"Have you seen the 200 foot long billboards in the country beyond town? Did you know that once billboards were only twenty feet long? But cars started rushing by so quickly they had to stretch the advertising out to make it last." -- Ray Bradbury, Fahrenheit 451

Fair solution! Might be cheaper to just cover the interior with advertising though?

On the Beijing subway there are adverts on LED displays on the tunnel walls that move along with the train, keeping pace outside the windows.