When does filtering become undertaking?
The letter of the law: Filtering is moving between two or more lanes of stationary or slow-moving traffic. Undertaking is passing a vehicle on their nearside (left), when the lane you're using is otherwise clear, Undertaking is illegal.
The reality: The ability to get through sticky traffic is one of the biggest practical advantages of motorcycling. Everyone acknowledges that, the police included. Since the difference between filtering and undertaking can be slight, the most important factor is how the undertaking/ filtering was conducted. It is a subjective issue, hence the confusion.
The speed differential between you and the traffic you're passing is important. If you re caught flying through gridlock at 50, there'll be little mercy, and rightly so. Moving cautiously through 40 mph traffic at 60 mph on a dual carriageway is unlikely to raise an eyebrow. As Thames Valley accident investigator Gary Baldwin puts it, "Once the speed differential becomes extreme, the sympathy vote quickly evaporates."
And you may need that sympathy vote. If there is an accident and you find yourself in court, you could face charges of careless or even dangerous riding. The verdict will be largely down to the jury who, going on pure statistics alone, are unlikely to have a motorcyclist among them. If you can convince them that your actions were both considered and reasonable, and that the me dent was the result of poor observation or sheer carelessness on the part of the third party, you should be all right. The more sensible you were, the more sympathetic they'll be. It's that simple.
Can you filter over zig zags on the approach to a pedestrian crossing?
The letter of the law: You can move up a line of stationary or slow moving traffic in a zigzag area, but not to the front. You must not pass the vehicle at the head of the queue.
The reality: One to stick to. Most riders treat passing through Zigzags with caution - by definition they're found in tricky areas.