Good morning from New Zealand..
Around our riding area we have a major forest and in that we also have a lot of log trucks travelling back and forth some 20 hours a day, so we have a choice... ride and get the horses used to all this or not ride the roadsides and have to float the horses to somewhere else to ride.
We choose to ride our roads and a system we found that suits us... is firstly with a new horse I will ride out on an experienced horse leading this new boy, and keep the new fella between the experienced horse and the traffic...
For a while as a truck approaches, the new horse will be scared and jig around, but after a few outings the new horse starts to see that the other horse is not phased at all so in time it learns to take its cues off the experienced horse..
Next I lead the new boy on the outside and again some jigging etc but soon learns that these monsters don't hurt it. Just big and noisy.
Next I ride the new horse and lead the other again with the other on the outside, so soon the new horse becomes used to the traffic..
I also do a lot of quiet talking to the boys to reassure them..
This works for me...
So you need to find and ride with someone who has a horse traffic desensertised initially ..
Ps..... take the number of vehicles that honk horns etc and call the police.. I have done this a couple of times and the police folks are more than happy to go door knocking to have a word with these people... around here anyway..!
Hope this helps..
Cheers for now..Tony