Kimberly Norton of Fort Worth offers this observation:
I had traveled TX 121 near Denton Tapp Road for over 20 years. Recently they made that part of the road a toll. It would be OK if you knew it was a toll (e.g. the paying booths) but it's a video toll. They take a photo of your license plate and send you a bill. I hope this is not starting a new trend in the DFW area!