Misbehaving around traffic is a dangerous thing and needs to be dealt with fairly aggressively, IMHO. I mean, giving it a time or two to see how bad it really is...a horse that jumps a little away from a car is one thing....but one that spins around, steps into the traffic lanes and tries to bolt is another.
I've seen three things that work....one was wearing spurs. I had a cutting horse mare who was a dream to ride, but if you weren't wearing spurs, she'd pull every sort of obnoxious behavior you can imagine, from refusing to step across tiny ditches, to boogering (hugely boogering) around mud puddles, to spooking so badly on the road that she almost smashed us both into an RV (who thoughtfully didn't slow down or anything).
I rode her out twice after I got her, and was broken hearted because she was supposed to be a family horse, one I could take my kids on, who was well trained, good to go for anything. She wasn't a dead head by any means (heavens no) but I was crushed because I took her out three times and three times I had to ride her back home because I couldn't get her past some ridiculous obstacle.
The third time I was almost in tears, and I told my mom, "I'm getting a pair of spurs, and if that doesn't work we're selling her."
I'd never used spurs before. It didn't even cross my mind that as a cutting horse, she'd probably RARELY been ridden without them.
So I got my spurs, and all I had to do was barely touch her with them when she dropped her head and started snorting. I mean the slightest touch. I never gootched her, never raked her. She just had to know they were there.
Mom had a horse that also spooked hideously at vehicles (randomly, you never knew when he would take exception) and spin and head for home. When I was riding (I was about 16 at the time and generally bareback) that meant he ran hell bent for leather all the way home and I just hung on. Not very pleasant.
When mom rode him, she over and undered him with her reins, and that was the end of that. When he's start to spook, she'd smack him and push him forward and that was it.
I've had other horses who would try to bolt or jump all over the road. Horses don't like to spin in circles. When they spin in circles what they're really trying to do is head back where they came from, and you're preventing them, so "teaching" them to spin in circles is really very unlikely in this situation. I generally have more luck circling a horse than I do with over and under, because I'm fairly uncoordinated and end up with reins wrapped around my head or hitting myself with them or whatever. It's okay with a romal or particularly long reins with clappers, but with regular split reins I just pull a nose over to my knee and the horse can spin around for as long as he likes. When he stops, we move forward. If he doesn't move forward, we go in circles again.
It does work, and it's generally pretty good at keeping the horse containing in one spot, i.e., out of the road. They have a hard time scooting all over the place when their nose is touching your knee (keep the outside rein LOOSE) and if they do start to veer somewhere you don't want to be, just stop, take a step or two and see where you're at. If the behavior has stopped, let him relax a minute and move forward. If it hasn't, circle him again.
When you're doing this it's important not to anticipate. You don't want to start circling your horse every time you see a car, or he will start to think that every time a car comes he's going to have to circle and it just makes it more unpleasant for him and makes him more likely to tense up and act like a booger. Wait until he spins back, and just bring him around and turn him around a few times and face in the direction you want to go and keep going. By then the car should be gone, and he'll have forgotten what the heck was going on and which direction he wanted to go. And eventually, he'll mellow out about it.
This would be a good thing to practice in a safe place, too, like a long driveway with a friend or two driving vehicles from different directions. Ultimately, the BEST way to deal with it is just to push him past it, let him booger to the side a little, if he spins just pull him back around and keep going, paying as little attention as possible and staying as calm as possible, but of course with random traffic whizzing along, where you may have limited shoulder area, the drivers aren't watching, this sometimes just isn't possible. The thing with being able to push him past it is he'll make the connection that it's a short lived scary thing, and he can just pass it and be on his way. Once he makes that connection, he'll spook less and less.
PS, some horses just are never very good with traffic. It might be something you have to face up to at some point in the future.