This is indeed a very great idea! To think of our busy world today, some have no extra time to check the web especially people whose on-travel from place to place.
I think the best Option is the #2nd one: Billboard Located Just off the Interstate
Since the Billboard is to promote Steemit to everyone, making it available to wide range of people is always the best choice. Not only the residents in this place can see it but also those who's coming from other places.