As previously indicated, a veterinarian can prescribe a sedative to help make the journey less stressful , and I would encourage you to have some on hand for use if necessary. There are also many homeopathic "de-stress" remedies on the market, but I have no experience with them so can't attest to their effectiveness.
But let me add, that since she has not had much experience in cars, she may well settle down on her own once you get on the freeway. Many dogs get excited or anxious when they first get in the car, or go on short city trips, but soon settle into the "vibe" of a longer excursion and will before long relax and go to sleep. Chances are, after every stop the time it takes her to settle down will lessen slightly, until she no longer sees it as a big deal at all. Good luck!
But let me add, that since she has not had much experience in cars, she may well settle down on her own once you get on the freeway. Many dogs get excited or anxious when they first get in the car, or go on short city trips, but soon settle into the "vibe" of a longer excursion and will before long relax and go to sleep. Chances are, after every stop the time it takes her to settle down will lessen slightly, until she no longer sees it as a big deal at all. Good luck!