So in landscape dioramas it’s a lot of math. To get the point of view right and perspective working there’s some work to do.
Looks like your Ford is 1/24.
That bridge I‘d more likely estimate at about 1/4 of a circle. And probably more than 100 feet across. That suggests a radius somewhat less let’s guess 75 feet. That’s 3 feet in your model scale. And about 4 feet end to end.
But unless the car is passing under it you don’t care. If that's the plan, you can scale it down a little. But still it'll be a sizable radius and length, bigger than anything out there tire wise, short of farm equipment.
As a background, there's a couple of advantages, one being that you only need to make one side. And it can be a lot smaller and simpler.
I would suggest you mock up your dio. Get a big piece of paper, draw the road on it and put a car about the same size where you want it. Then pour a beer, sit back and think about what makes sense. I'd think any distance more than 3 feet between the two is going to be a lot to deal with.
Draw the bridge at your first guess, a simple sector of a ring, and set it up, then get back and look at it from track level. Fool around with that until you get what looks good.
Then you can start thinking about how to make the thing. Look at stuff during your trips to Home Depot. Maybe a piece of 6" PVC sewer pipe in a 90 degree sweep would yield a section. Or a section of 4" dryer flex vent, although that seems problematic.
Oh, and it was raining too.
If you ever can get out here for Car Week in August, there's usually a chance to see an older GT40 run. Nothing like it.
Good luck, great project.