whether lane splitting is legal where you ride or drive, it isn't the job of car drivers to act as unpaid Traffic Policemen. Bikers are familiar with the scenario in which a driver sees a bike riding up the gap between queued cars. He becomes envious and resentful, and deliberately moves his car to block the biker's path. The biker simply rides around the other side of the car. If he's of the rude and vulgar type, the biker might choose to hurl abuse at, or to flip off the car driver for being a selfish pillock (which I cannot condone, but it happens). The car driver takes offence, having been rebuked for driving like an arse, and thinks, "Bikers are all ignorant, knuckle-dragging morons. QED."
Motorcyclists are, by and large, not knuckle-dragging morons. We are people, going about our daily lives the same as everyone else. Don't be envious if motorbikes pass you in a traffic jam. Just imagine how much worse that queue would be if each biker were driving a car.