The following opinion on filtering law has been supplied by Claims specialist Thomas Lawton.
Filtering - How it differs from an 'Overtake'.
Contrary to the belief of some motorists, filtering is entirely legal in the UK, providing that it is done safely. Once traffic speeds are high enough to suggest that the traffic is no longer queuing, your manoeuvre may then be regarded as a dangerous overtake.
Where is it illegal to filter?
As with any manoeuvre, you must not cause danger or force other vehicles to alter course or speed. Typically it's not a matter of where, but when. There are a few situations where it would be illegal to filter. Two that spring to mind is passing queuing traffic in a no overtaking zone (e.g. solid white lines or after a no overtaking sign) or on the approach to a crossing, with zigzags.
If the road you are on is split by a solid white line, it's still legal providing you follow the rules set out above and you DO NOT CROSS THE SOLID WHITE LINE! If you can safely pass (filter) on your side of the road, this is fine.
One of the problems the filtering motorcyclist faces in the event of an accident is the prejudice legal precedents that were established during the 1960s, 70s and 80s. Unfortunately, when a judge appoints blame for a particular incident, it forms what is known as a legal precedent. Legal precedents are the foundation of both civil and criminal law and allow the law to evolve over time. Essentially, a legal precedent means a future judge is bound to find liability in the same way when presented with a similar case.
There are a number of examples of bad legal precedent, which almost always appoint the majority of blame on the motorcyclist. Essentially the courts appear to have ruled that motorcycling is a risky and dangerous business and the rider as the vulnerable road user is typically to blame when an accident occurs.
For example the case of Powell vs. Moody (1966) found the motorcyclist to be 80% to blame for an accident where a motorist collided with the filtering motorcycle. In similar circumstances the case of Clarke vs. Whinchurch (1969) found the motorcyclist 100% at fault. In the case of Leeson vs. Bevis Transport (1972) the motorcyclist was found equally responsible for an accident where a van driver emerged from a side road. Most recently in the case of Worsford v Howe (1980) the motorcyclist was found 50% at fault when a vehicle changed lanes at the last moment in order to turn right and collided with motorcyclist. Clearly these are all examples of bad legal precedent, which prejudices the court against the motorcyclists. However, these precedents are not always definitive today. Essentially the courts will look at the speed and position of both vehicles, whether the traffic was stationary or moving and how fast the emerging vehicle was travelling. Most importantly the courts will listen to the opinions of any witness when apportioning blame.