This conversation is really interesting because Toll roads in Texas is just a very complex thing and weird when you think of them. While working for TXDOT, I have found this presentation and it kind of sets the standards of what toll roads should be like in the state.
Simple Rules for Toll Roads in Texas
1. Toll roads can constitute anything and don't have to follow interstate standards as long as it is an independent road.
-EXAMPLE: Dallas North Tollway, PGBT, etc
2. If a toll road is built on an existing highway, that highway must become the frontage road, if an interstate, the toll road becomes an express lane or lanes.
-EXAMPLES: I-635, I-10, US 290 etc.
3. Toll roads can't be tolled if next to Airports. They must somehow provide free access to them
-EXAMPLES: Beltway 8, SH 161
4: Overall, Interstates and Highways must somehow stay free*
*Exceptions of I-169?, Beltway 8
Overall these rules have mostly stayed consistent through Texas with all of the agencies. Its kind of weird