Like others have said, your horse will pick up on your state of mind. Relax, breath and stay calm, especially if you see something up ahead that might be scary to her ( a car, a bike, etc ). If she feels you're not stressed about it, she'll take your lead and will ignore it as well.
Its good that your friend can go along with you, it definitely helps to have a solid horse and rider with you. With my friend's younger horse, anytime we were out and there was something scary like an atv I'd just move my horse between them. This acted like a buffer, because Nugget is not bothered by that sort of thing, and it helped her horse relax as well.