The camera could be separate from the billboard, and point at the backs of the cars. The billboard would be a short distance past that.

Flock cameras are oriented to read rear plates. One would need a camera similarly configured + a billboard some distance in front, or perhaps 2 billboards, a 1-2 setup + payoff combo, the camera behind the first billboard, and the dynamic text on the second. Pulling up other public data correlated to the plate - where legal - may make a splash. I'm thinking addressing the car owner by their first name.