Robin Hindery:

The device would mimic a standard license plate when the vehicle is in motion but would switch to digital ads or other messages when it is stopped for more than four seconds, whether in traffic or at a red light. The license plate number would remain visible at all times in some section of the screen.

In emergencies, the plates could be used to broadcast Amber Alerts or traffic information.

First showing ads when you make a person pay for taxes on the vehicle is egregious. Second, allowing for broadcast of Amber Alerts is pretty cool.