hand walk the horse around a vehicle.
hand walk the horse around a vehicle that is running - have people in the vehicle TALK to the horse.
Then do it under saddle.
Then… have vehicle moving… but ALWAYS have the horse chase the bike, dog, vehicle so that the horse learns it can “scare” the thing away.
If you can, pasture the horse in SAFE paddock near a road. Have people ride bikes by the horse.
If riding with a friend - and there is enough room, have the Steady Eddy horse between the scary thing and your horse. This also works well when there are horse-eating mail boxes, discarded appliances, and tree stumps.
Tie some balloons to the fence for a week or so. Then tie balloons near where the horse eats (NOT the water trough though), hang a tarp so that it blows while horses are in pasture. Then work horse near it.
The MOST important thing the horse MUST know - is to stop and stand still. MUST. When I ride with others and we come to a scary place… it makes me crazy (and its happened to me and my horse too) when the horse cannot just stand or swings its butt around to back into whatever is coming.
And… play soccer in the field with your horse. just dribble the ball well away from your horse, then slowly work closer and closer. You eventually want to be able to kick the ball towards your horse without the horse making a big fuss… prepares them for birds popping up. or cats in the deep grass, etc.