The advantage of living with people who ride motorcycles.... My horses don't bat an eye at them. I'm surprised you've all had such bad experiences with them too. Most of the riders I know and have encountered on the road are extremely considerate of horses. They're much more aware of the damage a 1000+ lb animals can do to themselves and the bike.
Probably the most important thing is to have a horse that is calm and not all that spooky. It helps if your horse looks to you FIRST when he sees something "scary." You also need to be calm and not all that spooky.
If you have a smaller road or even a long driveway get someone to drive up/down it with you riding next to it. Not road speeds of course but will give you an idea of your horse's mindframe when encountered with a moving vehicle.
Watch out for broken glass and other debris on the road too.