Any form of trivially detected is going to cost millions. For what they assume is a tiny market, it's really an edge case that has little benifit to them. Don't forget they avoided ticketing most people who simply went through the EZ pass lanes without paying for years. Why, cost benifit analysis, they did not want to drive away users over what amounted to be a fairly small revenue stream.

Edit: Also, all it takes is a picture of your license plate as you go through a toll an any anonymity is gone which makes anon EZ passes somewhat silly.

OCR of license plate characters is far easier than distinguishing specific vehicle and weight classes. Fortunately, the license plate is connected to a database that tells you much of what you want anyway. Couple that with weight sensors (to count axles), and you've got the data you need for tolling without doing any "hard" visual recognition.