I have tried everything, from hi-viz vests, to blinky-lights, to bright items of clothing, and reflective arm and leg bands.
My considered opinion, based on five caminos is to wear a neon bright hat or Buff. You cannot see the color of the hat when it is on your head, assuming the brim has a darker fabric to absorb reflected light and glare...most do. But, oncoming drivers can see your bobbing head quite well.
As others have noted, your rucksack is going to cover perhaps half of the surface area of your hi-viz vest. Blinky lights for bicycle or runner use are excellent, but add weight and complexity...they are also tempting to light-fingered folks you might encounter.
I also obtained some 3M white reflective tape at my local DIY store. It is also available on Amazon in both 1" and 2" widths. A little bit goes a LONG way.
I applied one 1" wide turn around each section of my collapsible hiking poles. If you do this, consider how the poles will collapse FIRST. Then apply the tape to the small lower part of each tube that does not hide inside the next, larger diameter tube. The net result is a hiking pole with four, highly reflective white stripes.
When walking on a road, I flick my wrist up and down to articulate the poles in a 90 degree arc. I have experimented with this in low light and at night. The oncoming driver sees a whirring arc of four stripes, like an airplane propellor with stripes. It is VERY attention getting.
I verified this myself having a friend swing the poles in an arc with me some 100 meters away. It is not so much the strips themselves, it is the reflection of light on a moving stripe that produces the dazzling effect.
Using this on all my Caminos starting with Camino #2, it never fails to capture the attention of oncoming drivers.
So, in the end, I think I have a viable safety solution that costs nil additional weight and is convenient to use: an insanely bright hat, combined with striped poles for signalling. It works for me.
I have also used and advocated wearing very bright t-shirts or outer clothing. My rain parka is international orange, my poncho is lime green, and many of my Camino t-shirts have been neon colors.
Lately (after Caminio #3) I dropped the bright t-shirts. I got too many snarky comments from others, including my wife. So, I went back to utilitarian dark shirts to hide sweat and dirt. The other signal items have proven sufficient\ for safety purposes.
This year, I am experimenting with 2" long, bright 3M reflective tabs that will slide onto any 1" or smaller rucksack strap. I could not find them commercially, so I "invented" them. I need to make them myself, but, If it works as I think it will, I may make them available for a nominal price.
My plan is for two on the front shoulder straps, two on the back top cinching straps, and one to each side on a compression strap. If I sell them, I am thinking of offering pairs, OR possible a complete rucksack kit of six.
I also experimented with one of these slide on tabs on my ball cap. Works like a charm. It provides a reflective signal at the back of your head... At least they will see my bobbing head before they hit it.
Stay tuned...
cheers folks, buen Camino 
