You have touched on one of my pet subjects. Perhaps it is because it happend to me... twice... in my younger days. I also drive a LOT. (60,000 a year on average) When attacked as a teen, getting away was a very near thing, as the bg had quit playing games and started trying to ram me off the road. At any rate, I have given this a lot of thought over the years, and have thus far managed to avoid any further violent encounters. Here are some of my suggestions/observations.
Drive something large and preferably heavy. Yes, they burn more $ in the form of gas, but how much is your life worth?
When you purchase your next vehicle get a good older one. I am not talking abour dragging grandpa's packard out of the field, but something a few years old. Why spend a bunch of money on an new car which just end up being something to impress people you'll never meet at a stoplight. Besides, you'll save more than enough money to pay for the extra fuel, and quite a few other toys besides.
This is slightly off topic, but here is an interesting presentation on this very topic.
It is much to easy to get attached to that shiny new paintjob and allow a vehicle to become something other than a means of transportation. Don't get me wrong, I am a devoted gearhead, but it is easier to keep your perspective, on a 5 year old car than it is with one that you have 5 years worth of payments left on.
Before I go further, let me just say I am not avocating that you pit or ram anyone off the road, but let's face it, if you are driving a tank, a bg is less likely to try the same thing with you. Even if they do, the heavier your vehicle is in relation to the one trying to force you off the road, the easier it will be for you to maintain control.
Also if they do somehow box you in, you have much better odds of being able to ram your way out if it comes to it. Also in modern vehicles there are things such as airbags, and fuel pump inertia switches which are triggered by sudden deceleration/acceleration. The last thing you want to have happen is some nut ram you, trip the inertia switch and, suddenly disable your vehicle.
As was said earlier, I wouldn't display or heaven forbid fire your weapon unless you fear your life is about to end. Let's say you display it, and the bg decides to call in a "man with a gun" call to the police. He has just gotten a good look at it and will be able to describe it to them.
If you fire it, from a moving vehicle into another moving vehicle, and miss where is the bullet going to end up? Or lets say you hit them, and they panic, or become incapacitated. Where is their car going to end up? It would be entirely possible to make an otherwise "good shoot" and then have the bg's car go careening into the middle of someones living room, or wipe out some family in another vehicle.
Remain calm and try to calm the other driver down.
Make "I'm sorry" gestures, and ease over so they can pass. (whether you think you did anything wrong or not)
Stay on well traveled roads.
If they become threatening try to get away as safely as possible, and call 911. If you don't have a cell phone, buy one! If you can't afford one, get a friend's old one. Even without service you should be able to dial 911 as long as there is a tower nearby.
If they do somehow force you to stop, back up if there is room to do so, to keep distance between you and them. More distance = more time to react. If it becomes necessary to ram your way out of something like this, try to hit the other vehicle at or just behind the rear wheel. Most vehicles are lightest at this point, and will be easier to move with less damage to your vehicle. As a plus, most cars these days are front wheel drives and have fairly fragile rear axles. This means, that if you are lucky, you just might disable the vehicle or at least throw things out of kilter enough to prevent them from pursuing.