I read the article with interest and learned quite a lot from it. Thanks for posting @madscots
I'm a pillion on the back of someone who filters regularly. I admit that on the odd occasion I have been nervous but not by the rider's ability. It's the drivers (cars in the main) who, as @RB very clearly indicates above, are occupied by mobile phones (speaking and texting), tablets, satnavs, newspapers(!), and personal grooming regimes. Many of them don't even see motorcycles filtering until they pass, observation skills NIL! Where are the police patrol vehicles when you need them?
Strange as it may seem I never felt more safe filtering than last year when travelling through London. I was surprised to see the mentality of drivers in London are completely different to Northern (England) drivers. As soon as the London drivers were aware of a motorcycle travelling behind them they immediately repositioned their vehicle, in a safe manner, in order to allow the motorcycle to pass without a problem.
This certainly doesn't happen in Leeds/Bradford/Sheffield areas, in fact the opposite, gaps are closed down, often in a dangerously quick, last minute fashion.
Perhaps I should move