I haven't seen a billboard that would follow me around before. Placing them at popular location depends on existing data (or often just plain-ass thinking, like obviously a hub in the middle of the town will have more people see it compared to the middle of nowhere), and only very very indirectly generates any sort of new data (e.g. if you only place a single ad in a specific place and can correlate that placement with an increase in product sales.. but that is thoroughly unrealistic and I only write it to be nitpicky with myself).