Oh man, I just had a nasty experience not too long ago with roads.
Clementine's pasture used to border a quiet road, and no matter what passed the horses never got scared. I could ride her up and down it and the cars wouldn't faze her. So, a few months later at her new stable, me and another girl who boarded went on a ride. You had to go down a semi-busy street to get to the quiet back roads. Now, it wasn't like a highway, but there was a steady amount of cars. Based on how good she did before I thought she would be just fine.
Silly me. She was scared. She spooked at every car. I thought about going back, but while she was spooking, she listened to me and my cues and stayed going in a straight line. No bolting or skittering, so I thought she was good. I managed to keep her in line until the quieter roads, where she was fine, even when cars passed. Okay, so, she's fine with the occasional car, but not heavier car traffic. So, we went around the dirt roads, past a school, weaved through some roundabouts. She was fine. Didn't even spook at the white crosswalk paint. Until we approached this semi busy road on the way back. It was farther down the road, and so added about a quarter mile to the busy roads.
This is where it gets nasty. For some reason, even though on the way out she was mostly okay, she went into full, 100% panic mode. Eyes huge, head tossing. Skittering at every car. The last straw was when a firetruck absolutely blasted by - I felt her tense, and then she turned and bolted about 20 feet. I got off, unclipped one rein and clipped it to her rope halter, leaving the other clipped to the bridle. It's good that I did. It got to the point where she was CONSTANTLY tugging, hopping, running, and it took all I had to keep hold of her. She stepped on a soda can at one point and kicked her back legs out so high she almost fell forwards. Being such a stocky, heavy horse I didn't even think she could do that! It was a mile and a half of pure torture, trying to stop my 2000 pound machine from trampling me (Almost happened). Looking back, we should have just gone back right away, but she wasn't doing too bad, I figured it would be the same on the way back.
Several times she almost broke free of me, which would have been dangerous, because she would have bolted into the road. Mind you, while this was happening, the girl I was riding with rode along making comments about how badly my horse is trained and how had she known this would happen she wouldn't have come along (Hah, me too), and how I was doing everything wrong. (Then why don't you get off and show me, hmm?)
We made it back. Clem calmed down and I discovered her main vice - traffic. It was awful, and though it was pretty obvious we were having issues, cars didn't even bother slowing down for us. One even honked. Many times. Some people, I swear. If I ever see people on horses, I make sure to slow down to prevent such things from happening. I couldn't even be upset - She was scared. Granted, she could have been scared and continued to listen to me, I should have taken her back when I realized she was uncomfortable with the traffic to start out with. We lunged upon returning to reestablish the fact that she needs to listen to me, and then did some very basic groundwork to end on a good note. Needless to say, it was terrifying.