Billboards are audited by the Traffic Audit Bureau.  There are three components that go into the weekly impressions:  

1. The impressions start by counting the number of vehicles that pass each unit on a weekly basis.  
2. Then they are adjusted for vehicle load factor (1.5 passengers per car).  
3. Finally, the audit  takes into account the angle of the billboard, the size of the board, how far back it is from the road, obstructions and other factors to come up with a visibility adjustment factor - this adjusts the impressions for the actual likelihood of seeing the board.      

All of this work is done by a third party organization called the Traffic Audit Bureau.