Right -- there isn't any booking its just about who brings the money for that sign on that day. You can pre-declare how much money you want to assign to an hour and sign but can't know that you'll win for that sign and hour. If you don't win there, you keep your money to use elsewhere.

But we have ~1500 signs and each of them is playing a new ad every ~8 seconds. In other words, there's plenty of space. Those 8 seconds can range from a penny to a buck depending on the time and place and on the current competition. If you want to show up, you can.