my mare used to be really frightened of cars, and yesterday an eighteen wheeler passed by (not sure what it was doing in my neighborhood lol) and she barely quivered. what I did was walk her by cars, let her sniff them, look at them, I opened the doors and she looked inside, just showed her they wouldn't eat her. and I just continually rode her around my neighborhood, and when a car would start coming I'd turn her around and she would watch it come, pass, and then leave. I think she eventually figured out that they were not charging her, just passing by.
that's just what I did with my horse. good luck :)
"The only time I feel like I have my feet on the ground is when they're dangling three feet in the air.."