My friend is driving us in their Subaru. I forget the model, might be a forester. It's not very loud. It's fairly new. I would ride in my boyfriend's car when I was a bit less severe and it was a beat up old loud Toyota Camry with horrible brakes.You didn't say what vehicle you will be driving?
You said you haven't driven much. I doubt your tinnitus would worsen. Maybe a spike but if you wear hearing protection for part of the trip, you will probably be fine. I am only speculating and trying to use common sense in my assessment. Everyone is different but I find it difficult to believe someone will worsen tinnitus or hyperacusis from one day of driving.
I don't think 'double protection' will help and I think it's overkill. Your ears and/or ear canals might get sore if you do that over a long trip.
If you have a loud vehicle with a loud muffler, for e.g., then I will think differently or if you have a car lower to the road which can mean more road noise (going in the cabin).
I think a few breaks from wearing hearing protection and even pulling over to a truck stop or to a park might be good, a few times over the 15 hrs might help.
I would be more concerned about vehicles with sirens or loud trucks that drive near you. I got significant ear pain from a truck driver using the air brakes. I was outside though. But, a truck driver slamming on their air brakes beside you in traffic will probably be pretty loud even if you have the windows closed.
I drive a Chevy but haven't touched it in months. I tolerated road noise before with no protection but ever since randomly worsening in the last 2 weeks I don't know what will happen.
Tinnitus isn't really a concern for me more than hyperacusis. I took a 15-minute drive to a hotel yesterday and my left ear was in pain. I felt a spike when a truck went by. This was with double protection too. So I can't imagine what 15 hours will do for me.