My mare is afraid of traffic as well, I could never get her over it. She doesn't think twice about anything else, but she'll back out into the road full speed ahead if there's a car coming at her(yes, into the road). She had gotten better, and then a bit worse. She's ridable on the road now, but she has the occasional meltdown if a car speeds by.
I would recommend playing a game of "chase" with your horse, find a moderately empty road, and someone who can and will drive for you. Have your horse follow the car, from a distance at first, and get closer. You can choose your speed, I like to do it at a trot, and then have the car stop, and ride right up on it. I usually circle around to the front, and have the driver go in reverse while doing the same thing(at a slow pace). Most horses feel that there is nothing to fear when they are feared, and that's what this "game" teaches. It will take a while.
And if you want to play another fun game, ride circles in a field by a busy road, just work and don't pay any attention. Keep the horse focused on you, and when a car goes by, stop the work, turn the horse so the vehicle is visible, talk soothingly and pet the horse. Then return to work. Get closer and closer as time goes by, and if the horse gets scared, go back a bit.
Proud mommy of...
Bailey- Appendix Gelding|Satin- Morgan Mare| Teka- Reg. Paint Mare|Shiloh- Reg. QH Mare| Misty- Reg. Pinto Mare