Back in the day when I used to cycle regularly, although not far, and spent a while using my bike to nip into town to do a little shopping, I actually made a frame to hold my old Krauser panniers on the back of the bicycle. The most immediate response was cars giving me a much wider berth when overtaking, much more than they ever did with the boxes on the Honda
Then I discovered an orange plastic 'flag' thingummy that bolted to the seat stem and could be folded out on the offside (or flat against the top tube if not needed) and this made a huge difference. Even small cars would pass on the opposite side of the road; probably thought I was some kind of weirdo.
Out in the sticks where I am now, the biggest problem is the conflict between fast moving cars (read mine) and slow moving bicycles on the ever twisting roads. Even were I to stick to the mainly 50 or 60 limits it is way too fast when you come up behind someone round a blind bend on a sit up and beg with a basket on the front. And don't bother quoting the 'stopping on the same side of the road in the distance one can see to be clear' bit. Some folks round here do stick to that advice, although I'm not sure they have ever heard it quoted, they just drive reeeeeeally slowly and probably wouldn't see, or have the reactions to avoid, any unfortunate cyclists anyway.
One day, someone will invent the intelligent cat's eye which will flash red for 15 seconds after a cyclist has ridden past it.
Or perhaps we could make all cyclists wear safety helmets with very tall poles on top with flashing lights so we can see them over the hedges,
a la the prototype sports car by Frank Costin (?) from the mid '70's which was so low that he incorporated a tall tower on the boot lif with a flashing beacon on the top of it because he felt too vulnerable testing it in the lanes round his workshop!