Baby steps! Begin by leading/riding your horse near the road, where you are close enough to hear the traffic, but far enough that no amount of spooking/bolting will end up with you or your horse being in the road. Slowly work closer, over many days--not all in one day! When you are fairly confident that your horse will NOT spook at any amount of noise from the road, begin leading up by the road...little intervals at first. If things are still solid, go ahead and ride down the side of the road, beginning with small intervals and getting bigger. Slow process, I know...but who wants to get hit by a car??!?! I would really only advise it if there's a good side area to ride in where nobody's likely to hit you unless you spook big. Even then, might be best to get off and walk...I haven't personally seen this road, so I don't know really. I will say that I have to go up and cross the road in a TERRIBLE spot to get to some of my favorite trails, and to cross elsewhere I have to ride down a fairly busy road(busy for here anyway) and I do this to get my horse and myself prepared. Depending on how scary it is and how well your horse does with all this, maybe it's best to find an alternative route...but if that was possible you probably wouldn't have asked! Good luck!
A horse is the projection of peoples' views about themselves--strong, powerful, beautiful--and it has the capability of giving us escape from our mundane existance.