Road rage is something many people deal with. I'd imagine that it's worse for people with BPD, though, since problems with emotional regulation and self control are a large part of BPD. Personally I'm pretty level-headed but I've still had a couple of incidences when someone's cut me off or driven dangerously around me and I've seen red and responded aggressively.
It might help to try driving a large truck before you get too upset at them. If they pull out in front of you it's because it's the biggest gap they've seen for ten minutes and there's no way they'd get up to speed before a car came even if none were in sight when they pulled out, so they're always going to be cutting someone off. They're usually doing the best they can, their vehicle isn't as fast or manoeuvrable as yours, so cut them some slack. Definitely don't do things like deliberately cutting a truck off and then braking hard - you're not just pissing off a truckie here, you're trying to intimidate the laws of physics, and that doesn't work. Kinetic energy doesn't give a rats about you or whether you have a disorder or whether you were raging at the time. If your car gets into fisticuffs with a truck... You. Will. Lose.
Coping strategies, have you tried the usual anger management stuff? Count to ten, look out the window, admire the clouds, try to think what a nice day it is. Imagine gathering all your rage into a ball in front of you, squeeze it tight so none of it escapes and it's all trapped like some dark red pulsating vein covered tumour. Then throw that ball of anger at the truck that's pissed you off - it's now his problem, not yours, and you can let it go. If all that fails then visualise yourself being horribly crushed to death by a 5-foot-tall bull bar because you have no impulse control and you picked a fight you can't win.
Edit: Maybe I should be clearer - I'm not saying that you shouldn't feel rage like that. Quite the opposite, I understand exactly how you feel because I've been there myself, it's a very natural reaction when some asshole pulls a stunt that could have killed you. I'm just saying that acting on that rage could see you badly injured or dead, and none of us want that. It's certainly OK to talk about stuff like that here, that's why this forum exists!