I had the same problem with my pup. I also live out in the country with very little traffic. Also, I rarely walk on the road since I am fortunate to have 100+ acres of woods and fields to walk my dog.
But I did want to train my dog out of chasing cars because my wife likes to take walks on the road with our dog. So, I did what you did and made her sit as a car drove by. I started using rewards / treats to teach the behavior that sitting as a car goes by is good. However, because dogs have a natural instinct to chase stuff (Micheal Ellis calls this "Self Reinforcing Activity" - which is something that a dog does because of their natural instincts and/or they enjoy doing, such as chasing cars, squirrels, etc.), it is difficult to teach them out of this behavior using rewards based training. I am not saying that it is not possible, but it is difficult.
After my pup learned that sitting was good as a car drove by, she got better, but still lunged out of position towards the car as it drove by on some occasion. So then I used prong collar and leash pop to correct her when she lunged. I did this for a few days and she has been very good since then. Good luck and keep practicing.