Filtering is the act of overtaking slow-moving or stationary traffic by travelling between lanes and, despite causing scowls and abuse from many car owners, it's completely legal as long as it is done safely.
Why do riders filter?
There are lots of different names for filtering, including lane-splitting and white-lining, but what isn't up for debate is why motorcyclists do it -- to avoid congestion.
One of the many reasons we ride motorcyclesis the fact that you can avoid congestion on two wheels much easier than you can on four -- thanks to filtering.
Is filtering illegal?
Like most things in UK motoring law, whether it's illegal or not is in the eye of the beholder, the beholder in this case being the police.
Filtering, as an act, is legal and if you do it safely the police should not stop you. Filtering in an unsafe manner is illegal, however that very much depends on the speed of the traffic you are filtering past and the speed at which you pass them.
Filtering is also illegal in a no overtaking zone.
What about pedestrian crossings?
Things get a little more complicated when you approach a crossing with zig-zags.
Here filtering is illegal if you filter past a moving vehicle that is closest to a crossing, or the stationary vehicle that is closest to the crossing.
Essentially this means it is legal to filter past queuing traffic on the approach to a crossing, but not the vehicle that is closest to the crossing, whether it has stopped to allow passengers to cross or is moving through the crossing.
For an outline of the rules, see the useful comment from Graham beneath this post.
The 20:20 rule -- filtering safely
There is a school of thought that says filtering is safe as long as the traffic you are overtaking is travelling at no more than 20mph and you are overtaking them at no more than an additional 20mph.
This rule will cap filtering speeds at 40mph and it is thought that the police will not intervene with anyone filtering at that speed. Again, however, it is worth stressing that this is a school of thought rather than a hard and fast rule.
If a police officer determines that you are filtering unsafely, carelessly or too quickly, you could still be stopped, even if you haven't broached the 40mph limit.
How to filter safely
To filter safely is not to simply avoid police attention but to move through slow-moving or stationary traffic with minimal risk. This means you need to weigh up whether it is safe to filter and at what speed to minimise risk and maximise your progression.
Filtering safely is logical and, the faster you filter, the less time you or other road users have to react to you.
By filtering past slow-moving or stationary traffic, you will make time gains even at low speeds. To filter past faster moving traffic means we ourselves must up our speed and therefore the risk because lane-changing is more frequent at higher speeds.
As such, it is the speed of other traffic that should determine whether we filter or not, rather than the potential time gains.
