It seems amazing that a horse can know from being ridden what you are looking at. But when you look at something your seat and weight change even if it's the tiniest bit. I've found with spooky horses that when I see something I know they will probably spook at, it really helps to not look at it. It's like how when you're going over a jump, if you focus ON the jump your horse may stop instead of going over it. You have to look beyond to where you want to go.
If I saw a scary car my horse always spooked at, I would deliberately look in another direction and for many horses this will help them relax. Because if you're not even looking at the object, then how scary can it be? Often we unconsciously stare at objects we know will frighten our horses.
Another thing I do when going by a scary object or when something pops up in front of us is to immediately take some very deep, slow but overly loud breaths. You may notice when horses get frightened they hold their breath like we do or inhale sharply. So it can be reassuring for them to hear that you are breathing slowly and normally. They cue off of this like they would another horse: oh, they are not worried so maybe I don't need to be either.