I have ridden for miles on the roads, and from experience, hi viz on it's own is not enough. Welcome to the over the top safety thread!
Silicone flashing LED jogging armbands in red and white lights. Thread four red ones and two white ones onto nylon dog collars and use as leg bands - two white ones on front legs, two red ones for hind legs, and two red ones for your ankles. The dog collar means you will return home with them intact. A headtorch (flashing white light) for your hat, plus one for horse's browband. Attach a white flashing light to the front of a hi viz martingale. if you can source a lightweight red LED flashing armband, bandage the horse's tail and pin to the bandage, or put a plait in the tail and attach to that. Drivers travelling in the same direction as you tend to see the horse's hocks first (if they see you at all) so any hi viz below hock level won't be seen. White lights at the front and red to the rear indicate to other drivers which direction you are travelling. If you have got it right, you should stop traffic, or at least slow them down! I have found duct tape and cable ties are good for securing lights to saddlery.
