Yes, the system can measure speed by simply measuring the time the vehicle takes to travel over a set distance under the tower. I don't foresee Texas ever using this to mail tickets out. Not as long as the Republicans are in charge here. It would also discourage use of the toll road which is contrary to how they are going to pay for the road. But in other cities and states that need the money and with the wrong politicians who feel like the job of the government is to get into the lives of the citizens as much as possible, it could happen. The technology does exist.
Wouldn't surprise me. They have two sets of receiver antennae on the towers, if their equipment was accurate enough, they could measure the time a single tag takes to activate both receivers. The difference could be calculated to I stand speed. It would be much more reliable the conventional radar because there is no question about which car is measured and they already have the best license plate capture system in the country. It wouldn't be hard to collect some fines from there.