When traffic is present, ALWAYS walk facing oncoming traffic. In Spain, and most all continental Europe, this means on the LEFT. You commonwealth folks sometimes get it backwards. No problem, you will sort it out soon enough...or get mowed down by a vehicle coming up behind you.
The photos of pilgrims all over a road or path are likely because: the road is sufficiently rural that traffic is nil, the road is actually a farm access path on private property, or the pilgrims are just not paying full attention. In my experience, when an oncoming vehicle or piece of farm machinery is spotted folks will usually and quickly move into a single file, mostly on the correct side of the road. I usually find myself yelling "coche!" ('coach-aa' car in Spanish) to get people to look up from their phone screens...
But, seriously, on some very rural roads in some places, you might go 15 - 20 minutes, or more, without seeing a vehicle coming towards you. So, the photos are understandable.
Also, in Spain there is a law requiring pedestrians to wear hi-vis safety vests when walking against oncoming traffic. A viable alternative, of questionable legality, is to wear clothing items that are nearly as bright as the vest, without having to carry the additional vest. I do this. The police have not said anything yet. So, wearing bright clothing items is my considered recommendation.
My ball cap, shirts, outer wind shell parka, my hiking umbrella, and poncho are bright colors that stand out against the terrain and create contrast. You do not have to wear neon-lime yellow or international orange. Any stark and bright color that creates high contrast with the environment will work.
Lightweight clip on, blinky LED blinky lights, like used when riding a bicycle also work. I did this for a few years, but now eschew the added weight...everything adds up.
Headlamps also work in low visibility situations like on a rainy day, much like a bicycle front lamp. But, avoiding walking in the dark hours means I do not have to carry this weight whether... My iPhone provides more than enough light for finding the loo in the dark...
My bright red nylon wind and rain shell works. My bright red, yellow, or green t-shirts or polo shirts also work. So too, do my neon yellow ball cap work, as does my bright wasabi green poncho. I think you get the idea...
Conversely, subdued colors like black, navy blue, dark green, burgundy, brown, olive green, and any shade of grey etc. blend in with the rural and urban background. So, they do not create the high-contrast needed to provide a safety margin. Ironically most all of the rain gear one sees on Camino is usually black, navy blue, claret or burgundy, or a dark green... go figure...
THAT is what gets you seen...bright contrasting colors. I might look ridiculous, but an oncoming driver WILL see me. He or she may die laughing at the sight of me, but they WILL see me.
Hope this helps.