So I have a sign spanning the full width of the southbound tollway at this place:
And this is how I would have it for the present Florida's Turnpike Extension number (SR 821) with my suggested design for a tollway in Florida:
The purple denotes the main road's present TAGS ONLY / TOLL-BY-PLATE status. I'm going by Route Number and not the goofy fact that it's the Florida Turnpike Extension from original mile marker 4 of the Turnpike itself and not the original endpoint like Massachusetts did for the Mass. Pike.
Note the arrows underneath and divided by a line from the main board like in Australia.
Now here what it would like if it were an interstate:
I picked I-991 for two reasons: first, the existing Turnpike mainline is SR 91 which would become I-91 South in my scheme; second, if you look at a map you'll find most of the state highways in Miami-Dade County are in the 900s series.
Feel free to comment!