From what I've been able to tell the Varia changes flash mode whenever it detects an approaching object. I wasn't aware it had anything to do with the speed of the oncoming vehicle. When I test it in the garage I can trigger it by just walking up from behind the bike. It goes into what I call hyper-flash mode with increased frequency and increased brightness even though I'm only approaching at 3 mph.

And I don't know how much it helps but it can't hurt in being noticed. Of course if the driver isn't looking at the road nothing will help.