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The 20/20 'rule' is pretty good start point.
Only filter at a max speed of 20mph, while the traffic is doing <20mph?
That sounds about right.
Admittedly I'm fairly cautious, but I am always surprised by how fast people filter through stationary or slow moving traffic on my commute. Mainly I worry that if a car changes lanes without looking while traffic is moving then my insurance will find me at fault.
It's not sommuch a case of legal or illegal. But getting done for dangerous driving and driving without due car and attention.
The 20/20 'rule' is pretty good start point. And then use common sense and ride safe.