I never ride on roads with speed limits higher than 35 mph (I even avoid the 35 mph four lane that leads into my neighborhood because the average speed is closer to 45, so I take the sidewalk instead), and the rest of the time I'm on an MUP, so I don't feel the need for hi-vis clothing. I do run a DiNotte taillight, solid at night and flashing during the day, whenever I'm on a roadway, regardless of speed limit.

But, even though I don't wear it, I think that AlmostTrick is right; hi-vis works. I immediately notice every cyclist wearing hi-vis in the city as soon as they come within my field of vision; they just pop out from the background clutter. On one of the highways heading out of Anchorage there is a separated pathway that parallels the road for the first 20 miles, all the way to the south Birchwood exit. It's a popular cycling route in the summer (rollerbladers never make it past mile ten ), and again the hi-vis wearers are much more noticeable than the others, despite being more than a mile from the highway at times.

Wearing hi-vis is just like having bright lights; it helps in being noticed by those who are watching for others while they drive, and sometimes those extra couple of seconds of awareness can make a world of difference. As for those who aren't watching, there's nothing you can do about them anyways.