It terrifies me, the number of pedestrians and cyclists I see, when I am out in the car, who are well-nigh invisible. I do my best to drive safely, and if I see a cyclist or pedestrian, I will give them plenty of space and slow down as I pass them - then if they fall, or swerve or whatever, I will have the best chance possible of avoiding them.
If they are all in dark clothing, I may not see them until it is too late for me to pass them in that safe manner. I want to pass pedestrians and cyclists safely, and I assume they want the same - I will do all in my power to achieve this, and I think it is not unreasonable to want them to contribute to ensuring their safety by wearing something hi-viz. It could be as simple as a strap around their arm (something pretty small that could live in a pocket or the bottom of a handbag, when not needed) - is that an unreasonable thing to want?
If I cannot see someone, how am I supposed to avoid them? Crystal ball? The power to see into the future?
All three dses have or had paper rounds, and in the winter, they had to do them in the dark. We told them to use lights and hi viz clothing, to make themselves as visible as possible. They can't make all drivers into safe drivers, so they should control the things they can control - by making themselves as visible as possible.