One of the things I think about when filtering on the motorway (Which I'm guessing is very much Wrefords world to work) is the gap and not the car. So, almost like playing with a photo in some editing suite, with the 'negative' option where you flip the blacks and whites of the photo, Sometimes I flip in my brain the 'hazard' and look at gaps as hazards and not the cars them selves as hazards.
A car alongside a car, is actually relatively safe. They're not going anywhere. If you saw a biker slicing between two cars where if he'd have waited just a few seconds, they wouldn't have been opposite one another, it would be easy to think 'What a plumb - why do it when they're side by side'. When I see a car with no car opposite him, that to me
is the danger - the car, in a jiffy could swap lanes and fill that gap.
So I often apply the negative filter to my vision, and sometimes wait for two cars to be alongside one another before I make my move. Sounds a bit upside down, but in the right circumstances it's much safer than gambling with the gaps.
In the very slow and heavy traffic when it's M25, five pm and it's just two solid lanes, I very much identify breaks in the lines of cars. Again, that's where it's going to go wrong and again, my negative filter looks for those and not cars.
As a very rough bench mark, when I'm at work and am being corporate (i.e. the world is watching and judging the bloke on the marked bike!) then I cap my filtering when the moving traffic gets to 50mph. I'm not saying I wouldn't filter at traffic flow speeds higher than that on my own or an unmarked bike, but for corporate image ... 50 mph then I call it a day.