RFID car tags vs hand held tags, how exactly are they different?
In the toll roads you have a few things that help out:

Bigger antennas. The antenna over head is decently large compared to anything you would put to read someones badge walking into a building. The size of the antenna helps it to get better receive signal strength.

Inside the car, I have seen both active and passive tags, the active tags have longer range just because they have a battery in them, but they take up more space which you don't want to be carnying around. I have been told, by a letter sent out by my transit authority to all customers, that they take advantage of the windshield to help as an antenna. I am not exactly sure how this happens, but they say it does. It could also be that they use the metal of the car to help reflect.

Larger power. Yes, they do also send larger power down at you. But relative to the speed that you are driving through compared to walking through the door, you probably don't have much energy actually make it to you anyways.

Most car RFID stations don't actually care to get 100% of the reads so they are able to push the limits a little bit. The reason they don't care is that they can just take a picture of your license plate and realize what account it is and charge it. This process takes more computational power and many times includes a human so they try to limit it, but they can still fall back to it.

One thing you should also think about throwing into your thought process loop, the sensors that some stores use to but in items and then have an alarm go off if you try to steal them, those are all forms of RFID as well. In that situation they are able to take advantage of antennas that can be 4-5 feet tall and stick one on both sides of you.